## 2 Applications of Biosensors in Bacteria Detection
### Recognition of Bacteria Using Biosensors
#### 2.1.1 Optical Biosensors  
Optical biosensors utilize the changes in the optical properties of the sensor surface caused by the bound analyte, and these alterations are then transduced to a detector [23; 48; 49]. The mentioned methods are often divided into two categories, fluorescence- or nanoparticle-based labeling techniques or label-free ones [23]. Fluorescence-based biosensors can have high sensitivity [50; 51; 52; 53; 54; 55; 56]; for example, [PERSON] and co-authors applied a fluorescent dye-loaded micelle system to detect 15 cells/mL of _E. coli_[23; 57]. Colorimetric biosensors recently appear to be attractive for the researchers in the field of bacteria sensing due to their simplicity and rapidity (color reactions for detection in few minutes) [58; 59; 60; 61; 62]. These types of sensors can be used in point-of-care diagnostic platforms in hospitals and to improve the control of health-risks associated with contaminated food consumption [35]. However, the major disadvantage of using label-based optical biosensors is the requirement for sample labeling with the fluorescent reagents, which adds time and cost to the assay [23], furthermore, the dyes and labels may disturb the normal physiology of the cells [63]. Among label-free optical biosensors, surface plasmon resonance (SPR) has been employed for the detection of a range of materials since the first commercially available appliance  
Figure 1: Summary of the strategies of recognition of bacteria and ways of resistance sensing using biosensors. Sample preparation may be needed to lyse the bacteria (or otherwise disrupt them) to liberate the target bacterial components (first column); and preparation-free whole cell-based assays are in the second column. Few biosensors can sense antibiotic resistance as well. There are two possibilities: measuring and monitoring the growth of bacteria during antibiotic treatment (third column) or measuring resistance factor adhesion or bacteriophage–bacterium interaction. was released in 1990 by Biacore [23]. SPR systems contain a source of plane-polarized light which then passes through a glass prism, its surface contacts the bioreceptor-functionalized transducer interface, which is generally a thin gold film [23; 64; 65]. The binding of the material of interest to the transducer surface changes the local refractive index, which in turn alters the angle of light leaving the prism (the so-called \"SPR angle\") [23]. Approximately 50% of the biosensors used in the world are based on the SPR technique, which similarly dominates the antibiotic biosensing field [21]. These biosensors have been developed for the detection of whole bacterial cells using a variety of bioreceptors, for instance lectins [66], molecularly imprinted polymers [67], antimicrobial peptides [68], mucin [69], antibodies [70; 71; 72; 73; 74; 75], and bacteriophage [76; 77]. Unfortunately, the detection of whole bacteria using this technique has a low sensitivity compared to other methods, due especially to the small difference in refractive index between the bacterial cytoplasm and the aqueous medium, and the limited penetration of the electromagnetic field into the bacteria, which is what controls the signal transduction in the biosensor [23; 78]. In recent years, hybrid configuration SPR biosensors have been made by combining metals and dielectrics [79; 80]. In localized surface plasmon resonance (LSPR), metal nanoparticles are used to intensify the sensitivity of the system [23]. Fiber optic probes enhance sensitivity, too [68; 81]. Some other strategies to enhance the sensitivity of SPR-based bacterial sensors include transducer surface modifications, e.g., using nanorods [82], and sandwich-type assays with nanoparticles for analyte capture to boost the signal [23]. The use of gold-coated magnetic nanoparticles increased the sensitivity down to 3 CFU/mL for _E. coli_[78]. However, LSPR is reported to be less sensitive and sometimes limited by a sample with biological matrix [23; 83; 84].