## 2 Applications of Biosensors in Bacteria Detection
### Detection of Resistance Using Biosensors  
A fast biosensor for the detection of bacterial growth was developed by [PERSON] and co-workers using micromechanical oscillators coated by common nutritive layers [32]. The change in resonance frequency due to the increasing mass on a cantilever array is the basis of the detection scheme [32]. Single bacterial cells and virus particles can be detected in a dry environment with these types of sensors [188; 32; 189]. The calculated mass sensitivity according to the mechanical properties of the cantilever sensor is 50 pg/Hz, which corresponds to a sensitivity of circa 100 _E. coli_ cells [32]. The sensor is able to detect the active growth of _E. coli_ cells within 1 h [32]. In addition, this method allows the detection of selective growth of _E. coli_ within only 2 h by adding antibiotics such as kanamycin or tetracycline to the nutritive layers [32]. The authors confirmed the growth of _E. coli_ by scanning electron microscopy [32]. For this measurement, two cantilevers were coated differently; the growth-inhibiting cantilever was coated by a nutritive layer containing 10 g/mL kanamycin, which inhibits the growth of _E. coli_ XL1-Blue [32]. However, the growth-supporting cantilever was coated by the same nutritive layer, but tetracycline (10 g/mL) was added to it [32]. _E. coli_ XL1-Blue is resistant to tetracycline and is, therefore, able to grow on such a coating [32]. Both cantilevers were then exposed to equal amounts of _E. coli_ XL1-Blue bacteria (1000 cells/cantilever surface) [32]. There was no change in the resonance frequency of the growth-inhibiting cantilevers [32]. On the other hand, the frequency shifts of the growth-supporting cantilevers increased over the first 4 h of the experiment and then reached a constant value [32]. Due to the sensitivity of the sensor, it was possible to detect selective growth of _E. coli_ due to antibiotics in less than 2 h [32]. Magnesium zinc oxide (MZO) nanostructure-modified quartz crystal microbalance (MZOnano-QCM) biosensor monitored antimicrobial resistance in case of _E. coli_ and _S. cerevisiae_ from a small amount of sample (2 mL) within 10 min [190]. [PERSON] et al. designed biofunctionalized silicon micropillar arrays to provide both a preferable solid-liquid interface for bacteria networking and a simultaneous transducing element that monitors the response of pathogens when exposed to certain antibiotics in real time [191]. The micropillar architecture relays optical phase-shift reflectometric interference spectroscopic measurements as a platform for label-free, culture-free phenotypic antimicrobial susceptibility testing (AST) [191]. Within the interstitial space of the micropillar gratings, cells can freely move and colonize, while colonization on top of the transducing element (as opposed to within) is prevented, thus minimizing losses in optical response [191]. The responses of _E. coli_ to different concentrations of five clinically relevant antibiotics (gentamicin, ciprofloxacin, ampicillin, ceftaxone, sulfamethoxazol-trimethoprim) were optically tracked by PRISM and minimum inhibitory concentration (MIC) values determined and compared to both clinic-based automated AST system readouts and standard broth microdilution testing [191]. The capture of bacteria within these microtopologies followed by incubation of cells with the antibiotic solution yields fast (2-3 h total assay time versus 8 h with automated AST systems) and accurate determinations of antibiotic susceptibility [191] and can also differentiate between bactericidal and bacteriostatic antibiotics [191]. In the study by [PERSON] and co-workers, the authors presented the manufacturing and characterization of a low-cost carbon screen-printed electrochemical sensor on a ceramic substrate [33]. Applying label-free electrochemical impedance spectroscopy (EIS), the sensor is applicable for detection of blaNDM, which is one of the main antimicrobial resistance factors in carbapenem-resistant Enterobacteriaceaeae.