## 3 Biological Mechanisms in Bacterial Adhesion as Potential Subjects of Label-Free Monitoring
### Adhesive Molecules of Bacteria and Bacterial Entry into Host Cells
#### 3.1.5 Bacterial Entry into Host Cells  
The lifestyle inside the host cell has certain advantages to the pathogen bacteria; they no longer have to undertake energy-consuming adherence procedures to remain at the site of infection, they are further defended from the host immune system and antibiotics, and a wide range of nutrients are available in the cell cytoplasm [305]. Invasion offers the probability to penetrate and spread into deeper tissues and thus constitutes one of the most important strategies for bacteria to remain alive and be disseminated in their host organism [306]. In addition, intracellular pathogens need some virulence determinants, not just to pass into the cell, but also to withstand intracellular circumstances that may inhibit intracellular growth [17; 307; 308]. The host cell is not passive in the uptake process; moreover, pathogens usually apply special invasion mechanisms that exploit normal signaling pathways for their own benefit [306]. Although each species has adopted certain mechanisms to penetrate into eukaryotic cells, they generally share common elements, such as receptors (adhesion molecules), cytoskeleton-associated proteins, and signaling molecules (tyrosine kinases, GTPases, PI3-kinase) to confuse cellular functions [309]. During infection, phagocytosis is the main mechanism of the host to remove pathogenic microbes, by, for example, monocytes, neutrophils, and macrophages (see Figure 4) [17]. Phagocytosis is triggered by the cross-linking of various phagocytic receptors, resulting in the remodeling of the actin cytoskeleton and the cell membrane leading to the engulfment of the bacterium and formation of the phagosome, where eventually, through phagosome maturation, most bacteria will be killed and digested [310]. However, some bacteria can exploit this mechanism, surviving and proliferating in the host's professional phagocytes; _Mycobacterium tuberculosis_, for example, may inhibit phagosome maturation by preventing its acidification while _Legionella pneumonia_e blocks the fusion of the phagosome with the lysosome, among multiple other strategies [311]. _Salmonella_ and _Shigella_ use the triggering mechanism to penetrate into host cells, where a brief contact with the surface of the host cell actuates a fast cytoskeletal alteration in which actin polymerization in the cytoplasm pushes out large membrane extensions, called membrane ruffles (see Figure 4) [312; 313]. This Type III secretion-associated invasion mechanism leads to macropinocytosis, when the extended ruffles fold over and fuse back to the cell [17]. This procedure results in the inclusion of huge membrane pockets with the microorganism next to the in-folding ruffles [17]. This kind of invasion is very fast; the pathogen appears in the membrane-bound vacuole in a few minutes after contact with the host cell [314]. Salmonella triggers activation of host cell's phospholipase C, which provokes a temporary increase in the inositol triphosphate (IP3) concentration and mobilizes Ca2+ from intracellular stores [272]. Two actin binding proteins of Salmonella play a role in cytoskeletal redistribution; \(\mathrm{SipC}\) is necessary for F-actin bundling and nucleation of actin polymerization during the entry, whilst \(\mathrm{SipA}\) protein stimulates the function of \(\mathrm{SipC}\)[17; 315]. The zipper-type invasion procedure includes a single bacterial surface ligand that tightly contacts host cell adhesion molecules (see Figure 4) [17]. Thus, the entry process is driven by the tight binding between bacterial adhesins and their targets [270]. This connection leads to the clustering of receptors at the bacterium-cell interface that induces signal transduction cascades with cytoskeletal alterations (pseudopod-like structures). Consequently, circumferential binding of the receptors about the surface of the pathogen elicits  
Figure 4: Schematic illustration of the ways of bacterial entry into host cells.