A polysaccharide deacetylase enhances bacterial adhesion in high ionic strength environments

Abstract

The adhesion of organisms to surfaces in aquatic environments provides a diversity of benefits such as better access to nutrients or protection from the elements or from predation. Differences in ionic strength, pH, temperature, shear forces, and other environmental factors impact adhesion and organisms have evolved various strategies to optimize their adhesins for their specific environmental conditions. We know essentially nothing about how bacteria evolved their adhesive mechanisms to attach efficiently in environments with different physico-chemical conditions. Many species of Alphaproteobacteria, including members of the order Caulobacterales, use a polar adhesin, called holdfast, for surface attachment and subsequent biofilm formation in both freshwater and marine environments. Hirschia baltica, a marine member of Caulobacterales, produces a holdfast adhesin that tolerates a drastically higher ionic strength than the holdfast produced by its freshwater relative, Caulobacter crescentus. In this work, we show that the holdfast polysaccharide deacetylase HfsH plays an important role in adherence in high ionic strength environments. We show that deletion of hfsH in H. baltica disrupts holdfast binding properties and structure. Increasing expression of HfsH in C. crescentus improved holdfast binding in high salinity, whereas lowering HfsH expression in H. baltica reduced holdfast binding at high ionic strength. We conclude that HfsH plays a role in modulating holdfast binding at high ionic strength and hypothesize that this modulation occurs through varied deacetylation of holdfast polysaccharides.

HfaE is a component of the holdfast anchor complex that tethers the holdfast adhesin to the cell envelope

“…Little is known about how the holdfast anchor complex interacts with holdfast polysaccharides. Deacetylation of holdfast polysaccharides is required for strong interactions between the holdfast polysaccharides, anchor complex, and the thiol-containing component(s) of the holdfast (19, 20). Mutations in sugar-nucleotide synthesis genes have been shown to suppress holdfast shedding in hfaA and hfaD mutants, but not a hfaB mutant (29).…”

“…How holdfast polysaccharides are attached to the cell envelope via the anchor complex is still unknown. Polysaccharide deacetylase mutants shed holdfast similarly to anchor mutants, suggesting that deacetylation of holdfast polysaccharides is required for strong interactions between the holdfast polysaccharides and the anchor complex (19, 20). Mutations in the sugar-nucleotide synthesis genes, wbqV and rfbB, suppress holdfast shedding and restore biofilm formation in the hfaE mutant, similar to what has been observed in the hfaA and hfaD mutants (29).…”

“…Additional monosaccharide substituents are then added to form a repeat unit on the lipid carrier by three putative glycosyltransferases HfsG (16), HfsJ (17), and HfsL (18). Some of the sugar residues on the repeat units are enzymatically modified by a deacetylase, HfsH (19, 20), and a putative acetyltransferase, HfsK (21). The lipid carrier with the polysaccharide repeat unit is subsequently translocated across the inner membrane into the periplasm by the putative flippase HfsF (16).…”