## 3 Biological Mechanisms in Bacterial Adhesion as Potential Subjects of Label-Free Monitoring
### Adhesive Molecules of Bacteria and Bacterial Entry into Host Cells  
Bacteria have extensive tools--from single monomeric proteins to multimeric macromolecules--used for adhesion and invasion [239]. The infection starts with the adhesion of a bacterial pathogen to the host cell's surface. In general, pathogenic bacteria express surface-associated proteins, called adhesins, that recognize components of the eukaryotic cell surface or ECM and thus enable close contact with the host tissues [17; 240]. Bacterial adhesins may act synergistically, thus increasing their adhesive capacity [241]. Over the course of infection bacteria may express different adhesin types by gene regulation, depending on the site of infection, various environmental [242] and mechanical signals [243], and quorum sensing [244]. This gene regulation enables the bacteria to successfully colonize the host, first switching from reversible to irreversible adhesion, and later allows biofilm formation [245; 246]. Alterations in metabolic pathways (for example, induction of stress responses) usually affect biofilm formation and cell motility; furthermore, bacterial phenotype can also change during biofilm maturation [247]. Adhesins mediate attachment of pathogenic bacteria to host cells. These molecules are considered to be important virulence factors, but not all adhesins can be considered as essential virulence factors. Some pathogens produce more than just one adhesion, which can be expressed at different locations and times in the host and contributes individually to host cell attachment [17]. A variety of adhesins may also promote/initiate cell invasion [248]. Adhesins can be divided into two main groups: fimbrial and afimbrial [17; 249]. Fimbrial adhesins are filamentous polymeric structures in Gram-negative bacteria [241]. Initial adherence of microbes usually occurs via these structures (called fimbrie or pili), which extrude outwards from the bacterial cell surface [17; 250]. The formation of pili is complex and involves several auxiliary gene products that transport the pilus subunits to the cell surface and compose them into the organelle. The first step in their creation is the secretion of the pilin and adhesin subunits to the periplasmic space [17]. Host cell receptor proteins of pili are generally carbohydrate residues of glycolipids or glycoproteins [251]. For instance, in the upper urinary tract, P pili bind to the \(\alpha\)-d-galactopyranosyl-(1-4)-\(\beta\)-dgalactopyranoside moiety of glycolipids of cells [252]. Some bacteria, including Salmonella and Neisseria, have special environmentally controlled mechanisms to switch between nonpilated or pilated phases or different pilus types [17; 249; 253; 254]. Some bacteria use their pili as invasins [17; 255; 255]. The group of afimbial adhesins contains all surface proteins that are necessary for adherence but do not form themselves into pili-like structures [256]. According to some researchers, they are necessary to mediate a closer intimate attachment to cells that follows initial connection via fimbiae [242; 257]. These proteins usually contain parts that are homologous among each other or are homologous to eukaryotic sequences that enable cell-to-cell adhesion [17]. Afimbial adhesins of _E. coli_ (intimin), _Neisseria_ (Opas), _Yersinia_ (invasin, YadA), _Staphylococcus_/_Streptococcus_ (fibronectin-binding proteins, FnBPs), and _Listeria_ (internalin A, B, InIA, InIB) are the most characterized. _E. coli_ strains (enteropathogenic and enterohemorrhagic) produce the outer membrane protein intimin that recognizes minimum two types of receptors on the surface eukaryotic cell [17]. The first type includes those recognizing the \(\beta 1\) integrins, members of a big family of eukaryotic ECM receptor molecules [243]. The second type recognizes the translocated intimin receptor (Tir), which is produced by the bacteria and inserted into the host membrane [244]. The intimin molecules of different _E.