Mutations in Sugar-Nucleotide Synthesis Genes Restore Holdfast Polysaccharide Anchoring to Caulobacter crescentus Holdfast Anchor Mutants

Abstract

ABSTRACT Attachment is essential for microorganisms to establish interactions with both biotic and abiotic surfaces. Stable attachment of Caulobacter crescentus to surfaces requires an adhesive polysaccharide holdfast, but the exact composition of the holdfast is unknown. The holdfast is anchored to the cell envelope by outer membrane proteins HfaA, HfaB, and HfaD. H old f ast a nchor gene mutations result in holdfast shedding and reduced cell adherence. Translocation of HfaA and HfaD to the cell surface requires HfaB. The Wzx homolog HfsF is predicted to be a bacterial polysaccharide flippase. An hfsF deletion significantly reduced the amount of holdfast produced per cell and slightly reduced adherence. A Δ hfsF Δ hfaD double mutant was completely deficient in adherence. A suppressor screen that restored adhesion in the Δ hfsF Δ hfaD mutant identified mutations in three genes: wbqV , rfbB , and rmlA . Both WbqV and RfbB belong to a family of nucleoside-diphosphate epimerases, and RmlA has similarity to nucleotidyltransferases. The loss of wbqV or rfbB in the Δ hfsF Δ hfaD mutant reduced holdfast shedding but did not restore holdfast synthesis to parental levels. Loss of wbqV or rfbB did not restore adherence to a Δ hfsF mutant but did restore adherence and holdfast anchoring to a Δ hfaD mutant, confirming that suppression occurs through restoration of holdfast anchoring. The adherence and holdfast anchoring of a Δ hfaA ΔhfaD mutant could be restored by wbqV or rfbB mutation, but such mutations could not suppress these phenotypes in the Δ hfaB mutant. We hypothesize that HfaB plays an additional role in holdfast anchoring or helps to translocate an unknown factor that is important for holdfast anchoring. IMPORTANCE Biofilm formation results in increased resistance to both environmental stresses and antibiotics. Caulobacter crescentus requires an adhesive holdfast for permanent attachment and biofilm formation, but the exact mechanism of polysaccharide anchoring to the cell and the holdfast composition are unknown. Here we identify novel polysaccharide genes that affect holdfast anchoring to the cell. We identify a new role for the holdfast anchor protein HfaB. This work increases our specific knowledge of the polysaccharide adhesin involved in Caulobacter attachment and the general knowledge regarding production and anchoring of polysaccharide adhesins by bacteria. This work also explores the interactions between different polysaccharide biosynthesis and secretion systems in bacteria.