Coming and going versus staying put: adhesion to a surface, regulation of adhesion, and initiation of microcolony formation
Sticking to it: factors that mediate bacterial adherence to a surface
Bacteria generally harbor multiple adhesion mechanisms that allow them to maintain hold on a surface once contact is made. Representative adhesion mechanisms and appendages are described in this section.
Microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) are a family of bacterial cell surface adhesins that mediate the initial attachment to the surface of host cells. Described mostly in Gram-positive bacteria, MSCRAMMs recognize and bind specific host ECM components, such as fibronec- tin, lectin, and collagen (Patti et al., 1994a; Vengadesan and Narayana, 2011). MSCRAMMs are generally covalently attached to the peptidoglycan of the bacterial cell wall in a “sortase-dependent” manner, a pathway that is described in the following section (Chagnot et al., 2012). Bacteria can harbor multiple MSCRAMMs that are specific for certain ligands. For example, S. aureus encodes the following MSCRAMMs: FnBPA and FnBPB (fibronectin-binding proteins), Cna (collagen-binding adhesin), ClfA (fibrinogen-binding protein), as well as others (Foster and Hook, 1998; Clarke and Foster, 2006). S. aureus infection can lead to various disease outcomes, which are in part dependent on the panel of adhesins expressed on the bacterial surface. For example, the collagen adhesin gene, cna, is crucial for adherence during septic arthritis, but is not critical for osteomyelitis infections (Patti et al., 1994b).
In Gram-negative bacteria, the outer membrane can harbor adhesive proteins that belong to the outer membrane protein (OMP) group. Some OMPs facilitate the elaboration of adhesive pili (Soto and Hultgren, 1999; Koebnik et al., 2000) and have been reportedly found in the ECM of Pseudomonas aeruginosa, E. coli, and V cholerae (Orme et al., 2006; Fong and Yildiz, 2015; Toyofuku et al., 2012).
In addition to OMPs, Gram-negative bacteria can code for adhesins that reach the bacterial surface via a type V secretion pathway. These types of proteins known as autotransporters are large proteins with three distinct domains: a cleavable N-termi- nal signal sequence, an internal passenger domain that carries out the functions specific to each autotransporter, and a C-terminal transmembrane domain that allows for autotransporter insertion in the outer membrane (Kostakioti and Stathopoulos, 2006; Chagnot et al., 2013). Notably, not all autotransporters are adhesins; many contribute significantly to bacterial pathogenesis by acting as proteases or carrying out differential functions (Chagnot et al., 2013). Examples of adhesin autotransporters are the Neisseria meningitidis AutA that stimulates auto-aggregation on surfaces (Arenas et al., 2015) and the uropathogenic E. coli (UPEC) Antigen 43 (Ag43) that is critical for long-term persistence of bacteria in the urinary tract (Ulett et al., 2007).