Stimulation of the Caulobacter crescentus surface sensing pathway by deletion of a specialized minor pilin-like gene

Abstract

Bacteria colonize surfaces through complex mechanisms of surface sensing. Pili are dynamic bacterial appendages that play an important role in this process. In Caulobacter crescentus, tension on retracting, surface-bound pili triggers the rapid synthesis of the adhesive holdfast, which permanently attaches cells to surfaces. However, the detailed mechanisms of pilus-mediated surface sensing are unclear. In this study, we used a genetic screen to isolate mutants with altered pilus activity to identify genes that may be involved in pilus-mediated surface-sensing. This screen identified cpaL, whose deletion led to reduced piliation levels, and surprisingly, a threefold increase in surface adhesion due to increased holdfast production. To understand this finding, we compared holdfast synthesis in wild-type and cpaL mutant cells under conditions that block pilus retraction. While this treatment increased holdfast production in wild-type cells by triggering the surface-sensing pathway, no increase was observed in the cpaL mutant, suggesting that mutation of cpaL maximally stimulates surface-sensing. Furthermore, when the cpaL mutant was grown in a medium that blocks the surface sensing pathway, cells exhibited decreased surface attachment and holdfast production, consistent with a role for CpaL in pilus-dependent surface sensing in C. crescentus. To better understand the function of CpaL, we analyzed its predicted structure, which suggested that CpaL is a minor pilin fused to a mechanosensitive von Willebrand factor type A (vWA) domain that could be accommodated at the pilus tip. These results collectively position CpaL as a strong candidate for a mechanosensory element in pilus-mediated surface sensing. ImportanceSurface sensing is a crucial mechanism that allows bacteria to change their behaviors to adapt to life on a surface. Surface recognition by bacteria is the initial step toward surface colonization and biofilm formation. In Caulobacter crescentus, tight adherence (Tad) pili play a key role in surface recognition and adaptation. However, the mechanism of pilus-mediated surface sensing and the proteins that influence this process remain unknown. Here, we demonstrate that CpaL, a potential pilus tip mechanosensory protein, could be the major element of Tad pilus-mediated surface attachment and colonization in C. crescentus. CpaL plays an important role in the regulation of holdfast synthesis and production upon surface contact. By identifying CpaL as a key player in the process of surface recognition, our work offers valuable insights into the mechanisms of bacterial adhesion.