Bis-(3-5)-cyclic dimeric guanosine monophosphate (c-di-GMP)
The secondary messenger c-di-GMP is a key determinant in the change from a motile to sessile lifestyle, where it downregulates flagellar movement and actively promotes biofilm formation in many of the studied Gram-negative bacteria (Povolotsky and Hengge, 2015). In most cases, the transition from a motile state to initiating biofilm formation requires high levels of c-di-GMP. In E. coli, one of the critical proteins identified is a phosphodiesterase, called YhjH, that degrades c-di-GMP (Pesavento et al., 2008). YhjH is active during exponential phase of growth, but its expression decreases as cells enter stationary phase (Pesavento et al., 2008). Reduction in yhjH expression allows c-di-GMP levels to rise, facilitating interaction of c-di-GMP with the protein YcgR. YcgR is known as the flagellar “molecular brake,” and the complex of c-di-GMP and YcgR binds to the flagellar motor to slow flagellar rotation and promote adherence to the surface (Fang and Gomelsky, 2010).
A decrease in c-di-GMP levels can inhibit biofilm formation. In V. cholerae, the two-component system response regulator VieA regulates cholera toxin production, but also maintains phosphodiesterase activity that can decrease c-di-GMP levels. VieA decreases the cellular levels of c-di-GMP and has also been shown to repress expression of genes involved in V. cholerae exopolysaccharide synthesis to ultimately prevent biofilm formation (Tischler and Camilli, 2004, 2005). Downstream targets, proteins, and RNA riboswitches sense the relative abundance of c-di-GMP, and in turn, influence quorum sensing, production of extracellular matrix (ECM) components, and other virulence factors. In another example, levels of c-di-GMP regulate the expression of curli amyloid fibers in E. coli, which can serve as a component of the ECM in biofilms (Weber et al., 2006; Spurbeck et al., 2012).
Coupled with stochasticity, the hydrodynamic and nutritional stimuli surrounding the bacteria, along with appendages that mediate motility, guide the first interactions of a bacterial cell with a surface. For steadfast adherence to take place, a combination of contact-dependent signaling and the production of appropriate adherence factors must be present.