## 2 Applications of Biosensors in Bacteria Detection
### Resistance to Antibacterials and to Other Antimicrobial Agents
Other methods, such as electrochemical detection, UV, or fluorescence spectroscopy have minor significance because of their lower sensitivities. In the food industry, milk is one of the most controlled products, because typical antibiotics can easily accumulate in higher amounts due to its lipophilic properties. Because of the widespread consumption of milk, it is essential to control its threshold levels of antibiotics. It is necessary to detect the antibiotics prior to contamination of the food chain; for example, already at the farm [21]. To examine antibiotics, traditional microbiological tests based on the inhibition of bacterial growth can be applied [181]. These methods are simple and cheap; however, they must be carried out in the laboratory circumstances, and they are time-consuming [181; 182]. Alternative methods, such as liquid chromatography (LC) coupled to mass spectrometry (LC-MS) or UV (LC-UV) detection and capillary electrophoresis (CE)-based methods are expensive, need careful sample preparation, and can only be performed by trained staff [181; 182]. Compared to these analytical methods, biosensors can be used as cost-efficient, rapid, and simple methods that can be applied without further complicated sample preparation. They have been found to be comparable to conventional techniques with respect to sensitivity and specificity of antibiotic detection and thus fulfill international regulatory requirements [21]. [PERSON] and co-workers demonstrated a microfluidic platform enabling the electrochemical readout of up to eight enzyme-linked assays (ELAs) at the same time [181]. They applied highly sensitive biomolecular sensor systems for the detection of tetracycline and streptogram, the two commonly employed antibiotic classes. The limits of detection (LOD) are determined, with 6.33 for tetracycline and 9.22 ng/mL for pristinamycin. The employed channel material, dry film photoresist (DFR), allows an easy storage of preimobilized assays with a shelf life of minimum 3 months. Experiments in a medium are presented by the simultaneous detection of the antibiotics in spiked human plasma within a sample-to-result time of less than 15 min [181]. It is well known that fungi can produce antibacterial agents, but the bacteria themselves can be a robust source of novel antibiotics, as well; however, methods for screening large bacterial libraries for novel antibiotic production are required [183]. Recently, [PERSON] and co-workers showed a rapid and quantitative high-throughput liquid culture screening method for antibiotic production by bacteria. This technique screens both mono- and coculture mixtures of bacteria in vitro. The 5-day procedure starts after the colonies are quadrant-streaked from agar plates and allowed to grow for 36-48 h. Then, filter plates with the bacteria of interest are centrifuged and the filtrate treated with pathogenic bacteria. After 24 h incubation, pathogenic inhibition is assessed via optical density (at 590 nm) [183]. Hits are determined as any inhibition at least two standard deviations from the control in tryptic soy broth (TSB) [183]. Strong producers are categorized as those hits that have \(\geq\)30% inhibition. Over 260 bacterial species were screened in monoculture, and 38 and 34% were found to produce antibiotics inhibiting of _S. aureus_ or _E. coli_, respectively (with 8 and 4% strong producers, respectively). More than 270 cocultures were investigated, and 14 and 30% were found to generate antibiotics inhibiting _S. aureus_ or _E. coli_, respectively [183]. The number of biosensor applications in this field has risen since 2006, especially for the detection of antibiotics and bacteria in the environment [21]. In the design of sensors, magnetic nanoparticles can also be incorporated in order to increase their sensitivity in detecting antibacterials [180]. Despite many different antibiotic recognition principles, just a few biosensor techniques were used in the antibiotic field [21], but the recent developments and studies are very promising.