Obstruction of pilus retraction stimulates bacterial surface sensing

Abstract

One Sentence Summary: Bacteria use the tension imparted on retracting pilus fibers upon their binding to a surface for surface sensing. Abstract:Surface association provides numerous fitness advantages to bacteria. Thus, it is critical for bacteria to recognize surface contact and to consequently initiate physiological changes required for a surface-associated lifestyle (1). Ubiquitous microbial appendages called pili are involved in sensing surfaces and mediating downstream surface-associated behaviors (2-6). The mechanism by which pili mediate surface sensing remains unknown, largely due to the difficulty to visualize their dynamic nature and to directly modulate their activity without genetic modification. Here, we show that Caulobacter crescentus pili undergo dynamic cycles of extension and retraction that cease within seconds of surface contact, and this arrest of pilus activity coincides with surface-stimulated holdfast synthesis. By physically blocking pili, we show that imposing resistance to pilus retraction is sufficient to stimulate holdfast synthesis in the absence of surface contact. Thus, resistance to type IV pilus retraction upon surface attachment is used for surface sensing.

Feedback regulation of Caulobacter crescentus holdfast synthesis by flagellum assembly via the holdfast inhibitor HfiA

“…Caulobacter crescentus is a robust model organism for studying initial attachment because of its ability to rapidly synthesize the adhesive holdfast in direct response to surface contact (Li et al, 2012;Hoffman et al, 2015;Ellison et al, 2017). C. crescentus has a dimorphic lifestyle: at each division, two different cell types are produced, a motile swarmer cell and a sessile stalked cell (Fig.…”

“…Similarly, transcription of the holdfast inhibitor gene, hfiA, is controlled by cell cycle developmental regulators (Fiebig et al, 2014). Upon division, swarmer cells are preloaded with a complete set of holdfast biosynthesis proteins as evidenced by the observation that neither RNA nor protein synthesis is required for holdfast synthesis in swarmer cells (Levi and Jenal, 2006;Li et al, 2012;Ellison et al, 2017). In these cells, HfiA prevents the holdfast synthesis machinery from producing holdfast prematurely by inhibiting HfsJ, a glycosyltransferase required for holdfast synthesis (Fiebig et al, 2014).…”

“…In liquid cultures, holdfasts typically are not elaborated until the late swarmer stage (Levi and Jenal, 2006); however, holdfast synthesis can be stimulated earlier in direct response to surface contact (Li et al, 2012;Hoffman et al, 2015;Ellison et al, 2017). In both cases, post-transcriptional signals activate holdfast biosynthesis.…”

Monoderm bacteria: the new frontier for type IV pilus biology

“…However, an unexpected twist comes from the analysis of DNA uptake in M. luteus that was found to be mediated by Tfpc (Angelov et al, 2015), which were not previously known to be capable of mediating this property. This finding has important functional implications, especially since it suggests that Tfpc in M. luteus are retractile filaments capable of generating significant tensile force, which has recently been confirmed in diderms (Ellison et al, 2017). Owing to its simple pilus biogenesis machinery (Fig.…”

“…Important advances (although this is obviously subjective) concerning the molecular mechanisms of filament biogenesis would be to (i) precisely define the roles of PilM, PilN and PilO proteins in Tfpa assembly, (ii) determine whether the minor pilins sit at the filament tip and prime filament assembly (Cisneros et al, 2012;Nguyen et al, 2015), (iii) understand how Tfp cross the meshlike PG layer (could the non-canonical PilO protein be involved) and (iv) understand how the more rudimentary Tfpc machinery works. As for Tfp-mediated functions, monoderms could prove key for understanding how (i) Tfp mediate adhesion by studying PilB and PilC modular pilins (this would have implications reaching far beyond adhesion since modular pilins with hundreds of different domain architectures are widespread), (ii) Tfpa retraction/ extension dynamics orchestrate motility, possibly by using elegant labelling or label-free strategies that allow dynamic filament visualisation (Ellison et al, 2017;Tala et al, 2019) and (iii) Tfpc retraction occurs in the absence of PilT.…”