Patent Publication Number: US-11655494-B2

Title: Apparatus, systems, and methods for determining the concentration of microorganisms and the susceptibility of microorganisms to anti-infectives based on redox reactions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT Application No. PCT/US2018/054003 filed on Oct. 2, 2018, which claims the benefit of U.S. Provisional Application No. 62/567,648 filed on Oct. 3, 2017, the contents of which are incorporated herein by reference in their entities. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to in vitro detection of microorganisms or infectious agents and, more specifically, to apparatus, systems, and methods for determining the concentration of microorganisms or infectious agents and the susceptibility of such microorganisms or infectious agents to anti-infectives. 
     BACKGROUND 
     Infections caused by anti-infective resistant microorganisms or infectious agents are a significant problem for healthcare professionals in hospitals, nursing homes, and other healthcare environments. Rapid detection of such microorganisms is crucial in order to prevent the spread of their resistance profiles. When faced with such an infection, a preferred course of action is for a clinician to use anti-infective compounds judiciously, preferably only those necessary to alleviate the infection. However, what occurs most frequently today is that broad spectrum anti-infectives are given to the patient to ensure adequacy of treatment. This tends to result in microorganisms with multiple anti-infective resistances. Ideally, the sensitivity of the microorganism to anti-infectives would be detected soon after its presence is identified. 
     Existing methods and instruments used to detect anti-infective resistance in microorganisms include costly and labor intensive microbial culturing techniques to isolate the microorganism and include tests such as agar disk diffusion or broth microdilution where anti-infectives are introduced as liquid suspensions, paper disks, or dried gradients on agar media. However, those methods require manual interpretation by skilled personnel and are prone to technical or clinician error. 
     While automated inspection of such panels or media can reduce the likelihood of clinician error, current instruments used to conduct these inspections are often complex and require the addition of reporter molecules or use of costly components such as transparent indium tin oxide (ITO) electrodes. In addition, current instruments often rely on an optical read-out of the investigated samples, which require bulky detection equipment. 
     As a result of the above limitations and restrictions, there is a need for improved apparatus, systems, and methods to quickly and effectively detect anti-infective resistant microorganisms in a patient sample. 
     SUMMARY 
     Various apparatus, systems and methods for detecting the susceptibility of an infectious agent in a sample to one or more anti-infectives are described herein. In one embodiment a method of determining a concentration of an infectious agent can involve diluting a sample comprising the infectious agent with a dilutive solution to yield a diluted sample. The method can further involve introducing the diluted sample to a sensor such that the diluted sample is in fluid communication with a redox-active material of the sensor. The method can also involve monitoring an oxidation reduction potential (ORP) of the diluted sample over a period of time using at least one parameter analyzer coupled to the sensor to determine the concentration of the infectious agent in the sample. The ORP can be monitored in the absence of any added reporter molecules in the diluted sample. 
     In another embodiment, a system to determine a concentration of an infectious agent is disclosed comprising a metering conduit configured to deliver a dilutive solution to a sample comprising the infectious agent to yield a diluted sample. The system can comprise a redox-active material, a sample delivery conduit configured to introduce the diluted sample to the sensor, and at least one parameter analyzers coupled to the sensor. The parameter analyzer can be configured to monitor an ORP of the diluted sample over a period of time when the diluted sample is in fluid communication with the redox-active material of the sensor. The ORP can be monitored in the absence of any added reporter molecules in the diluted sample to determine the concentration of the infectious agent in the sample. 
     In another embodiment, a method of determining a susceptibility of an infectious agent to an anti-infective can involve diluting a sample comprising the infectious agent with a dilutive solution to yield a diluted sample. The method can also involve separating the diluted sample into a first aliquot and a second aliquot. The second aliquot can be used as a control solution. The method can also involve mixing an anti-infective at a first concentration into the first aliquot to yield a test solution and introducing the test solution to a first sensor such that the test solution is in fluid communication with a redox-active material of the first sensor. The method can further involve introducing the control solution to a second sensor such that the control solution is in fluid communication with the redox-active material of the second sensor. The method can also involve monitoring an ORP of the test solution and the control solution over a period of time using one or more parameter analyzers coupled to the first sensor, the second sensor, or a combination thereof. The ORPs can be monitored in the absence of any added reporter molecules in the test solution or the control solution. The method can further involve comparing the ORP of the test solution with the ORP of the control solution to determine the susceptibility of the infectious agent to the anti-infective. 
     In yet another embodiment, a system to determine a susceptibility of an infectious agent to one or more anti-infectives can comprise a metering conduit configured to deliver a dilutive solution to a sample comprising the infectious agent to yield a diluted sample. The metering conduit can separate the diluted sample into a first aliquot and a second aliquot. The second aliquot can be used as a control solution. The system can also comprise a first sensor comprising a redox-active material and a second sensor comprising the redox-active material. 
     The system can also comprise a first sample delivery conduit configured to introduce the first aliquot to the first sensor. The first sample delivery conduit can comprise a first anti-infective at a first concentration. The first aliquot can mix with the first anti-infective to form a first test solution. The system can also comprise a second sample delivery conduit configured to introduce the control solution to the second sensor. 
     The system can further comprise one or more parameter analyzers coupled to the first sensor and the second sensor. The one or more parameter analyzers can monitor an ORP of the first test solution over a period of time when the first test solution is in fluid communication with the redox-active material of the first sensor. The ORP can be monitored in the absence of any added reporter molecules in the first test solution. The one or more parameter analyzers can also monitor the ORP of the control solution over a period of time when the control solution is in fluid communication with the redox-active material of the second sensor. The ORP can be monitored in the absence of any added reporter molecules in the control solution. The one or more parameter analyzers or another device within the system can compare the ORP of the first test solution with the ORP of the control solution to determine the susceptibility of the infectious agent to the first anti-infective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates one embodiment of a method for determining the concentration of one or more infectious agents in a biological sample. 
         FIGS.  2 A to  2 C  illustrate embodiments of systems for determining the concentration of one or more infectious agents in a biological sample. 
         FIG.  3 A  illustrates example growth curves used to generate a standard curve for determining the concentration of one or more infectious agents in a biological sample. 
         FIG.  3 B  illustrates a fitted standard curve for determining the concentration of one or more infectious agents in a biological sample. 
         FIG.  4    illustrates example bacterial growth curves used to determine the concentration of the bacteria in a sample. 
         FIG.  5    illustrates one embodiment of a method for determining the susceptibility of one or more infectious agents to one or more anti-infectives. 
         FIG.  6    illustrates one embodiment of a multiplex system for determining the susceptibility of one or more infectious agents to one or more anti-infectives. 
         FIG.  7 A  illustrates a growth curve of an infectious agent resistant to one or more anti-infectives. 
         FIG.  7 B  illustrates a growth curve of an infectious agent susceptible to one or more anti-infectives. 
         FIG.  8    illustrates growth curves of bacteria in the presence of certain anti-infectives. 
         FIG.  9 A  illustrates a schematic of an embodiment of a sensor used as part of the methods and systems described herein. 
         FIG.  9 B  illustrates a schematic of another embodiment of the sensor used as part of the methods and systems described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Variations of the devices, systems, and methods described herein are best understood from the detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings may not be to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity and not all features may be visible or labeled in every drawing. The drawings are taken for illustrative purposes only and are not intended to define or limit the scope of the claims to that which is shown. 
       FIG.  1    illustrates an embodiment of a method  100  for determining the concentration of one or more infectious agents  102  in a sample  104 . The method  100  can comprise introducing one or more aliquots of the sample  104  into one or more reaction vessels  106  in step  1 A. The reaction vessels  106  can refer to one or more test tubes, reaction tubes, wells of a high throughput assay plate or well plate such as a 96-well plate, a 192-well plate, or a 384-well plate, culture plates or dishes, or other suitable containers for housing biological samples. One or more fluid delivery conduits  108  can inject, deliver, or otherwise introduce the aliquots of the sample  104  to the one or more reaction vessels  106 . 
     In other embodiments not shown in  FIG.  1   , a stimulus solution can be added to the sample  104  before introducing the sample  104  to the reaction vessel  106 . The stimulus solution can be a nutrient or growth solution. In these and other embodiments, the sample  104  can also be filtered before step  1 A. This filtering step can involve filtering the sample  104  using an instance of a filter, a microfluidic filter, or a combination thereof to filter out debris, inorganic material, and larger cellular components including blood cells or epithelial cells from the sample  104 . 
     The sample  104  can comprise at least one of a biological sample, a bodily fluid, a wound swab or sample, a rectal swab or sample, and a bacterial culture derived from the biological sample, the bodily fluid, the wound swab or sample, or the rectal swab or sample. The bodily fluid can comprise urine, blood, serum, plasma, saliva, sputum, semen, breast milk, joint fluid, spinal fluid, wound material, mucus, fluid accompanying stool, re-suspended rectal or wound swabs, vaginal secretions, cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, amniotic fluid, cultures of bodily which has been tested positive for bacteria or bacterial growth such as blood culture which has been tested positive for bacteria or bacterial growth (i.e., positive blood culture), or a combination thereof. 
     The infectious agents  102  that can be quantified using the methods or systems disclosed herein can be any metabolizing single- or multi-cellular organism including bacteria and fungi. In certain embodiments, the infectious agent  102  can be bacteria selected from the genera  Acinetobacter, Acetobacter, Actinomyces, Aerococcus, Aeromonas, Agrobacterium, Anaplasma, Azorhizobium, Azotobacter, Bacillus, Bacteroides, Bartonella, Bordetella, Borrelia, Brucella, Burkholderia, Calymmatobacterium, Campylobacter, Chlamydia, Chlamydophila, Citrobacter, Clostridium, Corynebacterium, Coxiella, Ehrlichia, Enterobacter, Enterococcus, Escherichia, Francisella, Fusobacterium, Gardnerella, Haemophilus, Helicobacter, Klebsiella, Lactobacillus, Legionella, Listeria, Methanobacterium, Microbacterium, Micrococcus, Morganella, Moraxella, Mycobacterium, Mycoplasma, Neisseria, Pandoraea, Pasteurella, Peptostreptococcus, Porphyromonas, Prevotella, Proteus, Providencia, Pseudomonas, Ralstonia, Raoultella, Rhizobium, Rickettsia, Rochalimaea, Rothia, Salmonella, Serratia, Shewanella, Shigella, Spirillum, Staphylococcus, Strenotrophomonas, Streptococcus, Streptomyces, Treponema, Vibrio, Wolbachia, Yersinia , or a combination thereof. In other embodiments, the infectious agent  102  can be one or more fungi selected from the genera  Candida  or  Cryptococcus  or mold. 
     Other specific bacteria that can be quantified using the methods and systems disclosed herein can comprise  Staphylococcus aureus, Staphylococcus lugdunensis , coagulase-negative  Staphylococcus  species (including but not limited to  Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus hominis, Staphylococcus capitis , not differentiated),  Enterococcus faecalis, Enterococcus faecium  (including but not limited to  Enterococcus faecium  and other  Enterococcus  spp., not differentiated, excluding  Enterococcus faecalis ),  Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus  spp., (including but not limited to  Streptococcus mitis, Streptococcus pyogenes, Streptococcus gallolyticus, Streptococcus agalactiae, Streptococcus pneumoniae , not differentiated),  Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella  spp. (including but not limited to  Klebsiella pneumoniae, Klebsiella oxytoca , not differentiated),  Escherichia coli, Enterobacter  spp. (including but not limited to  Enterobacter cloacae, Enterobacter aerogenes , not differentiated),  Proteus  spp. (including but not limited to  Proteus mirabilis, Proteus vulgaris , not differentiated),  Citrobacter  spp. (including but not limited to  Citrobacter freundii, Citrobacter koseri , not differentiated),  Serratia marcescens, Candida albicans , and  Candida glabrata.    
