## 3 Biological Mechanisms in Bacterial Adhesion as Potential Subjects of Label-Free Monitoring
### Adhesive Molecules of Bacteria and Bacterial Entry into Host Cells
coli_ strains are about 900 amino acids in length and have an N-terminal region that is very homologous to invasin. The cell-attaching function of invasin and intimin is localized within the last C-terminal 200 amino acids. Fibronectin, a substrate of \(\beta 1\) integrins, also contains two aspartate residues necessary for receptor binding. The similarity within the C-termini of intimin and invasin is the presence of a pair of cysteine residues separated by approximately 70 amino acids [17]. These residues form a disulfide loop in the invasin and intimin that is necessary for adhesion to \(\beta 1\) integrins [28]. _Neisseria meningitidis_ and _Neisseria gonorrhoeae_ express a number of Opas and Opcs, which are highly variable opacity-associated outer membrane proteins [258; 259]. These proteins impart tight interactions between various eukaryotic cells [245; 246]. A minimum of 12 diverse variants of Opa genes are in the gonococcal genome and they are expressed independently of each other [245; 246]. The Opa genes are transcripted from their own promoter [260]. Switching in Opa expression may contribute to antigen variability and various receptor tropisms [17; 249]. For example, the Opa50 variant recognizes heparan sulfate proteoglycan (HSPG), fibronectin, and vitronectin, which plays a role in the attachment of the bacterium to \(\beta 1\) integrin receptors [261]. Both \(\beta 1\) integrins and HSPG and are on the basolateral side of epithelial cells and primarily contribute to the basolateral invasion of epithelial cells [262]. Opa- and Opc-mediated cell invasion and interaction are inhibited when lipooligosaccharides are modulated by sialylation [17; 263]. Enteropathogenic _Yersinia_ has an additional adhesin, termed YadA, that helps adhesion to epithelial cells [264]. YadA also defends the bacterium against the defensin lysis and complement system [264]. YadA is a member of the family of adhesins that form lollipop-shaped surface projections (for example, UspA1 and UspA2 of _Moxarella catarrulnis_), and encoded on the _Yersinia_ virulence plasmid. Three or four YadAs form a fibrillar structure and cover the surface of the bacterium as a capsule-like fibrillous matrix. The C-termini of the YadAs are responsible for oligomerization, translocation, and fixing in the outer membrane [17]. The N-termini fold into the global head domain that plays a role in adhesion to cells and ECM molecules (collagens, fibronectin, laminins) [265]. It has been demonstrated that the eight repeats of the AsnSerValAlalleGlyXoxSer amino acids in the N-terminal part of YadA are principally involved in collagen binding. YadA also promotes the penetration into host cells via \(\beta 1\) integrins [266]. _L. monocytogenes_ is a foodborne pathogen that penetrates into different cell types during disease. This bacterium synthesizes various homologous surface proteins, termed internalins, which are significant for bacterial entry into eukaryotic cells [17; 267]. Internalins bind to ECM molecules with high affinity and, in general, the binding is irreversible. They are covalently linked to the cell wall at their C-terminus, and the N-terminus is exposed to the environment. Mutants are attenuated in virulence, which indicates that these molecules participate in colonization and disease as well [17]. [PERSON] and co-workers [268] have identified the corresponding receptor as E-cadherin, a transmembrane surface molecule of mammalian cells which plays a role in the formation of homophilic cell-cell junctions [17]. The InIA-E-cadherin-promoted penetration of Listeria corresponds to the invasin-1\(\beta\)-integrin-mediated cell entry by _Yersinia_[269]. Both bacteria penetrate via the 'zippering' mechanism [270] (see Figure 3). A schematic illustration of the parts of a bacterium and the adhesive molecules is found in Figure 1.