Abstract
Attachment is a necessary first step in bacterial commitment to surface-associated behaviors that include colonization, biofilm formation, and host-directed virulence. The Gram-negative opportunistic pathogen Pseudomonas aeruginosa can initially attach to surfaces via its single polar flagellum. Although many bacteria quickly detach, some become irreversibly attached and express surface-associated structures, such as Type IV pili, and behaviors, including twitching motility and biofilm initiation. P. aeruginosa that lack the GTPase FlhF assemble a randomly placed flagellum that is motile; however, we observed that these mutant bacteria show defects in biofilm formation comparable to those seen for non-motile, aflagellate bacteria. This phenotype was associated with altered behavior of ΔflhF bacteria immediately following surface-attachment. Forward and reverse genetic screens led to the discovery that FlhF interacts with FimV to control flagellar rotation at a surface, and implicated cAMP signaling in this pathway. Although cAMP controls many transcriptional programs in P. aeruginosa, known targets of this second messenger were not required to modulate flagellar rotation in surface-attached bacteria. Instead, alterations in switching behavior of the motor appeared to result from direct or indirect effects of cAMP on switch complex proteins and/or the stators associated with them.
Highlights
Biofilms are clinically relevant, surface-associated multicellular bacterial communities
We observed that ΔflhF bacteria, which assemble a randomly placed but functional single flagellum, were as defective in biofilm formation as bacteria lacking a flagellum (ΔfliC) (Fig 1A)
As the flagellum influences initial steps in biofilm formation, we developed a single-cell tethering assay to observe bacteria after surface attachment (S1 Fig)
Summary
Reversible, flagellar-mediated attachment of bacterial cells to a surface is somehow sensed, resulting in cessation of flagellar rotation and a transition to irreversible attachment [2, 3]. This step precedes surface-associated growth and T4P-mediated surface colonization, precursors to the formation of a matrix-encased biofilm community. The initial event of surface sensing is essential for this process, but how this bacterial sense of “touch” is perceived and transmitted remains unknown. A favored hypothesis is that increased flagellar load associated with surface tethering serves as a signal for surface attachment [4], but mediators that could transmit such a signal have not been definitively identified
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