     Other more specific bacteria that can be quantified can comprise  Acinetobacter baumannii, Actinobacillus  spp.,  Actinomycetes, Actinomyces  spp. (including but not limited to  Actinomyces israelii  and  Actinomyces naeslundii ),  Aeromonas  spp. (including but not limited to  Aeromonas hydrophila, Aeromonas veronii  biovar  sobria  ( Aeromonas sobria ), and  Aeromonas caviae ),  Anaplasma phagocytophilum, Alcaligenes xylosoxidans, Actinobacillus actinomycetemcomitans, Bacillus  spp. (including but not limited to  Bacillus anthracis, Bacillus cereus, Bacillus subtilis, Bacillus thuringiensis , and  Bacillus stearothermophilus ),  Bacteroides  spp. (including but not limited to  Bacteroides fragilis ),  Bartonella  spp. (including but not limited to  Bartonella bacilliformis  and  Bartonella henselae, Bifidobacterium  spp.,  Bordetella  spp. (including but not limited to  Bordetella pertussis, Bordetella parapertussis , and  Bordetella bronchiseptica ),  Borrelia  spp. (including but not limited to  Borrelia recurrentis , and  Borrelia burgdorferi ),  Brucella  sp. (including but not limited to  Brucella abortus, Brucella canis, Brucella melintensis  and  Brucella suis ),  Burkholderia  spp. (including but not limited to  Burkholderia pseudomallei  and  Burkholderia cepacia ),  Campylobacter  spp. (including but not limited to  Campylobacter jejuni, Campylobacter coli, Campylobacter lari  and  Campylobacter fetus ),  Capnocytophaga  spp.,  Cardiobacterium hominis, Chlamydia trachomatis, Chlamydophila pneumoniae, Chlamydophila psittaci, Citrobacter  spp.  Coxiella burnetii, Corynebacterium  spp. (including but not limited to,  Corynebacterium diphtheriae, Corynebacterium jeikeum  and  Corynebacterium ),  Clostridium  spp. (including but not limited to  Clostridium perfringens, Clostridium difficile, Clostridium botulinum  and  Clostridium tetani ),  Eikenella corrodens, Enterobacter  spp. (including but not limited to  Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae  and  Escherichia coli , including opportunistic  Escherichia coli , including but not limited to enterotoxigenic  E. coli , enteroinvasive  E. coli , enteropathogenic  E. coli , enterohemorrhagic  E. coli , enteroaggregative  E. coli  and uropathogenic  E. coli )  Enterococcus  spp. (including but not limited to  Enterococcus faecalis  and  Enterococcus faecium )  Ehrlichia  spp. (including but not limited to  Ehrlichia chafeensia  and  Ehrlichia canis ),  Erysipelothrix rhusiopathiae, Eubacterium  spp.,  Francisella tularensis, Fusobacterium nucleatum, Gardnerella vaginalis, Gemella morbillorum, Haemophilus  spp. (including but not limited to  Haemophilus influenzae, Haemophilus ducreyi, Haemophilus aegyptius, Haemophilus parainfluenzae, Haemophilus haemolyticus  and  Haemophilus parahaemolyticus, Helicobacter  spp. (including but not limited to  Helicobacter pylori, Helicobacter cinaedi  and  Helicobacter fennelliae ),  Kingella kingii, Klebsiella  spp. (including but not limited to  Klebsiella pneumoniae, Klebsiella granulomatis  and  Klebsiella oxytoca ),  Lactobacillus  spp.,  Listeria monocytogenes, Leptospira interrogans, Legionella pneumophila, Leptospira interrogans, Peptostreptococcus  spp.,  Moraxella catarrhalis, Morganella  spp.,  Mobiluncus  spp.,  Micrococcus  spp.,  Mycobacterium  spp. (including but not limited to  Mycobacterium leprae, Mycobacterium tuberculosis, Mycobacterium intracellulare, Mycobacterium avium, Mycobacterium bovis , and  Mycobacterium marinum ),  Mycoplasm  spp. (including but not limited to  Mycoplasma pneumoniae, Mycoplasma hominis , and  Mycoplasma genitalium ),  Nocardia  spp. (including but not limited to  Nocardia asteroides, Nocardia cyriacigeorgica  and  Nocardia brasiliensis ),  Neisseria  spp. (including but not limited to  Neisseria gonorrhoeae  and  Neisseria meningitidis ),  Pasteurella multocida, Plesiomonas shigelloides. Prevotella  spp.,  Porphyromonas  spp.,  Prevotella melaninogenica, Proteus  spp. (including but not limited to  Proteus vulgaris  and  Proteus mirabilis ),  Providencia  spp. (including but not limited to  Providencia alcalifaciens, Providencia rettgeri  and  Providencia stuartii ),  Pseudomonas aeruginosa, Propionibacterium acnes, Rhodococcus equi, Rickettsia  spp. (including but not limited to  Rickettsia rickettsii, Rickettsia akari  and  Rickettsia prowazekii, Orientia tsutsugamushi  (formerly:  Rickettsia tsutsugamushi ) and  Rickettsia typhi ),  Rhodococcus  spp.,  Serratia marcescens, Stenotrophomonas maltophilia, Salmonella  spp. (including but not limited to  Salmonella enterica, Salmonella typhi, Salmonella paratyphi, Salmonella enteritidis, Salmonella cholerasuis  and  Salmonella typhimurium ),  Serratia  spp. (including but not limited to  Serratia marcesans  and  Serratia liquifaciens ),  Shigella  spp. (including but not limited to  Shigella dysenteriae, Shigella flexneri, Shigella boydii  and  Shigella sonnei ),  Staphylococcus  spp. (including but not limited to  Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus hemolyticus, Staphylococcus saprophyticus ),  Streptococcus  spp. (including but not limited to  Streptococcus pneumoniae  (for example chloramphenicol-resistant serotype 4  Streptococcus pneumoniae , spectinomycin-resistant serotype 6B  Streptococcus pneumoniae , streptomycin-resistant serotype 9V  Streptococcus pneumoniae , erythromycin-resistant serotype 14  Streptococcus pneumoniae , optochin-resistant serotype 14  Streptococcus pneumoniae , rifampicin-resistant serotype 18C  Streptococcus pneumoniae , tetracycline-resistant serotype 19F  Streptococcus pneumoniae , penicillin-resistant serotype 19F  Streptococcus pneumoniae , and trimethoprim-resistant serotype 23F  Streptococcus pneumoniae , chloramphenicol-resistant serotype 4  Streptococcus pneumoniae , spectinomycin-resistant serotype 6B  Streptococcus pneumoniae , streptomycin-resistant serotype 9V  Streptococcus pneumoniae , optochin-resistant serotype 14  Streptococcus pneumoniae , rifampicin-resistant serotype 18C  Streptococcus pneumoniae , penicillin-resistant serotype 19F  Streptococcus pneumoniae , or trimethoprim-resistant serotype 23F  Streptococcus pneumoniae ),  Streptococcus agalactiae, Streptococcus mutans, Streptococcus pyogenes , Group A streptococci,  Streptococcus pyogenes , Group B streptococci,  Streptococcus agalactiae , Group C streptococci,  Streptococcus anginosus, Streptococcus equismilis , Group D streptococci,  Streptococcus bovis , Group F streptococci, and  Streptococcus anginosus  Group G streptococci),  Spirillum minus, Streptobacillus moniliformi, Treponema  spp. (including but not limited to  Treponema carateum, Treponema petenue, Treponema pallidum  and  Treponema endemicum, Tropheryma whippelii, Ureaplasma urealyticum, Veillonella  sp.,  Vibrio  spp. (including but not limited to  Vibrio cholerae, Vibrio parahemolyticus, Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio vulnificus, Vibrio alginolyticus, Vibrio mimicus, Vibrio hollisae, Vibrio fluvialis, Vibrio metchnikovii, Vibrio damsela  and  Vibrio furnisii ),  Yersinia  spp. (including but not limited to  Yersinia enterocolitica, Yersinia pestis , and  Yersinia pseudotuberculosis ) and  Xanthomonas maltophilia  among others. 
     Furthermore, other infectious agents  102  that can be quantified can comprise fungi or mold including, but not limited to,  Candida  spp. (including but not limited to  Candida albicans, Candida glabrata, Candida tropicalis, Candida parapsilosis , and  Candida krusei ),  Aspergillus  spp. (including but not limited to  Aspergillus fumigatous, Aspergillus flavus, Aspergillus clavatus ),  Cryptococcous  spp. (including but not limited to  Cryptococcus neoformans, Cryptococcus gattii, Cryptococcus laurentii , and  Cryptococcus albidus ),  Fusarium  spp. (including but not limited to  Fusarium oxysporum, Fusarium solani, Fusarium verticillioides , and  Fusarium proliferatum ),  Rhizopus oryzae, Penicillium marneffei, Coccidiodes immitis , and  Blastomyces dermatitidis.    
     The fluid delivery conduits  108  can include tubes, pumps, containers, or microfluidic channels for delivering buffers, reagents, fluid samples including the sample  104  or solubilized solutions thereof, other solutions, or a combination thereof to and between devices, apparatus, or containers in the system. For example, as shown in  FIG.  1   , the fluid delivery conduits  108  can refer to parts of a pump such as a syringe pump. In other embodiments, the fluid delivery conduits  108  can include or refer to at least part of a hydraulic pump, a pneumatic pump, a peristaltic pump, a vacuum pump or a positive pressure pump, a manual or mechanical pump, or a combination thereof. In additional embodiments, the fluid delivery conduits  108  can include or refer to at least part of an injection cartridge, a pipette, a capillary, or a combination thereof. The fluid delivery conduits  108  can also be part of a vacuum system configured to draw fluid to or through channels, tubes, or passageways under vacuum. Moreover, the fluid delivery conduits  108  can include or refer to at least part of a multichannel delivery system or pipette. 
     The method  100  can comprise diluting the sample  104  comprising the infectious agent  102  with a dilutive solution  110  to yield a diluted sample  112  in step  1 B. In one embodiment, the dilutive solution  110  can comprise growth media or a growth inducer. In this and other embodiments, the dilutive solution  110  can be a solution containing bacto-tryptone, yeast extract, beef extract, cation-adjusted Mueller Hinton Broth (CAMHB), Mueller Hinton growth media (MHG), starch, acid hydrolysate of casein, calcium chloride, magnesium chloride, sodium chloride, blood or lysed blood including lysed horse blood (LHB), CAMHB-LHB, glucose, or a combination thereof. The growth inducer can comprise a carbon-based inducer, a nitrogen-based inducer, a mineral, a trace element, a biological growth factor, or any combination thereof. For example, the growth inducer can include but is not limited to glucose, ammonia, magnesium, blood, or a combination thereof. In one example embodiment, the dilutive solution  110  can comprise Tryptone, yeast extract, sodium chloride, and glucose. The dilutive solution  110  can be used to counteract the buffering effects of ions or substances present in the sample  104 . 
     In one embodiment, diluting the sample  104  with the dilutive solution  110  in step  1 B can involve diluting the sample  104  to a dilution ratio between about 1:1 to about 1:10. In another embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:10 to about 1:100. In yet another embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:100 to about 1:1000. In a further embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:1000 to about 1:10000. Although  FIG.  1    illustrates one reaction vessel  106  or one aliquot of the sample  104  being diluted, it is contemplated by this disclosure that multiple aliquots of the sample  104  can be diluted to different dilution ratios such that one or more diluted samples  112  can act as internal controls. 
     As will be discussed in the following sections in relation to  FIGS.  2 A,  2 B, and  2 C , in alternative embodiments, the method  100  can comprise diluting the sample  104  comprising the infectious agent  102  with deionized water, a saline solution, or a combination thereof serving as the dilutive solution  110 . In these embodiments, the diluted sample(s)  112  can be introduced to one or more sensors through sample delivery conduits comprising growth media or a growth inducer such that the diluted sample  112  is mixed with the growth media or growth inducer. More details concerning these embodiments will be discussed in the following sections. 
     The method  100  can also comprise incubating the diluted sample  112  at an elevated temperature for a period of time in step  1 C. The diluted sample  112  can be incubated in the same reaction vessel  106  or transferred to a different reaction vessel  106  or container. For example, the diluted sample  112  can be heated to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) and allowed to incubate for an incubation period  114 . The incubation period  114  can range from 15 minutes to over one hour. In other embodiments, the incubation period  114  can be less than 15 minutes or up to 48 hours. 
     The method  100  can further comprise introducing the diluted sample  112  to a sensor  116  or exposing the sensor  116  to the diluted sample  112  such that the diluted sample  112  is in fluid communication with a redox-active material  908  (see  FIGS.  9 A and  9 B ) of the sensor  116  in step  1 D. In one or more embodiments, the sensor  116  can be an oxidation reduction potential (ORP) sensor configured to respond to a change in a solution characteristic (e.g., the ORP) of a measured solution. In the example embodiment shown in  FIG.  1   , exposing the sensor  116  to the diluted sample  112  can involve directly immersing at least part of a handheld or probe instance of the sensor  116  into the diluted sample  112 . In this embodiment, the handheld or probe instance of the sensor  116  can be a handheld OPR sensor coupled to a standalone parameter analyzer  118  such as a voltmeter or multimeter. In another example embodiment shown in  FIG.  2   , introducing the diluted sample  112  to the sensor  116  can involve injecting, delivering, or otherwise introducing the diluted sample  112  to a well or container comprising the sensor  116  fabricated on a substrate. The sensor  116  will be discussed in more detail in the following sections. 
     The method  100  can further comprise monitoring the ORP of the diluted sample  112  with at least one parameter analyzer  118  coupled to the sensor  116  in step  1 E. The ORP of the diluted sample  112  can be monitored in the absence of any added reporter molecules or exogenous reporter molecules in the diluted sample  112  in order to determine the concentration of the infectious agent  102  in the original sample  104 . 
     The diluted sample  112  can have a solution characteristic. The solution characteristic of the diluted sample  112  can change as the amount of electro-active redox species changes due to the energy use, oxygen uptake or release, growth, or metabolism of the infectious agents  102  in the diluted sample  112 . For example, the amount of electro-active redox species in the diluted sample  112  can change as a result of cellular activity (e.g., microbial aerobic or anaerobic respiration) undertaken by the infectious agents  102 . As a more specific example, the amount of electron donors from Table 1 below (e.g., the amount of energy carriers such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH 2 )) in the diluted sample  112  can change due to the growth of the infectious agents  102  in the diluted sample  112  within the reaction vessel  106 . Also, as another more specific example, the amount of oxygen depleted in the diluted sample  112  due to aerobic respiration can change due to the growth of the infectious agents  102  in the diluted sample  112  within the reaction vessel  106 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Below is a “redox tower” visualizing potential electron donors  
               
