## 2 Applications of Biosensors in Bacteria Detection
### Resistance to Antibacterials and to Other Antimicrobial Agents  
Inevitably, antibacterials rapidly lose their effectiveness as bacteria adapt and develop resistance to them; some bacteria even resist multiple drugs [172; 173]. Sequential chromosomal mutations can occur during continuous exposure to antibiotics, which lead to the appearance of resistance mechanisms step by step [21]. Various factors can participate in the development of antibiotic resistance: (i) extensive and sometimes unnecessary applications of antibiotics in pet care and agriculture; (ii) overprescribing of antibiotics for humans; (iii) patients not taking antibiotics as prescribed; (iv) the increased presence of antibiotics or their metabolites in the environment; and (v) lack of rapid laboratory tests for the identification of bacteria leading to the application of broad-spectrum antibiotics [21; 174]. Although the efficacy of antibiotic treatment can never be permanently guaranteed, the practice of medicine will be greatly helped if the further progress of bacterial resistance can be slowed down. To this end, it will be useful to identify natural compounds or develop new antibiotics that can attack novel target structures or overcome the existing resistance mechanisms. Unfortunately, the development of novel antibacterial compounds is difficult. Until the early 1980s, various antibacterial classes had been identified. After that, until the end of the 20 th century, there was no further discovery of truly novel antibacterials [21; 175]. Then, in recent years, three novel antibacterials (daptomycin (lipopeptide) [21; 176], tigecycline (glycycline), and retapamulin (pleuromutilin)) have been validated [21; 177; 178]. Other accepted antibacterials are merely derivatives of known ones. There are currently only five real novel antibacterial compounds in clinical development [21]. These compounds are extensively used agents against microbial infection in animals as well [179]. However, inappropriate doses may result in antibacterial residues in food of animal origin and possible side effects may influence human health [179]. The accumulation of these chemicals in natural waters, beverages in both their metabolized and unmetabolized forms are of interest, too [180]. Antibiotics and other type of antimicrobial compounds are in some cases probably not able to overcome the binding capacity of the glycocalyx and therefore simply cannot reach their bacterial target [18]. Adhesion has an important role in the success of pathogens, so an effective way to prevent or combat bacterial infection can be the prevention of their adhesion. Three paths to antibiotics that interfere with glycocalyx formation or function in specific pathogens may be:  
* occupation and blockage of the active site of a lectin that mediates the adhesion of bacterial glycocalyx fibers to the fibers of host cells,* blockage of the \"receptor\" sites on host cells, the glycoprotein fibers to which bacterial fibers adhere directly, or
* disruption of the production of glycocalyx fibers (mimics its normal substrate and therefore occupies the enzyme's active site) [18]. Vaccines against the surface components of these pathogenic bacteria have been shown to be protective, and a range of vaccines against the pili/glycocalyx are being developed. The ion exchange barrier, the distal polysaccharide portion, influences antibiotic sensitivity. Furthermore, divalent cation concentrations affect bacterial sensitivity to antibiotics. The effect of the glycocalyx is complicated to isolate. In the absence of human and bacterial polysaccharides, examination of the antibacterial effect of a compound using laboratory cultures is hardly predictive [19]. Currently, the main analytical tools for antibiotic detection are liquid and gas chromatographic techniques, mostly combined with mass spectrometric analysis. For example, the detection of antibacterial agents in environmental waters is generally performed by HPLC-MS or HPLC-MS/MS [21].