## 4 Conclusions  
We have reviewed the status of label-free biosensors in bacteria monitoring and summarized potential novel application directions with biological relevancies. Optical, mechanical, and electrical sensing technologies were all discussed with their detailed capabilities in monitoring bacterial species. In order to review interesting potential future applications of the outlined techniques in bacteria research, we summarized the most important kinetic processes relevant to the adhesion and survival of bacterial cells. These processes are potential targets of kinetic investigations employing modern label-free technologies in order to reveal new fundamental aspects and to initiate new directions for future research and development. As a potential future area to be targeted by these systems, we have outlined how microscopic pathogenic creatures enter much bigger mammalian cells and infect them.  
Not less importantly, we highlighted that numerous illnesses caused by pathogenic bacteria can be cured by antibiotics or other types of antimicrobial compounds, but excessive and sometimes incorrect usage has led to the proliferation of (multi)resistant bacteria, causing a worldwide problem. Pathogenic bacteria and antibacterial agents are important targets for detection--i.e., both cells and molecules. This review, however, focuses above all on the detection of bacterial cells (the detection of eukaryotic cells has been reviewed elsewhere [170]). The use of biosensors to detect molecules is covered elsewhere [337; 338]. We summarized the roles of adhesive molecules, receptors, and glycocalyx of the bacteria, which are highly important factors in biofilm formation. To reduce and inhibit biofilm formation and maturation, bacteria-repellent surfaces are being developed and tested. For a long time, culture-based conventional methods were the only way to identify bacteria from samples from, for instance, the food industry or medicine. Although they are reliable methods, they have long processing times and professional operators are required. In contrast, biosensors provide a rapid, simpler way to detect and identify bacteria and antibiotic effects, and modern commercial instruments can be used as a \"black box\" by untrained staff ignorant of their inner workings, often with acceptably reliable results. We have reviewed the most promising types of label-free biosensors, including those that use bacteriophages. Recently, the examination of antibiotic resistance by using these techniques has been implemented. Further improvements of biosensors should make the detection of pathogens easier and cheaper, and they may become used routinely in clinical medicine, the food industry, security, and public health more effectively [339]. Moreover, thanks to their unique capabilities, novel label-free sensing technologies will potentially find applications in basic research along the lines summarized here.  
Conceptualization, B.P. and R.H.; resources, B.P. and R.H.; data curation, B.P. and E.F.; writing--original draft preparation, B.P., E.F., J.R., Z.S., I.S., S.B. and S.K.; writing--review and editing, B.P., R.H. and J.R.; visualization, B.P.; supervision, B.P. and R.H.; project administration, B.P. and R.H.; funding acquisition, B.P., S.B. and R.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Lendulet (Momentum) Programme of the Hungarian Academy of Sciences as well as the ERC_HU, KH_17, KKP_19, PD 131543 (for B.P.) and PD 134195 (for Z.S.) Programmes of the National Research, Development and Innovation Office (NKFIH). S.B. acknowledges the support of the ELTE Thematic Excellence Programme 2020 supported by the National Research, Development and Innovation Office (TKP2020-IKA-05) and thanks for the support of grant EFOP-1.8.0-VEKOP-17-2017-00001.  
Not applicable.  
All relevant data are available in the manuscript.  
The authors declare no conflict of interest.