               
                 and acceptors which can be utilized by microorganisms or infectious  
               
               
                 agents during the course of metabolism. An electron donor will  
               
               
                 have a greater negative potential than the electron acceptor.  
               
               
                 In aerobic respiration for example, O 2  can serve as a terminal  
               
               
                 electron acceptor whereas in anaerobic respiration, the terminal  
               
               
                 electron acceptor can comprise NO 3   − , Fe 3+ , Mn 4+ , SO 4   2− , or CO 2 . 
               
            
           
           
               
               
               
            
               
                   
                 Measured  
                   
               
               
                   
                 Standard 
                 Standard  
               
               
                   
                 Reduction  
                 Reduction 
               
               
                   
                 Potential E′ 0   
                 Potential E′ 0    
               
               
                 Electron Donor and Acceptor Pairs 
                 (mV) 
                 (mV) range 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Glucose    2 Pyruvate + 2e −   
                 −720 
                 −700 
               
               
                   
                   
                 −600 
               
               
                 Glucose    6 CO 2  + 24e −   
                 −500 
                 −500 
               
               
                 H 2      2H +  + 2e −   
                 −420 
                 −400 
               
               
                 NADH    NAD +  + 2e −   
                 −320 
                 −300 
               
               
                 2 GSH    GSSG + 2e −   
                 −240 
                 −200 
               
               
                 H 2 S    SO 4   2-  + 8e −   
                 −220 
                   
               
               
                 FADH 2      FAD + 2H +  + 2e −   
                 −220 
                   
               
               
                 Lactate    Pyruvate + 2e −   
                 −190 
                 −100 
               
               
                 Succinate    Fumarate + 2e −   
                 33 
                 0 
               
               
                 Cyt b (red)    Cyt b (ox) + e −   
                 80 
                   
               
               
                 Ubiquinol    Ubiquinone + 2e −   
                 110 
                 100 
               
               
                 Cyt c (red)    Cyt c (ox) + e −   
                 250 
                 200 
               
               
                 Cyt a (red)    Cyt a (ox) + e −   
                 290 
                   
               
               
                   
                   
                 300 
               
               
                 NO 2   −  + H 2 O    NO 3   −  + 2e −   
                 420 
                 400 
               
               
                 NH 4   +  + H 2 O    NO 2   −  + 6e −   
                 440 
                   
               
               
                 Mn 2+  + H 2 O    MnO 2  + 2e −   
                 460 
                   
               
               
                   
                   
                 500 
               
               
                   
                   
                 600 
               
               
                 ½ N 2  + 3H 2 O    NO 3   −  + 5e −   
                 740 
                 700 
               
               
                 Fe 2+     Fe 3+  + 1e −   
                 770 
                   
               
               
                 H 2 O    ½ O 2  + 2H +  + 2e −   
                 820 
                 800 
               
               
                   
                   
                 900 
               
               
                   
               
            
           
         
       
     
