Environmental stimuli that influence bacterial movement
In response to their surrounding environments, bacteria constantly cue changes in their direction of motion. Thus, the composition and hydrodynamics of the environment surrounding a surface are critical factors influencing bacterial movement and adherence. There are three main signaling pathways that control movement, though they have diverse outcomes in different bacteria: chemotaxis, quorum sensing, and the secondary signaling molecule bis-(3-5')-cyclic dimeric guanosine monophosphate (c-di-GMP).
A typical chemosensory system comprises transmembrane chemoreceptors that are generally found at the cell poles (Briegel et al., 2009). These chemoreceptors bind distinct ligands and in response undergo differential methylation (via the action of cytoplasmic accessory proteins) and a conformational change that allows for the formation of complexes with a cytoplasmic histidine kinase, CheA. In E. coli, reduction of a chemoattractant in the medium leads to CheA autophosphorylation and subsequent phosphotransfer to another cytoplasmic functional regulator, CheY. Phosphorylated CheY (CheY ~ P) binds to the FliM component of the flagellar motor and forces a change in the rotation of the flagella from counterclockwise to clockwise direction (Porter et al., 2011; Wadhams and Armitage, 2004; Brown et al., 2011). As described in Section 3.2.2, a counterclockwise to clockwise rotation results in bacterial tumbling, and thereby a switch in the direction of movement until bacteria encounter high concentrations of the chemoattractant they are lacking. In other bacterial species, the binding of CheY ~ P to FliM can lead to “molecular braking,” causing the flagella to slow down or stop entirely (Porter et al., 2011). Reduction of flagellar rotation due to nutritional cues can provide enough stasis for the bacteria to engage a surface and initiate the formation of a biofilm.