## 2 Applications of Biosensors in Bacteria Detection
### Recognition of Bacteria Using Biosensors  
Timely detection and identification of pathogenic bacteria is very important for different fields including medicine, food safety, and security [23; 34; 35]. Infectious illnesses cause millions of deaths and hospitalizations each year. _Salmonella_, _Campylobacter_, _Enteroharmoragic Escherichia coli_, _Listeria_, and _Vibrio cholerae_ are the most common cause of bacterial food- and water-borne infection according to the World Health Organization [23], and _Staphylococcus aureus_ is one of the most common etiological agents in hospital-acquired infections [35]. As discussed above, conventional laboratory-based methods for bacterial detection and identification typically require specialized equipment, often non-automated, have long processing times, are costly, and some of them lack sensitivity and specificity. On-site identification of bacteria and expected response to antibiotic treatment may be critical for clinical diagnosis and treatment. Application of biosensors for bacterial detection has been summarized in a recent book [36]. In the coming sections, we shall highlight some important points from earlier research and review the most important new developments.  
Biosensors provide a cost-effective, rapid, and sensitive method of detection. Recent strategies for detection of bacteria involve biosensor techniques based on recognition of bacterial components or whole bacterial cells. Nucleic acid-, intracellular protein- (enzymes), and bacteria-produced exotoxin-based biosensors include sample preparation steps to achieve target bacterial component separation by lysis or other disruption [23; 37; 38; 39; 40]. The main disadvantage of these methods is the necessity of component isolation and purification, which raises the cost and time of these measurements. Thus, biosensors for the direct detection of whole bacteria is preferable for point-of-care diagnostic. The main challenge in biosensor detection is to obtain a concentrated sample from a high-volume flowing environment. This is critical because the infectious dose of bacteria for many human pathogens is remarkably low [23]. The concentration of bacteria can be achieved through centrifugation or filtration of the sample, concentration by using nanoparticles, and some biosensor techniques have the possibility to concentrate the target cells, for example with electrodes [41]. Dielectrophoresis is often used in biosensors employing microfluidics [42; 43; 44].  
A bioreceptor provides the specificity of the analytical determination in the biosensor technique. The surface of the transducer must be modified with any molecule that bacteria of interest are known to bind to. It can be mono- or polyclonal recombinant antibodies [45], glycans and lectin proteins [46], and bacteriophage [47]. Bacteria have different surface antigens presented on the cell or the flagellae, such as surface proteins, glycoproteins, peptidoglycans, and lipopolysaccharidesaccharides that can act as targets for biorecognition. Antibodies are the most widely used bioreceptors. Aside from their high specificity, their production and purification is time-consuming and costly, cross-reactivity may occur, their proper orientation on the sensor surface is difficult to achieve, and they are generally rather unstable [23]. Bacteria exhibit many epitopes that can also cause so-called nonspecific interactions with the unmodified sensor surface. However, appropriate design of the surface can be exploited to achieve effective differential binding of bacteria [23]. Improvement of the detection limits (signal-to-noise ratio) for bacteria is a key assignment for the next generation of biosensor techniques. Figure 1 schematically shows possible ways to recognize bacteria and sense antibiotic resistance using biosensors.