     As illustrated in  FIG.  1   , the parameter analyzer  118  can be connected to or communicatively coupled to a device having a display  122  or a display component configured to display a read-out of the electrical characteristic of the sensor  116  representing the solution characteristic of the diluted sample  112 . Such a device can be referred to as a reader  120 . In certain embodiments, the reader  120  can be a mobile device, a handheld device, a tablet device, or a computing device such as a laptop or desktop computer and the display  122  can be a mobile device display, a handheld device display, a tablet display, or a laptop or desktop monitor. In these and other embodiments, the parameter analyzer  118  can wirelessly communicate a signal or result to the reader  120  or another computing device having the display  122 . In other embodiments, the parameter analyzer  118  and the reader  120  can be integrated into one device. 
     The method  100  can further comprise monitoring the ORP of the diluted sample  112  over a period of time with the at least one parameter analyzer  118 , the reader  120 , or a combination thereof in step  1 F. The parameter analyzer  118 , the reader  120 , or a combination thereof can also determine the concentration of the infectious agent  102  in the sample  104  within this period of time in step  1 F. The period of time within which the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the concentration of the infectious agent  102  can be referred to as a quantification window  124 . In one embodiment, the quantification window  124  can be between 60 minutes and 120 minutes. In other embodiments, the quantification window  124  can be between 5 minutes and 60 minutes. In additional embodiments, the quantification window  124  can be greater than 120 minutes. 
     The parameter analyzer, the reader  120 , or a combination thereof can determine the concentration of the infectious agent  102  in the sample  104  using measured ORP signals (e.g., measured output voltages) and a standard curve  126  generated by monitoring the ORPs of prepared cultures of the infectious agent in different concentrations. In some embodiments, the standard curve  126  can be generated before step  1 A. In other embodiments, the standard curve  126  can be generated at any time prior to step  1 F. 
     In one example embodiment, the standard curve  126  can be generated using different concentrations of bacteria (e.g., from about 1*10 4  CFU/mL to about 1*10 8  CFU/mL) grown at 35° C. in growth media. The ORPs of growth media comprising such bacterial concentrations can be monitored over time for a change in their ORPs using an ORP sensor. A threshold voltage can be set (e.g., between about −100 mV and 100 mV) and a standard curve can be generated by plotting the various bacterial concentrations against the time it took the monitored ORP of each such bacterial concentration to reach the threshold voltage (also known as the time-to-detection (TTD)). Generation of the standard curve is discussed in more detail in the following sections. 
     With the standard curve  126  generated, the method  100  can involve comparing the measured or monitored ORP of the diluted sample  112  over time against the values obtained from the standard curve  126 . For example, as shown in  FIG.  1   , a growth curve  128  for the infectious agent  102  within the sample  104  under investigation can be generated using the change in ORP of the diluted sample  112  over time measured by the parameter analyzer  118 , the reader  120 , or a combination thereof. The same threshold voltage  130  can be applied to the growth curve  128  as the threshold voltage  130  used to generate the standard curve  126 . The time-to-detection  132  or the time it took the monitored ORP of the diluted sample  112  to reach the threshold voltage  130  can be ascertained from the growth curve  128 . The reader  120 , the parameter analyzer  118 , or another device can then determine the concentration of the infectious agent in the sample  104  under investigation by using the time-to-detection  132  and the values obtained from the standard curve  126 . For example, the concentration can be calculated using the time-to-detection  132  and an equation derived from the standard curve  126 . 
     In some embodiments, one or more of the aforementioned steps of the method  100  can be stored as machine-executable instructions or logical commands in a non-transitory machine-readable medium (e.g., a memory or storage unit) of the parameter analyzer  118 , the reader  120 , or another device communicatively or electrically coupled to the parameter analyzer  118  or the reader  120 . Any of the parameter analyzer  118 , the reader  120 , or another device coupled to the parameter analyzer  118  or the reader  120  can comprise one or more processors or controllers configured to execute the aforementioned instructions or logical commands. 
     The steps depicted in  FIG.  1    do not require the particular order shown to achieve the desired result. Moreover, certain steps or processes may be omitted or occur in parallel in order to achieve the desired result. In addition, any of the systems or devices disclosed herein can be used in lieu of devices or systems shown in the steps of  FIG.  1   . 
       FIGS.  2 A,  2 B, and  2 C  illustrate embodiments of systems  200  for determining the concentration of one or more infectious agents  102  in a sample  104  (see  FIG.  1   ). It is contemplated by this disclosure (and it should be understood by one or ordinary skill in the art) that any of the systems  200  described in connection with  FIG.  2 A,  2 B , or  2 C can be used to undertake one or more steps of the method  100  described in the preceding sections.  FIG.  2 A  illustrates that the system  200  can comprise one or more sensors  116  fabricated or positioned on a surface of a substrate  202 , one or more parameter analyzers  118  electrically or communicatively coupled to the one or more sensors  116 , and one or more readers  120  electrically or communicatively coupled to the one or more parameter analyzers  118 . In some embodiments, the reader  120  and the parameter analyzer  118  can be integrated into one device. 
     In some embodiments, the substrate  202  and the sensors  116  can be part of a cartridge, a test strip, an integrated circuit, a micro-electro-mechanical system (MEMS) device, a microfluidic chip, or a combination thereof. In these and other embodiments, the substrate  202  can be part of a lab-on-a-chip (LOC) device. In all such embodiments, the sensors  116  can comprise components of such circuits, chips, or devices including, but not limited to, one or more transistors, gates, or other electrical components. The sensors  116  can be micro- or nano-scale ORP sensors. Each of the sensors  116  can comprise an active electrode and a reference electrode (see  FIGS.  9 A and  9 B ). Each of the sensors  116  can also comprise a redox-active material  908  (see  FIGS.  9 A and  9 B ) or layer such as a gold layer, a platinum layer, a metal oxide layer, carbon layer, or a combination thereof. The sensors  116  will be discussed in more detail in the following sections. 
     In one embodiment, the sample  104  comprising the infectious agent  102  can be diluted using growth media or growth inducers representing the dilutive solution  110 . The growth media or growth inducers can be the same growth media or growth inducers described with respect to step  1 B of method  100 . In this embodiment, the diluted sample  112  can be injected, pipetted, delivered, or otherwise introduced to the one or more sensors  116  such that the diluted sample  112  is in fluid communication with the redox-active material  908  (see  FIGS.  9 A and  9 B ) of the sensors  116 . 
     The system  200  can also comprise an incubating component configured to incubate the diluted sample  112  in fluid communication with the sensor  116  by heating the diluted sample  112  to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) for a period of time (e.g., the incubation period  114 ). 
     In another embodiment, the sample  104  comprising the infectious agent  102  can be diluted using deionized water, a saline solution, or a combination thereof representing the dilutive solution  110  to yield the diluted sample  112 . In this embodiment, the one or more sensors  116  on the substrate  202  can be covered or coated by a lyophilized or dried form of the growth media or growth inducer. For example, the one or more sensors  116  can comprise a layer of lyophilized or dried growth media or growth inducer covering or coating the one or more sensors  116 . In another embodiment, the lyophilized or dried growth inducer can cover or coat a surface in a vicinity of the one or more sensors  116 . In yet another embodiment, the one or more sensors  116  can be disposed within a well or a container defined on the substrate  202  and the well or container can comprise an aqueous form of the growth media or growth inducer. In all such embodiments, the diluted sample  112  can mix with the growth media or growth inducer. 
     The incubating component can then incubate the diluted sample  112  mixed with the growth media or growth inducer by heating the mixture to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) for a period of time (e.g., the incubation period  114 ). 
       FIG.  2 B  illustrates another embodiment of a system  200  for determining the concentration of one or more infectious agents  102  in a sample  104 . The system  200  can comprise a sample receiving surface  204  defined on a substrate  202 , one or more metering conduits  206  in fluid communication with the sample receiving surface  204 , a sensor  116  fabricated or otherwise disposed on the substrate  202 , one or more sample delivery conduits  208  fluidly connecting or extending in between the sample receiving surface  204  and the sensor  116 , a parameter analyzer  118  electrically or communicatively coupled to the sensor  116 , and a reader  120  electrically or communicatively coupled to the parameter analyzer  118 . In some embodiments, the reader  120  and the parameter analyzer  118  can be integrated into one device. 
     In one or more embodiments, the sample receiving surface  204  can be a flat surface for receiving the sample  104 . In other embodiments, the sample receiving surface  204  can be a concave or tapered surface of a well, divot, dish, or container. For example, the sample  104  can be injected, pipetted, pumped, spotted, or otherwise introduced to the sample receiving surface  204 . 
     The one or more metering conduits  206  can be channels, passageways, capillaries, tubes, parts therein, or combinations thereof for delivering the dilutive solution  110  to the sample  104  on the sample receiving surface  204 . For example, the one or more metering conduits  206  can refer to channels, passageways, capillaries, or tubes defined on the substrate  202 . Also, for example, the one or more metering conduits  206  can refer to channels, passageways, capillaries, or tubes serving as part of hydraulic pump, a pneumatic pump, peristaltic pump, a vacuum or positive pressure pump, a manual or mechanical pump, a syringe pump, or a combination thereof. For example, the one or more metering conduits  206  can be microfluidic channels or tubes or channels serving as part of a vacuum system. 
     In some embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:1 to about 1:10. In other embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:10 to about 1:100. In additional embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:100 to about 1:1000. In yet additional embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:1000 to about 1:10000. 
     The one or more sample delivery conduits  208  can be channels, passageways, capillaries, tubes, parts therein, or combinations thereof for delivering the diluted sample  112  to the sensor  116 . For example, the one or more sample delivery conduits  208  can fluidly connect the sample receiving surface  204  with the sensor  116  such that the diluted sample  112  or fluid on the sample receiving surface  204  is in fluid communication with at least part of the sensor  116 . 
     As shown in the example embodiment of  FIG.  2 B , the one or more sample delivery conduits  208  can comprise growth media  210  or growth inducer. The growth media  210  or growth inducer can be the same growth media or growth inducer discussed in connection with  FIG.  2 A  and  FIG.  1   . 
     In one or more embodiments, the sample delivery conduits  208  can be covered or coated by a lyophilized or dried form of the growth media  210  or the growth inducer. In other embodiments, the sample delivery conduits  208  can contain growth media  210  or grow inducer in an aqueous form. In these and other embodiments, the dilutive solution  110  delivered by the one or more metering conduits  206  can be a saline solution, deionized water, or a combination thereof. The dilutive solution  110  can dilute the sample  104  and deliver the sample  104  through the sample delivery conduits  208  to the sensor  116  such that the diluted sample  112  mixes with the growth media  210  en route to the sensor  116 . In other embodiments not shown in the figures, at least one layer of the sensor  116  or a surface in a vicinity of the sensor  116  can be coated or covered by the growth media  210  in lyophilized or dried form and the diluted sample  112  can mix with the growth media  210  when the diluted sample  112  is in fluid communication with the part of the sensor  116  or part of the area covered by the growth media  210 . 
     In all such embodiments, the diluted sample  112  can mix with the growth media  210  or growth inducer. 
     The incubating component can then incubate the diluted sample  112  mixed with the growth media  210  or growth inducer by heating the mixture to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) for a period of time (e.g., the incubation period  114 ). 
     In some embodiments, the substrate  202  and sensors  116  can be part of a cartridge, a test strip, an integrated circuit, a micro-electro-mechanical system (MEMS) device, a microfluidic chip, or a combination thereof. In these and other embodiments, the substrate  202  can be part of a lab-on-a-chip (LOC) device. In all such embodiments, the sensor  116  can comprise components of such circuits, chips, or devices including, but not limited to, one or more transistors, gates, or other electrical components. The sensor  116  can be a micro- or nano-scale ORP sensor. The sensor  116  can comprise an active electrode and a reference electrode. The sensor  116  can also comprise a redox-active material  908  (see  FIGS.  9 A and  9 B ) or layer such as a gold layer, a platinum layer, a metal oxide layer, carbon layer, or a combination thereof. The sensor  116  will be discussed in more detail in the following sections. 
       FIG.  2 C  illustrates a multiplex version of the system  200  shown in  FIG.  2 B . For example, the system  200  of  FIG.  2 C  can have multiple sensors  116 , multiple metering conduits  206 , and multiple sample delivery conduits  208 . In one embodiment, different samples comprising different types of infectious agents can be delivered, injected, or otherwise introduced to the various sample receiving surfaces  204  on one substrate  202 . 
     The substrate  202  can be comprised of a polymeric material, a metal, a ceramic, a semiconductor layer, an oxide layer, an insulator, or a combination thereof. The substrate  202  can be part of a test strip, cartridge, chip or lab-on-a-chip, microfluidic device, multi-well container, or a combination thereof. The sensors  116  can be fabricated or located on a surface of the substrate  202 . In some embodiments, the one or more parameter analyzers  118  can also be fabricated or located on the substrate  202 . In other embodiments, the one or more parameter analyzers  118  can be standalone devices such as a voltmeter or a multimeter electrically coupled to the sensors  116 . 
     In this embodiment, the system  200  shown in  FIG.  2 C  can be used to determine the concentrations of infectious agents  102  in multiple samples concurrently. In other embodiments, aliquots of the same sample  104  can be introduced to the various sample receiving surfaces  204  on one substrate  202  and different amounts of the dilutive solution  110  can be delivered to the various sample receiving surfaces  204  through the metering conduits  206 . In this embodiment, the multiplex system  200  of  FIG.  2 C  can be used to dilute aliquots of the same sample  104  to different dilution ratios so as to use certain dilutions as internal controls and to determine the minimum amount of dilution needed to quantify a certain sample. 
     In the example embodiments shown in  FIGS.  2 A,  2 B, and  2 C , the one or more parameter analyzers  118  can be disposed or fabricated on the substrate  202  or the parameter analyzers  118  can also be standalone devices coupled to the one or more sensors  116 . The parameter analyzers  118  can be electrically or communicatively coupled to one or more readers  120  having a display  122  or display component. The display  122  or display component can be configured to display a read-out of the electrical characteristic of the one or more sensors  116  representing the solution characteristic of the diluted sample  112 . In certain embodiments, the reader  120  can be a mobile device, a handheld device, a tablet device, or a computing device such as a laptop or desktop computer and the display  122  can be a mobile device display, a handheld device display, a tablet display, or a laptop or desktop monitor. In some embodiments, the parameter analyzer  118  can wirelessly communicate a signal or result to the reader  120  or another computing device having the display  122 . 
     Similar to step  1 F of method  100 , the systems  200  of  FIGS.  2 A,  2 B, and  2 C  can monitor the ORP of the diluted sample  112  and determine the concentration of the infectious agent  102  in the sample  104  within a period of time (e.g., the quantification window  124  of method  100 ). This period of time can be between 60 minutes and 120 minutes. In other embodiments, this period of time can be between 5 minutes and 60 minutes. In additional embodiments, this period of time can be greater than 120 minutes. 
     The parameter analyzer  118 , the reader  120 , or another device in communication with the parameter analyzer  118  or the reader  120  can determine the concentration of the infectious agent  102  in the sample  104  using measured ORP signals (e.g., measured output voltages) and a standard curve (such as the standard curve  126  described in connection with method  100  of  FIG.  1   ). In one example embodiment, a standard curve can be generated using different concentrations of bacteria (e.g., from about 1*10 4  CFU/mL to about 1*10 8  CFU/mL) grown at 35° C. in growth media. The ORPs of growth media comprising such bacterial concentrations can be monitored over time for a change in their ORPs using one or more ORP sensors. A threshold voltage can be set (e.g., between about −100 mV and 100 mV) and a standard curve can be generated by plotting the various bacterial concentrations against the time it took the monitored ORP of each such bacterial concentration to reach the threshold voltage (also known as the time-to-detection (TTD)). Generation of the standard curve is discussed in more detail in the following sections. 
     The reader  120 , the parameter analyzer  118 , or another device in communication with either the reader  120  or the parameter analyzer  118  can compare the measured or monitored ORP of the diluted sample  112  over time against the values obtained from the standard curve. The reader  120 , the parameter analyzer or another device in communication with either the reader  120  or the parameter analyzer  118  can then determine the concentration of the infectious agent  102  in the sample  104  under investigation by using the time-to-detection and the values obtained from the standard curve. For example, the concentration can be calculated using the time-to-detection and an equation derived from the standard curve. 
     In some embodiments, one or more of the aforementioned steps can be stored as machine-executable instructions or logical commands in a non-transitory machine-readable medium (e.g., a memory or storage unit) of the parameter analyzer  118 , the reader  120 , or another device communicatively or electrically coupled to the parameter analyzer  118  or the reader  120 . Any of the parameter analyzer  118 , the reader  120 , or another device coupled to the parameter analyzer  118  or the reader  120  can comprise one or more processors or controllers configured to execute the aforementioned instructions or logical commands. In addition, any of the devices or systems shown in the example embodiments of  FIGS.  2 A,  2 B , and  2 C can be used to perform steps or operations of methods disclosed herein including, but not limited to, methods  100  and  500 . 
       FIG.  3 A  illustrates bacterial growth curves obtained by monitoring the change in ORP of growth media comprising different concentrations (e.g., from about 1*10 4  CFU/mL to about 1*10 8  CFU/mL) of a type of bacteria. For example,  FIG.  3 A  illustrates growth curves of different concentrations of  Pseudomonas aeruginosa  (PAe) bacteria grown at 35° C. in Mueller Hinton growth media (MHG). The ORPs of growth media exposed to the various PAe concentrations were monitored using ORP sensors (for example, any of the sensors  116  of  FIGS.  1 ,  2 A,  2 B, and  2 C ). A threshold voltage  130  was set at −100 mV and the time it took the monitored ORPs to reach the threshold voltage  130  (i.e., the TTDs  132 ) were used to generate the standard curve  126 . 
       FIG.  3 B  illustrates a standard curve  126  generated using certain experimental data from the experiments described above. As shown in  FIG.  3 B , a threshold ORP level was set at −100 mV. The various TTDs  132  were plotted as a function of the logarithm of the known concentration of the infectious agent  102  present in the various samples. A standard curve  126  can then be generated using curve fitting techniques such as logarithmic regression and least-squares. In other embodiments, polynomial and logarithmic curve fitting techniques can also be used. 
     As shown in  FIG.  3 B , a logarithmic standard curve  126  can be generated using values obtained from monitoring the ORP of growth media exposed to various concentrations of an infectious agent  102 . Deriving an equation for this logarithmic standard curve  126  can then allow us to interpolate unknown concentrations of infectious agents  102  in a sample using only the time it took such a solution to reach the ORP threshold voltage  130 . 
       FIG.  4    illustrates bacterial growth curves used in the quantification of PAe from positive blood cultures. The positive blood cultures were prepared by adding 10 CFU/mL of PAe to 25 mL of human blood. The resulting blood comprising PAe was then added to 30 mL of blood culture media (e.g., 30 mL of BD BACTEC™ Plus Aerobic Medium). The combined mixture of human blood containing PAe and blood culture media was then grown to positivity. Three aliquots of the positive blood culture were then diluted with growth media to dilution ratios of 1:10, 1:100, and 1:1000, respectively. Such diluted samples were then introduced to an ORP sensor comprising a redox-active material.  FIG.  4    illustrates changes in the ORP signals of the three diluted samples over time (commonly referred to as bacterial growth curves). As shown in  FIG.  4   , a threshold voltage of −100 mV was set and the time-to-detection of each curve was measured and compared to the PAe standard curve of  FIG.  3 B . The concentration of the PAe (in CFU/mL) can then be determined using the standard curve and by taking into account the amount of dilution. Diluting the positive blood culture with growth media to different dilution ratios can be helpful in determining the minimum amount of dilution needed to quantify a certain sample and ensuring that all such concentration determinations ultimately align. 
       FIG.  5    illustrates an embodiment of a method  500  for determining the susceptibility of one or more infectious agents  102  in a sample  104  to one or more anti-infectives  502 . The method  500  can comprise introducing one or more aliquots of the sample  104  into one or more reaction vessels  106  in step  5 A. The reaction vessels  106  can refer to one or more test tubes, reaction tubes, wells of a high throughput assay plate or well plate such as a 96-well plate, a 192-well plate, or a 384-well plate, culture plates or dishes, or other suitable containers for housing biological samples. One or more fluid delivery conduits  108  can introduce, deliver, or otherwise introduce the aliquots of the sample  104  to the one or more reaction vessels  106 . 
     In other embodiments not shown in  FIG.  5   , a stimulus solution can be added to the sample  104  before introducing the sample  104  to the reaction vessel  106 . The stimulus solution can be a nutrient or growth solution. In these and other embodiments, the sample  104  can also be filtered before step  5 A. This filtering step can involve filtering the sample  104  using an instance of a filter, a microfluidic filter, or a combination thereof to filter out debris, inorganic material, and larger cellular components including blood cells or epithelial cells from the sample  104 . 
     The sample  104  can comprise at least one of a biological sample, a bodily fluid, a wound swab or sample, a rectal swab or sample, and a bacterial culture derived from the biological sample, the bodily fluid, the wound swab or sample, or the rectal swab or sample. The bodily fluid can comprise urine, blood, serum, plasma, saliva, sputum, semen, breast milk, joint fluid, spinal fluid, wound material, mucus, fluid accompanying stool, re-suspended rectal or wound swabs, vaginal secretions, cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, amniotic fluid, cultures of bodily which has been tested positive for bacteria or bacterial growth such as blood culture which has been tested positive for bacteria or bacterial growth (i.e., positive blood culture), or a combination thereof. 
     The infectious agents  102  that can be assayed for anti-infective susceptibility using the methods or systems disclosed herein can be any metabolizing single- or multi-cellular organism including bacteria and fungi. In certain embodiments, the infectious agent  102  can be bacteria selected from the genera  Acinetobacter, Acetobacter, Actinomyces, Aerococcus, Aeromonas, Agrobacterium, Anaplasma, Azorhizobium, Azotobacter, Bacillus, Bacteroides, Bartonella, Bordetella, Borrelia, Brucella, Burkholderia, Calymmatobacterium, Campylobacter, Chlamydia, Chlamydophila, Citrobacter, Clostridium, Corynebacterium, Coxiella, Ehrlichia, Enterobacter, Enterococcus, Escherichia, Francisella, Fusobacterium, Gardnerella, Haemophilus, Helicobacter, Klebsiella, Lactobacillus, Legionella, Listeria, Methanobacterium, Microbacterium, Micrococcus, Morganella, Moraxella, Mycobacterium, Mycoplasma, Neisseria, Pandoraea, Pasteurella, Peptostreptococcus, Porphyromonas, Prevotella, Proteus, Providencia, Pseudomonas, Ralstonia, Raoultella, Rhizobium, Rickettsia, Rochalimaea, Rothia, Salmonella, Serratia, Shewanella, Shigella, Spirillum, Staphylococcus, Strenotrophomonas, Streptococcus, Streptomyces, Treponema, Vibrio, Wolbachia, Yersinia , or a combination thereof. In other embodiments, the infectious agent  102  can be one or more fungi selected from the genera  Candida  or  Cryptococcus  or mold. 
     Other specific bacteria that can be assayed for anti-infective susceptibility using the methods and systems disclosed herein can comprise  Staphylococcus aureus, Staphylococcus lugdunensis , coagulase-negative  Staphylococcus  species (including but not limited to  Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus hominis, Staphylococcus capitis , not differentiated),  Enterococcus faecalis, Enterococcus faecium  (including but not limited to  Enterococcus faecium  and other  Enterococcus  spp., not differentiated, excluding  Enterococcus faecalis ),  Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus  spp., (including but not limited to  Streptococcus mitis, Streptococcus pyogenes, Streptococcus gallolyticus, Streptococcus agalactiae, Streptococcus pneumoniae , not differentiated),  Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella  spp. (including but not limited to  Klebsiella pneumoniae, Klebsiella oxytoca , not differentiated),  Escherichia coli, Enterobacter  spp. (including but not limited to  Enterobacter cloacae, Enterobacter aerogenes , not differentiated),  Proteus  spp. (including but not limited to  Proteus mirabilis, Proteus vulgaris , not differentiated),  Citrobacter  spp. (including but not limited to  Citrobacter freundii, Citrobacter koseri , not differentiated),  Serratia marcescens, Candida albicans , and  Candida glabrata.    
     Other more specific bacteria that can be assayed for anti-infective susceptibility can comprise  Acinetobacter baumannii, Actinobacillus  spp.,  Actinomycetes, Actinomyces  spp. (including but not limited to  Actinomyces israelii  and  Actinomyces naeslundii ),  Aeromonas  spp. (including but not limited to  Aeromonas hydrophila, Aeromonas veronii  biovar  sobria  ( Aeromonas sobria ), and  Aeromonas caviae ),  Anaplasma phagocytophilum, Alcaligenes xylosoxidans, Actinobacillus actinomycetemcomitans, Bacillus  spp. (including but not limited to  Bacillus anthracis, Bacillus cereus, Bacillus subtilis, Bacillus thuringiensis , and  Bacillus stearothermophilus ),  Bacteroides  spp. (including but not limited to  Bacteroides fragilis ),  Bartonella  spp. (including but not limited to  Bartonella bacilliformis  and  Bartonella henselae, Bifidobacterium  spp.,  Bordetella  spp. (including but not limited to  Bordetella pertussis, Bordetella parapertussis , and  Bordetella bronchiseptica ),  Borrelia  spp. (including but not limited to  Borrelia recurrentis , and  Borrelia burgdorferi ),  Brucella  sp. (including but not limited to  Brucella abortus, Brucella canis, Brucella melintensis  and  Brucella suis ),  Burkholderia  spp. (including but not limited to  Burkholderia pseudomallei  and  Burkholderia cepacia ),  Campylobacter  spp. (including but not limited to  Campylobacter jejuni, Campylobacter coli, Campylobacter lari  and  Campylobacter fetus ),  Capnocytophaga  spp.,  Cardiobacterium hominis, Chlamydia trachomatis, Chlamydophila pneumoniae, Chlamydophila psittaci, Citrobacter  spp.  Coxiella burnetii, Corynebacterium  spp. (including but not limited to,  Corynebacterium diphtheriae, Corynebacterium jeikeum  and  Corynebacterium ),  Clostridium  spp. (including but not limited to  Clostridium perfringens, Clostridium difficile, Clostridium botulinum  and  Clostridium tetani ),  Eikenella corrodens, Enterobacter  spp. (including but not limited to  Enterobacter aerogenes, Enterobacter agglomerans, Enterobacter cloacae  and  Escherichia coli , including opportunistic  Escherichia coli , including but not limited to enterotoxigenic  E. coli , enteroinvasive  E. coli , enteropathogenic  E. coli , enterohemorrhagic  E. coli , enteroaggregative  E. coli  and uropathogenic  E. coli )  Enterococcus  spp. (including but not limited to  Enterococcus faecalis  and  Enterococcus faecium )  Ehrlichia  spp. (including but not limited to  Ehrlichia chafeensia  and  Ehrlichia canis ),  Erysipelothrix rhusiopathiae, Eubacterium  spp.,  Francisella tularensis, Fusobacterium nucleatum, Gardnerella vaginalis, Gemella morbillorum, Haemophilus  spp. (including but not limited to  Haemophilus influenzae, Haemophilus ducreyi, Haemophilus aegyptius, Haemophilus parainfluenzae, Haemophilus haemolyticus  and  Haemophilus parahaemolyticus, Helicobacter  spp. (including but not limited to  Helicobacter pylori, Helicobacter cinaedi  and  Helicobacter fennelliae ),  Kingella kingii, Klebsiella  spp. (including but not limited to  Klebsiella pneumoniae, Klebsiella granulomatis  and  Klebsiella oxytoca ),  Lactobacillus  spp.,  Listeria monocytogenes, Leptospira interrogans, Legionella pneumophila, Leptospira interrogans, Peptostreptococcus  spp.,  Moraxella catarrhalis, Morganella  spp.,  Mobiluncus  spp.,  Micrococcus  spp.,  Mycobacterium  spp. (including but not limited to  Mycobacterium leprae, Mycobacterium tuberculosis, Mycobacterium intracellulare, Mycobacterium avium, Mycobacterium bovis , and  Mycobacterium marinum ),  Mycoplasm  spp. (including but not limited to  Mycoplasma pneumoniae, Mycoplasma hominis , and  Mycoplasma genitalium ),  Nocardia  spp. (including but not limited to  Nocardia asteroides, Nocardia cyriacigeorgica  and  Nocardia brasiliensis ),  Neisseria  spp. (including but not limited to  Neisseria gonorrhoeae  and  Neisseria meningitidis ),  Pasteurella multocida, Plesiomonas shigelloides. Prevotella  spp.,  Porphyromonas  spp.,  Prevotella melaninogenica, Proteus  spp. (including but not limited to  Proteus vulgaris  and  Proteus mirabilis ),  Providencia  spp. (including but not limited to  Providencia alcalifaciens, Providencia rettgeri  and  Providencia stuartii ),  Pseudomonas aeruginosa, Propionibacterium acnes, Rhodococcus equi, Rickettsia  spp. (including but not limited to  Rickettsia rickettsii, Rickettsia akari  and  Rickettsia prowazekii, Orientia tsutsugamushi  (formerly:  Rickettsia tsutsugamushi ) and  Rickettsia typhi ),  Rhodococcus  spp.,  Serratia marcescens, Stenotrophomonas maltophilia, Salmonella  spp. (including but not limited to  Salmonella enterica, Salmonella typhi, Salmonella paratyphi, Salmonella enteritidis, Salmonella cholerasuis  and  Salmonella typhimurium ),  Serratia  spp. (including but not limited to  Serratia marcesans  and  Serratia liquifaciens ),  Shigella  spp. (including but not limited to  Shigella dysenteriae, Shigella flexneri, Shigella boydii  and  Shigella sonnei ),  Staphylococcus  spp. (including but not limited to  Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus hemolyticus, Staphylococcus saprophyticus ),  Streptococcus  spp. (including but not limited to  Streptococcus pneumoniae  (for example chloramphenicol-resistant serotype 4  Streptococcus pneumoniae , spectinomycin-resistant serotype 6B  Streptococcus pneumoniae , streptomycin-resistant serotype 9V  Streptococcus pneumoniae , erythromycin-resistant serotype 14  Streptococcus pneumoniae , optochin-resistant serotype 14  Streptococcus pneumoniae , rifampicin-resistant serotype 18C  Streptococcus pneumoniae , tetracycline-resistant serotype 19F  Streptococcus pneumoniae , penicillin-resistant serotype 19F  Streptococcus pneumoniae , and trimethoprim-resistant serotype 23F  Streptococcus pneumoniae , chloramphenicol-resistant serotype 4  Streptococcus pneumoniae , spectinomycin-resistant serotype 6B  Streptococcus pneumoniae , streptomycin-resistant serotype 9V  Streptococcus pneumoniae , optochin-resistant serotype 14  Streptococcus pneumoniae , rifampicin-resistant serotype 18C  Streptococcus pneumoniae , penicillin-resistant serotype 19F  Streptococcus pneumoniae , or trimethoprim-resistant serotype 23F  Streptococcus pneumoniae ),  Streptococcus agalactiae, Streptococcus mutans, Streptococcus pyogenes , Group A streptococci,  Streptococcus pyogenes , Group B streptococci,  Streptococcus agalactiae , Group C streptococci,  Streptococcus anginosus, Streptococcus equismilis , Group D streptococci,  Streptococcus bovis , Group F streptococci, and  Streptococcus anginosus  Group G streptococci),  Spirillum minus, Streptobacillus moniliformi, Treponema  spp. (including but not limited to  Treponema carateum, Treponema petenue, Treponema pallidum  and  Treponema endemicum, Tropheryma whippelii, Ureaplasma urealyticum, Veillonella  sp.,  Vibrio  spp. (including but not limited to  Vibrio cholerae, Vibrio parahemolyticus, Vibrio vulnificus, Vibrio parahaemolyticus, Vibrio vulnificus, Vibrio alginolyticus, Vibrio mimicus, Vibrio hollisae, Vibrio fluvialis, Vibrio metchnikovii, Vibrio damsela  and  Vibrio furnisii ),  Yersinia  spp. (including but not limited to  Yersinia enterocolitica, Yersinia pestis , and  Yersinia pseudotuberculosis ) and  Xanthomonas maltophilia  among others. 
     Furthermore, other infectious agents  102  that can be assayed for anti-infective susceptibility can comprise fungi or mold including, but not limited to,  Candida  spp. (including but not limited to  Candida albicans, Candida glabrata, Candida tropicalis, Candida parapsilosis , and  Candida krusei ),  Aspergillus  spp. (including but not limited to  Aspergillus fumigatous, Aspergillus flavus, Aspergillus clavatus ),  Cryptococcous  spp. (including but not limited to  Cryptococcus neoformans, Cryptococcus gattii, Cryptococcus laurentii , and  Cryptococcus albidus ),  Fusarium  spp. (including but not limited to  Fusarium oxysporum, Fusarium solani, Fusarium verticillioides , and  Fusarium proliferatum ),  Rhizopus oryzae, Penicillium marneffei, Coccidiodes immitis , and  Blastomyces dermatitidis.    
     The fluid delivery conduits  108  can include tubes, pumps, containers, or microfluidic channels for delivering buffers, reagents, fluid samples including the sample  104  or solubilized solutions thereof, other solutions, or a combination thereof to and between devices, apparatus, or containers in the system. For example, as shown in  FIG.  5   , the fluid delivery conduits  108  can refer to parts of a pump such as a syringe pump. In other embodiments, the fluid delivery conduits  108  can include or refer to at least part of a hydraulic pump, a pneumatic pump, a peristaltic pump, a vacuum pump or a positive pressure pump, a manual or mechanical pump, or a combination thereof. In additional embodiments, the fluid delivery conduits  108  can include or refer to at least part of an injection cartridge, a pipette, a capillary, or a combination thereof. The fluid delivery conduits  108  can also be part of a vacuum system configured to draw fluid to or through channels, tubes, or passageways under vacuum. Moreover, the fluid delivery conduits  108  can include or refer to at least part of a multichannel delivery system or pipette. 
     The method  500  can comprise diluting the sample  104  comprising the one or more infectious agents  102  with a dilutive solution  110  to yield a diluted sample  112  in step  5 B. In one embodiment, the dilutive solution  110  can comprise growth media or a growth inducer. In this and other embodiments, the dilutive solution  110  can be a solution containing bacto-tryptone, yeast extract, beef extract, cation-adjusted Mueller Hinton Broth (CAMHB), Mueller Hinton growth media (MHG), starch, acid hydrolysate of casein, calcium chloride, magnesium chloride, sodium chloride, blood or lysed blood including lysed horse blood (LHB), CAMHB-LHB, glucose, or a combination thereof. The growth inducer can comprise a carbon-based inducer, a nitrogen-based inducer, a mineral, a trace element, a biological growth factor, or any combination thereof. For example, the growth inducer can include but is not limited to glucose, ammonia, magnesium, blood, or a combination thereof. In one example embodiment, the dilutive solution  110  can comprise Tryptone, yeast extract, sodium chloride, and glucose. The dilutive solution  110  can be used to counteract the buffering effects of ions or substances present in the sample  104 . 
     In one embodiment, diluting the sample  104  with the dilutive solution  110  in step  5 B can involve diluting the sample  104  to a dilution ratio between about 1:1 to about 1:10. In another embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:10 to about 1:100. In yet another embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:100 to about 1:1000. In a further embodiment, diluting the sample  104  with the dilutive solution  110  can involve diluting the sample  104  to a dilution ratio between about 1:1000 to about 1:10000. Although  FIG.  5    illustrates one reaction vessel  106  or one aliquot of the sample  104  being diluted, it is contemplated by this disclosure that multiple aliquots of the sample  104  can be diluted to different dilution ratios such that one or more diluted samples  112  can act as internal controls. 
     As will be discussed in the following sections in relation to  FIG.  6   , in alternative embodiments, the method  500  can comprise diluting the sample  104  comprising the infectious agent  102  with deionized water, a saline solution, or a combination thereof serving as the dilutive solution  110 . In these embodiments, the diluted sample(s)  112  can be introduced to one or more sensors through sample delivery conduits comprising growth media/growth inducers and anti-infectives such that the diluted sample  112  is mixed with the growth media/growth inducers and anti-infectives. More details concerning these embodiments will be discussed in the following sections. 
     The method  500  can further comprise separating the diluted sample  112  into multiple aliquots such as, for example, a first aliquot and a second aliquot in step  5 C. The method  500  can also comprise introducing and mixing an anti-infective  502  at a first concentration into the first aliquot of the diluted sample  112 . The mixture comprising the first aliquot and the anti-infective  502  at the first concentration can be referred to as a test solution  506 . The second aliquot of the diluted sample  112  without the anti-infective  502  can be used as a control solution  504 . Although  FIG.  5    illustrates only one test solution  506  comprising the first aliquot and the anti-infective  502 , it is contemplated by this disclosure and should be understood by one of ordinary skill in the art that the method  500  and systems disclosed herein can assay multiple test solutions and some such test solutions can comprise a different anti-infective  502 , the same anti-infective  502  at a different concentration, or a different anti-infective  502  at a different concentration. For example, the anti-infective  502  can be diluted to two different concentrations before being introduced to two different reaction vessels  106  containing aliquots of the diluted sample  112 . 
     The anti-infective  502  used in the systems and methods disclosed herein can comprise a bacteriostatic anti-infective, a bactericidal anti-infective, an anti-fungal anti-infective, or a combination thereof. 
     In certain embodiments, the bacteriostatic anti-infective can comprise β-lactams (including but not limited to penicillins such as ampicillin, amoxicillin, flucloxacillin, penicillin, amoxicillin/clavulanate, and ticarcillin/clavulanate and monobactams such as aztreonam), β-lactam and β-lactam inhibitor combinations (including but not limited to piperacillin-tazobactam and ampicillin-sulbactam), Aminoglycosides (including but not limited to amikacin, gentamicin, kanamycin, neomycin, netilmicin, paromomycin, streptomycin, spectinomycin, and tobramycin), Ansamycins (including but not limited to rifaximin), Carbapenems (including but not limited to ertapenem, doripenem, imipenem, and meropenem), Cephalosporins (including but not limited to ceftaroline, cefepime, ceftazidime, ceftriaxone, cefadroxil, cefalotin, cefazolin, cephalexin, cefaclor, cefprozil, fecluroxime, cefixime, cefdinir, cefditoren, cefotaxime, cefpodoxime, ceftibuten, and ceftobiprole), Chloramphenicols, Glycopeptides (including but not limited to vancomycin, teicoplanin, telavancin, dalbavancin, and oritavancin), Folate Synthesis Inhibitors (including but not limited to trimethoprim-sulfamethoxazole), Fluoroquinolones (including but not limited to ciprofloxacin), Lincosamides (including but not limited to clindamycin, lincomycin, azithromycin, clarithromycin, dirithromycin, roxithromycin, telithromycin, and spiramycin), Lincosamines, Lipopeptides, Macrolides (including but not limited to erythromycin), Monobactams, Nitrofurans (including but not limited to furazolidone and nitrofurantoin), Oxazolidinones (including but not limited to linezolid, posizolid, radezolid, and torezolid), Quinolones (including but not limited to enoxacin, gatifloxacin, gemifloxacin, levofloxacin, lomefloxacin, moxifloxacin, naldixic acid, norfloxacin, trovafloxacin, grepafloxacin, sparfloxacin, and temafloxacin), Rifampins, Streptogramins, Sulfonamides (including but not limited to mafenide, sulfacetamide, sulfadiazine, sulfadimethoxine, sulfamethizole, sulfamethoxazole, sulfasalazine, and sulfisoxazole), Tetracyclines (including but not limited to oxycycline, minocycline, demeclocycline, doxycycline, oxytetracycline, and tetracycline), polypeptides (including but not limited to bacitracin, polymyxin B, colistin, and cyclic lipopeptides such as daptomycin), phages, or a combination or derivative thereof. 
     In other embodiments, the anti-infective  502  can comprise clofazimine, ethambutol, isoniazid, rifampicin, arsphenamine, chloramphenicol, fosfomycin, metronidazole, tigecycline, trimethoprim, or a combination or derivative thereof. 
     In certain embodiments, the anti-fungal can comprise Amphotericin B, Anidulafungin, Caspofungin, Fluconazole, Flucytosine, Itraconazole, Ketoconazole, Micafungin, Posaconazole, Ravuconazole, Voriconazole, or a combination or derivative thereof. 
     The method  500  can also comprise incubating the first aliquot and the second aliquot at an elevated temperature for a period of time in step  5 E. The first aliquot and the second aliquot can be incubated in their respective reaction vessels  106  or transferred to different reaction vessels  106  or containers. For example, the first aliquot and the second aliquot can be heated to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) and allowed to incubate for an incubation period  114 . The incubation period  114  can range from 15 minutes to over one hour. In other embodiments, the incubation period  114  can be less than 15 minutes or up to 48 hours. 
     The incubation period  114  can be adjusted based on the type of infectious agent  102  suspected in the sample  104 , such as the type of bacteria or fungus. The incubation period  114  can also be adjusted based on the type of anti-infective  502 , the mechanism of action of the anti-infective  502 , the amount of the sample  104 , or a combination thereof. The incubation period  114  can be start-delayed or a pre-incubation time period can be added before the start of the incubation period  114 . The start-delay or the pre-incubation time period can be added for slower acting drugs or anti-infectives  502  (e.g., β-lactams). In some embodiments, the start-delay or the pre-incubation time period can be between 10 minutes and 2 hours. In other embodiments, the start-delay or the pre-incubation time period can be as long as needed for the drug or anti-infective  502  to take effect. During the start-delay or pre-incubation time period, readings or measurements from the sensor(s) would not be used or would not be included as part of any growth curves generated (ORP signals monitored). The start-delay or the pre-incubation time period is particularly useful for instances where higher inoculums or a higher concentration of infectious agents  102  is present in the sample  104  or aliquots and where the signal is generated relatively fast in comparison to the mode of action of the drug or anti-infective  502 . 
     The method  500  can further comprise introducing the test solution  506  to a first sensor  508  or exposing the first sensor  508  to the test solution  506  such that the test solution  506  is in fluid communication with a redox-active material of the first sensor  508  in step  5 F(i). The method  500  can also comprise introducing the control solution  504  to a second sensor  510  or exposing the second sensor  510  to the control solution  504  such that the control solution  504  is in fluid communication with the redox-active material of the second sensor  510  in step  5 F(ii). 
     In certain embodiments, the first sensor  508  and the second sensor  510  can be oxidation reduction potential (ORP) sensors configured to respond to a change in a solution characteristic (e.g., the ORP) of a measured solution. In the example embodiment shown in  FIG.  5   , exposing the first sensor  508  and the second sensor  510  to the test solution  506  and the control solution  504 , respectively, can involve directly immersing at least part of a handheld or probe instance of the first sensor  508  and the second sensor  510  into the test solution  506  and the control solution  504 , respectively. In this embodiment, the handheld or probe instance of the first sensor  508  or the second sensor  510  can be a handheld OPR sensor coupled to a standalone parameter analyzer  118  such as a voltmeter or multimeter. In alternative example embodiments also shown in  FIG.  5   , introducing the test solution  506  and the control solution  504  to the first sensor  508  and the second sensor  510 , respectively, can involve injecting, delivering, or otherwise introducing the test solution  506  to a well or container comprising the first sensor  508  and introducing the control solution  504  to another well or container comprising the second sensor  510 . In these embodiments, the first sensor  508  and the second sensor  510  can be fabricated on one substrate  202  or different substrates  202 . 
     The substrate  202  can be comprised of a polymeric material, a metal, a ceramic, a semiconductor layer, an oxide layer, an insulator, or a combination thereof. The substrate  202  can be part of a test strip, cartridge, chip or lab-on-a-chip, microfluidic device, multi-well container, or a combination thereof. In some embodiments, the one or more parameter analyzers  118  can also be fabricated or located on the substrate  202 . In other embodiments, the one or more parameter analyzers  118  can be standalone devices such as a voltmeter or a multimeter electrically coupled to the sensors. 
     As will be discussed in more detail in the following sections, each of the first sensor  508  and the second sensor  510  can comprise an active electrode and a reference electrode. In addition, the redox-active material  908  can comprise a gold layer, a platinum layer, a metal oxide layer, a carbon layer, or a combination thereof. 
     The method  500  can further comprise monitoring the ORP of the test solution  506  over a period of time using one or more parameter analyzers  118  coupled to the first sensor  508  in step  5 F(i). The method  500  can also comprise monitoring the ORP of the control solution  504  over a similar period of time using one or more parameter analyzers  118  coupled to the second sensor  510  in step  5 F(ii). In one or more embodiments, the ORPs of the test solution  506  and the control solution  504  can be monitored in the absence of any added or exogenous reporter molecules present in the test solution  506  or the control solution  504 . 
     The test solution  506  and the control solution  504  can each have a solution characteristic. The solution characteristic of the test solution  506  and the solution characteristic of the control solution  504  can change as the amount of electro-active redox species changes due to the energy use, oxygen uptake or release, growth, or lack thereof of the infectious agents  102  in the test solution  506  and the control solution  504 . For example, the amount of electro-active redox species in the test solution  506  can change as a result of increasing or diminishing cellular activity undertaken by the infectious agents  102  in the test solution  506 . Also, for example, the amount of electro-active redox species in the control solution  504  can change as a result of cellular activity undertaken by the infectious agents  102  in the control solution  504 . As a more specific example, the amount of electron donors from Table 1 (e.g., the amount of energy carriers such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH 2 )) in the test solution  506  or the control solution  504  can change due to the growth or lack thereof of the infectious agents  102  in the test solution  506  or the control solution  504 . Also, as another more specific example, the amount of oxygen depleted in the test solution  506  or the control solution  504  due to aerobic respiration can change due to the growth or lack thereof of the infectious agents  102  in the test solution  506  or the control solution  504 . 
     The method  500  can further comprise comparing the ORP of the test solution  506  with the ORP of the control solution  504  to determine the susceptibility of the infectious agent  102  to the anti-infective  502  in step  5 G. In some embodiments, comparing the ORP of the test solution  506  with the ORP of the control solution  504  can be done using one or more parameter analyzers  118  coupled to the first sensor  508 , the second sensor  510 , or a combination thereof. In other embodiments, comparing the ORP of the test solution  506  with the ORP of the control solution  504  can be done using another device electrically or communicatively coupled to the parameter analyzer  118  such as the reader  120 . In yet additional embodiments, comparing the ORP of the test solution  506  with the ORP of the control solution  504  can be done using a combination of one or more parameter analyzers  118  and the reader  120 . 
     In certain embodiments, the reader  120  can be a mobile device, a handheld device, a tablet device, or a computing device such as a laptop or desktop computer having a display  122 . For example, the display  122  can be a mobile device display, a handheld device display, a tablet display, or a laptop or desktop monitor. In some embodiments, the parameter analyzer  118  can also comprise a display or can wirelessly communicate a signal or readout to a device having a display. 
     The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor and compare the ORP of the test solution  506  with the ORP of the control solution  504  over a period of time. The period of time can be referred to as a detection window  512 . The parameter analyzer  118 , the reader  120 , or a combination thereof can assess the susceptibility of the infectious agent  102  to the anti-infective  502  within this detection window  512 . In one embodiment, the detection window  512  can be between 60 minutes and 120 minutes. In other embodiments, the detection window  512  can be between 5 minutes and 60 minutes. In additional embodiments, the detection window  512  can be greater than 120 minutes. 
     In one embodiment, the parameter analyzer  118 , the reader  120 , or a combination thereof can comprise one or more controllers or processors to execute logical commands concerning the comparison of the ORP of the test solution  506  with the ORP of the control solution  504 . In this and other embodiments, the parameter analyzer  118 , the reader  120 , or a combination thereof can generate or instruct another device to generate a read-out, graph, or signal concerning a result of the comparison on a display such as the display  122 . 
     For example, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine or assess the susceptibility of the infectious agent  102  in the sample  104  as resistant to an anti-infective  502  when the parameter analyzer  118 , the reader  120 , or a combination thereof fails to detect a statistically significant difference between the ORP of the test solution  506  and the ORP of the control solution  504 . This statistically significant difference can be a difference exceeding a threshold value or range. Conversely, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine or assess the susceptibility of the infectious agent  102  as susceptible to an anti-infective  502  when the parameter analyzer  118 , the reader  120 , or a combination thereof detects certain statistically significant differences between the ORP of the test solution  506  and the ORP of the control solution  504  within the detection window  512 . 
     Although not shown in  FIG.  5   , the method  500  can also comprise separating the diluted sample  112  into a third aliquot and introducing the anti-infective  502  at a second concentration into the third aliquot to form another test solution. In some embodiments, the second concentration of the anti-infective  502  can be less than the first concentration of the anti-infective  502  added to the first aliquot. In these embodiments, the second concentration of the anti-infective  502  can be obtained by diluting the first concentration of the anti-infective  502 . In other embodiments, the second concentration can be greater than the first concentration. 
     The method  500  can further comprise introducing the other test solution to a third sensor such that the other test solution is in fluid communication with the redox-active material of the third sensor. The ORP of the other test solution  506  can be monitored over a period of time using one or more parameter analyzers  118  coupled to the third sensor. The ORP can be monitored in the absence of any added reporter molecules in the other test solution. The method  500  can also comprise comparing the ORP of the other test solution with the ORPs of the test solution  506  formed from the first aliquot and the control solution  504 . The ORPs can be compared to determine a degree of susceptibility of the infectious agent  102  to the anti-infective  502 . For example, the parameter analyzer  118 , the reader  120 , or a combination thereof can assess the degree or level of susceptibility of the infectious agent  102  in the sample  104  on a tiered scale. As a more specific example, the parameter analyzer  118 , the reader  120 , or a combination thereof can assess the susceptibility of the infectious agent  102  in the sample  104  as being resistant, of intermediate susceptibility, or susceptible to the anti-infective  502  based on a comparison of the ORPs of the two test solutions with each other and comparisons of the ORPs of the two test solutions with the control solution  504 . 
     In some embodiments, one or more of the aforementioned steps of the method  500  can be stored as machine-executable instructions or logical commands in a non-transitory machine-readable medium (e.g., a memory or storage unit) of the parameter analyzer  118 , the reader  120 , or another device communicatively or electrically coupled to the parameter analyzer  118  or the reader  120 . Any of the parameter analyzer  118 , the reader  120 , or another device coupled to the parameter analyzer  118  or the reader  120  can comprise one or more processors or controllers configured to execute the aforementioned instructions or logical commands. 
     The steps depicted in  FIG.  5    do not require the particular order shown to achieve the desired result. Moreover, certain steps or processes may be omitted or occur in parallel in order to achieve the desired result. In addition, any of the systems or devices disclosed herein can be used in lieu of devices or systems shown in the steps of  FIG.  5   . 
       FIG.  6    illustrates an embodiment of a multiplex system  600  for determining a susceptibility an infectious agent  102  as well as a level of susceptibility of an infectious agent  102  to one or more anti-infectives  502 . In some embodiments, the multiplex system  600  can be part of a cartridge, a test strip, an integrated circuit, a micro-electro-mechanical system (MEMS) device, a microfluidic system or chip, or a combination thereof. 
     The system  600  can be another embodiment of the system  200  illustrated in  FIG.  2 C  with many of the same components as the system  200 . The system  600  can comprise the same sample delivery surface  204  defined on the same substrate  202 . In one or more embodiments, the sample receiving surface  204  can be a flat surface for receiving the sample  104 . In other embodiments, the sample receiving surface  204  can be a concave or tapered surface of a well, divot, dish, or container. For example, the sample  104  can be injected, pipetted, pumped, spotted, or otherwise introduced to the sample receiving surface  204  for analysis. 
     The system  600  can also comprise the one or more metering conduits  206  in fluid communication with the sample receiving surface  204 . In some embodiments, the one or more metering conduits  206  can be channels, passageways, capillaries, tubes, parts therein, or combinations thereof for delivering the dilutive solution  110  to the sample  104  on the sample receiving surface  204 . For example, the one or more metering conduits  206  can refer to channels, passageways, capillaries, or tubes defined on the substrate  202 . Also, for example, the one or more metering conduits  206  can refer to channels, passageways, capillaries, or tubes serving as part of hydraulic pump, a pneumatic pump, peristaltic pump, a vacuum or positive pressure pump, a manual or mechanical pump, a syringe pump, or a combination thereof. For example, the one or more metering conduits  206  can be microfluidic channels or tubes or channels serving as part of a vacuum system. 
     In some embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:1 to about 1:10. In other embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:10 to about 1:100. In additional embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:100 to about 1:1000. In yet additional embodiments, the one or more metering conduits  206  can be configured to dilute the sample  104  with the dilutive solution  110  to a dilution ratio between about 1:1000 to about 1:10000. 
     The system  600  can also comprise a plurality of sensors and a plurality of sample delivery conduits connecting and extending in between each of the sensors and the sample receiving surface  204 , the one or more metering conduits  206 , or a combination thereof. In certain embodiments, the one or more metering conduits  206  can also separate the diluted sample into multiple aliquots including at least a first aliquot, a second aliquot, a third aliquot, a fourth aliquot, and a fifth aliquot. In these embodiments, aliquots of the diluted sample can automatically flow from the one or more metering conduits  206  into the sample delivery conduits leading to the sensors. 
     In other embodiments, the sample  104  can be diluted by a user or technician in a separate reaction vessel, test tube, or container. In these embodiments, the user can separate the diluted sample into multiple aliquots and introduce each of the aliquots to either the sample delivery conduits or the sensors directly. 
     In the example embodiment shown in  FIG.  6   , the plurality of sensors can comprise at least the first sensor  508 , the second sensor  510 , a third sensor  602 , a fourth sensor  604 , and a fifth sensor  606 . Although five sensors are described herein it should be understood by one of ordinary skill in the art that the system  600  can comprise more than five sensors. 
     In some embodiments, the sensors (including any of the first sensor  508 , the second sensor  510 , the third sensor  602 , the fourth sensor  604 , and the fifth sensor  606 ) can be the sensors  900  described in connection with  FIGS.  9 A and  9 B . For example, the sensors can be micro- or nano-scale ORP sensors. The sensors can be fabricated or located on a surface of the substrate  202 . For example, the substrate  202  can be part of a circuit, chip, or device and the sensors can comprise components of such circuits, chips, or devices including, but not limited to, one or more transistors, gates, or other electrical components. In some embodiments, the sensors can be positioned within a well, divot, cut-out, or groove defined along the substrate  202 . In these and other embodiments, the diluted samples can be injected, directed, or otherwise introduced into each of the wells, divots, cut-outs, or grooves. 
     Each of the sensors can comprise an active electrode and a reference electrode. Each of the sensors can also comprise a redox-active material  908  (see  FIGS.  9 A and  9 B ) or layer such as a gold layer, a platinum layer, a metal oxide layer, carbon layer, or a combination thereof. The sensors will be discussed in more detail in the following sections. 
     The sample delivery conduits (e.g., the first sample delivery conduit  608 , the second sample delivery conduit  610 , the third sample delivery conduit  612 , the fourth sample delivery conduit  614 , and the fifth sample delivery conduit  616 ) can extend in between the sample receiving surface  204  and the plurality of sensors or extend in between the one or more metering conduits  206  and the plurality of sensors. The sample delivery conduits can be channels, passageways, capillaries, tubes, microfluidic channels, parts therein, or combinations thereof for delivering the diluted sample to the sensors. The sample delivery conduits can allow aliquots of the diluted sample to be in fluid communication the sensors. For example, each of the sample delivery conduits can allow an aliquot of the diluted ample to be in fluid communication with a redox-active material or layer of a sensor. 
     In the example embodiment shown in  FIG.  6   , each of the sample delivery conduits can be covered or coated by a lyophilized or dried form of an anti-infective. The anti-infective can be any of the anti-infectives  502  discussed in connection with  FIG.  5   . The sample delivery conduits can be configured such that aliquots of the diluted sample flow through the sample delivery conduits and mix with the lyophilized or dried forms of the anti-infective en route to the sensors. In this and other example embodiments, the dilutive solution  110  used to dilute the sample  104  can comprise growth media such as Mueller Hinton growth media (MHG), a growth inducer, or a combination thereof. 
     In other embodiments, the dilutive solution  110  used to dilute the sample  104  can be deionized water or saline solution and the sample delivery conduits  208  can be covered or coated by both a lyophilized or dried form of the anti-infective and a lyophilized or dried form of the growth media. In these embodiments, aliquots of the diluted sample flowing through the sample delivery conduits can mix with the lyophilized or dried forms of the anti-infective and the growth media en route to the sensors. 
     In additional embodiments not shown in  FIG.  6   , the sample delivery conduits  208  can contain anti-infectives, growth media, or a combination thereof in aqueous form. In these embodiments, aliquots of the diluted sample can mix with the aqueous forms of the anti-infective, the growth media, or a combination thereof en route to the sensors. 
     In yet additional embodiments, some of the sensors themselves (e.g., one or more layers of the sensor) can be covered or coated by lyophilized or dried forms of the anti-infective, the growth media, or a combination thereof. In these embodiments, aliquots of the diluted sample can mix with the anti-infective, the growth media, or a combination thereof when the aliquots reach or are in fluid communication with the sensors. Moreover, in other embodiments not shown in the figures, a surface in the vicinity of the sensors can be covered or coated by lyophilized or dried forms of the anti-infective, the growth media, or a combination thereof. In these embodiments, aliquots of the diluted sample can mix with the lyophilized or dried forms of the anti-infective, the growth media, or a combination thereof when the diluted sample is in fluid communication with the surface covered or coated by the lyophilized anti-infective or growth media. 
     In all such embodiments, at least one of the sample delivery conduits leading up to at least one of the sensors can be free or devoid of anti-infectives. In these embodiments, the diluted sample flowing through this sample delivery conduit can act as a control solution. Also, in these embodiments, each of the aliquots of the diluted sample mixed with the anti-infective can be referred to as a test solution. 
     The system  600  shown in  FIG.  6    can be used to determine the susceptibility of a sample  104  comprising the infectious agent  102  to multiple anti-infectives (as well as multiple concentrations of one or more anti-infectives) concurrently. As such, one benefit of the multiplex system  600  of  FIG.  6    is the ability to perform high-throughput antibiotic susceptibility testing. 
     In further alternative embodiments not shown in the figures, a user can dilute the sample  104  with growth media and mix one or more anti-infectives into aliquots of the diluted sample prior to introducing the mixture to the system  600 . In these embodiments, the user can introduce the mixture comprising the diluted sample and the anti-infectives to the sample receiving surface  204  or the sensors directly. 
     In some embodiments, the system  600  can further comprise an incubating component configured to incubate the diluted sample mixed with the anti-infective, the growth media, or a combination thereof by heating the mixture to a temperature of between about 30° C. and about 40° C. (e.g., 35° C.±2° C.) for a period of time (e.g., the incubation period  114 ). 
     The system  600  can also comprise one or more parameter analyzers  118  electrically or communicatively coupled to the sensors and a reader  120  electrically or communicatively coupled to the one or more parameter analyzers  118 . In some embodiments, the one or more parameter analyzers  118  can be fabricated or located on the substrate  202 . In other embodiments, the one or more parameter analyzers  118  can be standalone devices such as a voltmeter or a multimeter electrically coupled to the sensor. In some embodiments, the reader  120  and the parameter analyzer(s)  118  can be integrated into one device. The parameter analyzer  118  and the reader  120  depicted in  FIG.  6    can be the same parameter analyzers  118  and reader  120  depicted in  FIG.  5   . 
     The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor and compare the ORP of the test solution with the ORP of one or more control solutions over a period of time. This period of time can be between 60 minutes and 120 minutes. In other embodiments, this period of time can be between 5 minutes and 60 minutes. In additional embodiments, this period of time can be greater than 120 minutes. 
     In some embodiments, the parameter analyzer  118 , the reader  120 , or a combination thereof can comprise one or more controllers or processors to execute logical commands concerning the comparison of the ORPs of the test solutions with the ORP of the control solution. In this and other embodiments, the parameter analyzer  118 , the reader  120 , or a combination thereof can generate or instruct another device to generate a read-out, graph, or signal concerning a result of the comparison on a display such as the display  122 . 
     For example, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine or assess the susceptibility of the infectious agent  102  in the sample  104  as resistant to an anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof fails to detect a statistically significant difference between the ORP of one of the test solutions and the ORP of the control solution. This statistically significant difference can be a difference exceeding a threshold value or range. Conversely, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine or assess the susceptibility of the infectious agent  102  as susceptible to an anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof detects certain statistically significant differences between the ORP of one of the test solutions and the ORP of the control solution. 
     As will be discussed in the following sections, the system  600  can also assess the degree or level of susceptibility of the infectious agent  102  in the sample  104  on a tiered scale. As a more specific example, the parameter analyzer  118 , the reader  120 , or a combination thereof can assess the susceptibility of the infectious agent  102  in the sample  104  as being resistant, of intermediate susceptibility, or susceptible to the anti-infective  502  based on a comparison of the ORPs of two test solutions with each other and comparisons of the ORPs of the two test solutions with the control solution  504 . 
     For example, as shown in  FIG.  6   , the system  600  can comprise at least a first sample delivery conduit  608 , a second sample delivery conduit  610 , a third sample delivery conduit  612 , a fourth sample delivery conduit  614 , and a fifth sample delivery conduit  616 . The metering conduit  206  can also separate the diluted sample into a first aliquot, a second aliquot, a third aliquot, a fourth aliquot, and a fifth aliquot. The system  600  can direct the first aliquot to the first sample delivery conduit  608 , the second aliquot to the second sample delivery conduit  610 , the third aliquot to the third sample delivery conduit  612 , the fourth aliquot to the fourth sample delivery conduit  614 , and the fifth aliquot to the fifth sample delivery conduit  616 . 
     The first sample delivery conduit  608  can comprise a first anti-infective at a first concentration and the third sample delivery conduit  612  can comprise the first anti-infective at a second concentration. In some embodiments, the second concentration can be less than the first concentration and can be obtained by diluting a solution comprising the first anti-infective at the first concentration. 
     The fourth sample delivery conduit  614  can comprise a second anti-infective at a first concentration and the fifth sample delivery conduit  616  can comprise the second anti-infective at a second concentration. The second anti-infective can be a different anti-infective than the first anti-infective. 
     The second sample delivery conduit  610  can be free or devoid of any anti-infective such that the second aliquot of the diluted sample introduced through the second sample delivery conduit  610  can be considered a control solution. The first sample delivery conduit  608  can be configured to introduce the first aliquot of the diluted sample to the first sensor  508 . The first aliquot can mix with the lyophilized or dried first anti-infective at the first concentration to form a first test solution. The third sample delivery conduit  612  can be configured to introduce the third aliquot of the diluted sample to the third sensor  602 . The third aliquot can mix with the lyophilized or dried first anti-infective at the second concentration to form a second test solution. The fourth sample delivery conduit  614  can be configured to introduce the fourth aliquot of the diluted sample to the fourth sensor  604 . The fourth aliquot can mix with the lyophilized or dried second anti-infective at the first concentration to form a third test solution. The fifth sample delivery conduit  616  can be configured to introduce the fifth aliquot of the diluted sample to the fifth sensor  606 . The fifth aliquot can mix with the lyophilized or dried second anti-infective at the second concentration to form a fourth test solution. 
     The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor the ORP of the first test solution when the first test solution is in fluid communication with the redox-active material of the first sensor  508 . The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor the ORP of the control solution when the control solution is in fluid communication with the redox-active material of the second sensor  510 . The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor the ORP of the second test solution when the second test solution is in fluid communication with the redox-active material of the third sensor  602 . The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor the ORP of the third test solution when the third test solution is in fluid communication with the redox-active material of the fourth sensor  604 . The parameter analyzer  118 , the reader  120 , or a combination thereof can monitor the ORP of the fourth test solution when the fourth test solution is in fluid communication with the redox-active material of the fifth sensor  606 . The ORPs of the first test solution, the second test solution, the third test solution, the fourth test solution, and the control solution can be monitored in the absence of any added reporter or exogenous reporter molecules in the first test solution, the second test solution, the third test solution, the fourth test solution, and the control solution, respectively. 
     The parameter analyzer  118 , the reader  120 , or a combination thereof can compare the ORP of the second test solution with the ORPs of the first test solution and the control solution to determine a degree of susceptibility of the infectious agent  102  to the first anti-infective. For example, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as susceptible to the first anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof detects both a statistically significant difference between the ORP of the first test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the first test solution) and a statistically significant difference between the ORP of the second test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the second test solution). Alternatively, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as resistant to the first anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof fails to detect a statistically significant difference between the ORP of the first test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the first test solution) and fails to detect a statistically significant difference between the ORP of the second test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the second test solution). As a further alternative example, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as of intermediate susceptibility to the first anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof detects a statistically significant difference between the ORP of the first test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the first test solution or the first anti-infective at a higher concentration) but fails to detect a statistically significant difference between the ORP of the second test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the second test solution or the first anti-infective at the lower concentration). 
     The parameter analyzer  118 , the reader  120 , or a combination thereof can also compare the ORP of the fourth test solution with the ORPs of the third test solution and the control solution to determine a degree of susceptibility of the infectious agent  102  to the second anti-infective. For example, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as susceptible to the second anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof detects both a statistically significant difference between the ORP of the third test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the third test solution (which is the second anti-infective at the higher concentration)) and a statistically significant difference between the ORP of the fourth test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the fourth test solution (which is the second anti-infective at the lower concentration)). Alternatively, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as resistant to the second anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof fails to detect a statistically significant difference between the ORP of the third test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the third test solution (which is the second anti-infective at the higher concentration)) and fails to detect a statistically significant difference between the ORP of the fourth test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the fourth test solution (which is the second anti-infective at the lower concentration)). Furthermore, the parameter analyzer  118 , the reader  120 , or a combination thereof can determine the infectious agent  102  as of intermediate susceptibility to the second anti-infective when the parameter analyzer  118 , the reader  120 , or a combination thereof detects a statistically significant difference between the ORP of the third test solution and the ORP of the control solution (i.e., the infectious agents  102  are dead or dying in the third test solution (which is the second anti-infective at the higher concentration)) but fails to detect a statistically significant difference between the ORP of the fourth test solution and the ORP of the control solution (i.e., the infectious agents  102  are alive and growing in the fourth test solution (which is the second anti-infective at the lower concentration)). 
       FIG.  7 A  illustrates an example growth curve  700  of an infectious agent  102  not susceptible or resistant to an anti-infective (such as anti-infective  502 ) in solution. The growth curve  700  can be recorded by monitoring the sensor output of an ORP sensor (including, but not limited to, the first sensor  508  or the second sensor  510 ) in fluid communication with the sampled solution. In one embodiment, the sensor output can be a potential difference between an active electrode and a reference electrode (see  FIGS.  9 A and  9 B ). The sensor output of the ORP sensor can change as the ORP of the sampled solution (e.g., any of the test solutions or the control solution  504 ) changes. 
     The voltage output of the ORP sensor can change over time. For example, as shown in  FIG.  7 A , the voltage output of the sensor can decrease over time as the solution characteristic of the sampled solution changes due to the energy use, oxygen uptake or release, growth, or metabolism of the infectious agents  102  in solution. In some embodiments, the change (e.g., decrease) in the voltage output of the sensor can follow a sigmoidal pattern or shape, a step function or shape, or other patterns or shapes. Over longer time scales, the sensor output or voltage can begin to increase or become more positive. 
     For example, the voltage output of the sensor can decrease over time as the solution characteristic of the sampled solution changes as a result of cellular activity undertaken by the infectious agents  102  in solution. As a more specific example, the solution characteristic of the sampled solution can change as the amount of energy carriers (such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH 2 )) in the sampled solution changes due to the growth of anti-infective resistant infectious agents  102 . Also, as another more specific example, the amount of oxygen depleted in the sampled solution can change due to the growth or lack thereof of the infectious agents  102  in solution. 
       FIG.  7 B  illustrates an example growth curve  702  of an infectious agent  102  susceptible to or not resistant to an anti-infective (such as anti-infective  502 ) in solution. The growth curve  702  can be recorded by monitoring the sensor output of an ORP sensor in fluid communication with the sampled solution. As shown in  FIG.  7 B , the growth curve  702  can be relatively constant (e.g., a substantially flat line) or change very little. In other embodiments not shown in  FIG.  7 B , the growth curve  702  can exhibit changes within a predetermined threshold range. The sensor output of the ORP sensor can stay relatively constant as the ORP of the sampled solution (e.g., any of the test solutions or the control solution  504 ) stays relatively constant. 
     In one embodiment, the voltage output of the ORP sensor can be a potential difference between an active electrode and a reference electrode such as the external reference electrode, the on-chip reference electrode, or another reference electrode. 
     The voltage output of the ORP sensor can stay relatively constant as the solution characteristic of the sampled solution stays relatively constant due to the inhibitive effects of the anti-infective  502  on the infectious agents  102  in solution. 
       FIG.  8    illustrates example growth curves of  Pseudomonas aeruginosa  (PAe) from positive blood culture in the presence of various anti-infectives  502 . Blood culture positive for PAe was diluted into Mueller Hinton growth media (MHG) to a concentration of 5*10 5  CFU/mL and probed with different antibiotics at their susceptibility breakpoints. As shown in  FIG.  8   , the antibiotics include (1) imipenem (IMI), (2) ceftazidime (CAZ), (3) doripenem (DOR), (4) cefepime (CPM), (5) levofloxacin (LVX), (6) ciprofloxacin (CIP), (7) norfloxacin (NOR), and (8) gentamicin (GEN). PAe and antibiotic mixtures were exposed to ORP sensors (for example, any of the sensors discussed in connection with  FIGS.  5  and  6   ) and changes in the ORP of the mixture were assessed over time and compared to the bacterial sample without antibiotic (curve labeled MHG in  FIG.  8   ). A flat or substantially flat line over the entire detection period can indicate elimination of the bacteria or susceptibility to the antibiotic. A flat or substantially flat line followed by a delayed change in ORP can indicate partial elimination of the bacteria (i.e., time-shifted regrowth in the presence of the antibiotic) or intermediate susceptibility to the antibiotic. 
       FIG.  9 A  illustrates a side view of one embodiment of a sensor  900 . The sensor  900  can be or refer to any of the sensors depicted in  FIGS.  1 ,  2 A,  2 B,  2 C,  5 , and  6    (including but not limited to sensor  116  of  FIG.  1 ,  2 A,  2 B , or  2 C; the first sensor  508  or the second sensor  510  of  FIG.  5  or  6   ; and the third sensor  602 , the fourth sensor  604 , or the fifth sensor  606  of  FIG.  6   ). The sensor  900  can be an electrochemical cell comprising an active electrode  901  and an external reference electrode  902 . In some embodiments of the sensor  900 , the active electrode  901  and the external reference electrode  902  are the only electrodes of the sensor  900 . 
     The active electrode  901  can extend from or be disposed on a substrate layer  904 . The substrate layer  904  can be composed of, but is not limited to, any non-conducting material such as a polymer, an oxide, a ceramic, or a composite thereof. The electrochemical cell can be surrounded or contained by walls  906  configured to retain a sampled solution  910 . The walls  906  can be made of an inert or non-conductive material. 
     The sampled solution  910  can refer to any of the diluted sample  112 , the test solutions, the control solution  504 , or an aliquot thereof. At least part of external reference electrode  902  can be in fluid communication or fluid contact with the sampled solution  910 . For example, the external reference electrode  902  can extend into or be immersed in the sampled solution  910 . The external reference electrode  902  can also have a stable or well-known internal voltage and the sensor  900  can use the external reference electrode  902  to determine or measure a relative change in the potential of the active electrode  901 . In one embodiment, the external reference electrode  902  can be a standalone probe or electrode. In other embodiments, the external reference electrode  902  can be coupled to the parameter analyzer  118 . In some embodiments, multiple sensors (including but not limited to any of the first sensor  508 , the second sensor  510 , the third sensor  602 , the fourth sensor  604 , or the fifth sensor  606 ) can share and use the same external reference electrode  902 . 
     In one embodiment, the external reference electrode  902  can be a silver/silver chloride (Ag/AgCl) electrode. In other embodiments, the external reference electrode  902  can comprise a saturated calomel reference electrode (SCE) or a copper-copper (II) sulfate electrode (CSE). The external reference electrode  902  can also be a pseudo-reference electrode including any metal that is not part of the active electrode such as platinum, silver, gold, or a combination thereof; any metal oxide or semiconductor oxide material such as aluminum oxide, iridium oxide, silicon oxide; or any conductive polymer electrodes such as polypyrrole, polyaniline, polyacetylene, or a combination thereof. 
     The active electrode  901  can comprise multiple conductive layers (e.g., a stack of metallic layers) and a redox-active material  908  or layer such as a gold layer, a platinum layer, a metal oxide layer, a carbon layer, or a combination thereof on top of the multiple conductive layers. In some embodiments, the metal oxide layer can comprise an iridium oxide layer, a ruthenium oxide layer, or a combination thereof. The parameter analyzer  118  can be coupled to the active electrode  901  and the external reference electrode  902 . 
     The parameter analyzer  118  can determine the ORP of the sampled solution  910  by measuring the potential difference between the external reference electrode  902  and the active electrode  901  instantly or over a period of time. As shown in  FIG.  9 A , the parameter analyzer  118  can be a voltmeter or any other type of high-impedance amplifier or sourcemeter. The voltmeter can measure a relative change in an equilibrium potential at an interface between the redox-active material  908  of the active electrode  901  and the sampled solution  910  containing electro-active redox species. The solution characteristic of the sampled solution  910  can change as the amount of electro-active redox species changes due to the energy use, oxygen uptake or release, growth, or metabolism of the infectious agents  102  in solution. For example, the amount of electro-active redox species in the sampled solution  910  can change as a result of cellular activity undertaken by the infectious agents  102  in solution. As a more specific example, the amount of electron donors from Table 1 (e.g., the amount of energy carriers such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH 2 )) in the sampled solution  910  can change due to the growth or lack thereof of the infectious agents  102  in solution. Also, as another more specific example, the amount of oxygen depleted in the sampled solution  910  can change due to the growth or lack thereof of the infectious agents  102  in solution. 
     In one embodiment, the active electrode  901  can comprise a metallic layer. The metallic layer can comprise a gold layer, a platinum layer, or a combination thereof. The active electrode  901  can also comprise multiple layers comprising a semiconductor layer having a redox-active metal oxide layer, such as iridium oxide or ruthenium oxide on top of the multiple layers. In other embodiments, the active electrode  901  can comprise one or more metallic layers, one or more redox-active metal oxide layers, one or more semiconductor layers, or any combination or stacking arrangement thereof. 
       FIG.  9 B  illustrates a side view of another embodiment of the sensor  900  having an on-chip reference electrode  912  disposed on the substrate layer  904  in lieu of the external reference electrode  902  of  FIG.  9 A . In some embodiments of the sensor  900 , the active electrode  901  and the on-chip reference electrode  912  are the only electrodes of the sensor  900 . 
     In these and other embodiments, the on-chip reference electrode  912  can be coated by a polymeric coating. For example, the on-chip reference electrode  912  can be coated by a polyvinyl chloride (PVC) coating, a perfluorosulfonate coating (e.g., Nafion™), or a combination thereof. 
     The on-chip reference electrode  912  can serve the same purpose as the external reference electrode  902  except be fabricated on or integrated with the substrate layer  904 . The on-chip reference electrode  912  can be located adjacent to or near the active sensor  120 . The sensor  900  of  FIG.  9 B  can serve the same function as the sensor  900  of  FIG.  9 A . Similar to the active electrode  901  of  FIG.  9 B , the on-chip reference electrode  912  can also be in fluid communication or communication with the sampled solution  910  retained within walls  906 . 
     The on-chip reference electrode  912  can be comprised of a metal, a semiconductor material, or a combination thereof. The metal of the on-chip reference electrode  912  can be covered by an oxide layer, a silane layer, a polymer layer, or a combination thereof. In another embodiment, the on-chip reference electrode  912  can be a metal combined with a metal salt such as an Ag/AgCl on-chip reference electrode. In another embodiment, the on-chip reference electrode can be a miniaturized electrode with a well-defined potential. In some embodiments, multiple sensors can share and use the same on-chip reference electrode  912 . The on-chip reference electrode  912  can comprise a saturated calomel reference electrode (SCE) or a copper-copper (II) sulfate electrode (CSE). The on-chip reference electrode  912  can also comprise a pseudo-reference electrode including any metal that is not part of the active electrode such as platinum, silver, gold, or a combination thereof; any metal oxide or semiconductor oxide material such as aluminum oxide, iridium oxide, silicon oxide; or any conductive polymer electrodes such as polypyrrole, polyaniline, polyacetylene, or a combination thereof. 
     Each of the individual variations or embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other variations or embodiments. Modifications may be made to adapt a particular situation, material, composition of matter, process, process act(s) or step(s) to the objective(s), spirit or scope of the present invention. 
     Methods recited herein may be carried out in any order of the recited events that is logically possible, as well as the recited order of events. For example, the flowcharts or process flows depicted in the figures do not require the particular order shown to achieve the desired result. Moreover, additional steps or operations may be provided or steps or operations may be eliminated to achieve the desired result. 
     It will be understood by one of ordinary skill in the art that all or a portion of the methods disclosed herein may be embodied in a non-transitory machine readable or accessible medium comprising instructions readable or executable by a processor or processing unit of a computing device or other type of machine. 
     Furthermore, where a range of values is provided, every intervening value between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. Also, any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. 
     All existing subject matter mentioned herein (e.g., publications, patents, patent applications and hardware) is incorporated by reference herein in its entirety except insofar as the subject matter may conflict with that of the present invention (in which case what is present herein shall prevail). The referenced items are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such material by virtue of prior invention. 
     Reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “an,” “said” and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     This disclosure is not intended to be limited to the scope of the particular forms set forth, but is intended to cover alternatives, modifications, and equivalents of the variations or embodiments described herein. Further, the scope of the disclosure fully encompasses other variations or embodiments that may become obvious to those skilled in the art in view of this disclosure. The scope of the present invention is limited only by the appended claims.