Abstract
Biofilms are community structures of bacteria enmeshed in a self-produced matrix of exopolysaccharides. The biofilm matrix serves numerous roles, including resilience and persistence, making biofilms a subject of research interest among persistent clinical pathogens of global health importance. Our current understanding of the underlying biochemical pathways responsible for biosynthesis of these exopolysaccharides is largely limited to Gram-negative bacteria. Clostridia are a class of Gram-positive, anaerobic and spore-forming bacteria, and include the important human pathogens Clostridium perfringens, Clostridium botulinum, and Clostridioides difficile, among numerous others. Clostridia have been reported to form biofilms composed of cellulose, although the specific loci which encode the cellulose synthase have not been identified. Here, we report the discovery of a gene cluster, which we named ccsABZCD, among selected bacteria within class Clostridia that appears to encode a synthase complex responsible for polymerization, modification, and export of an O-acetylated cellulose exopolysaccharide. To test this hypothesis, we subcloned the putative glycoside hydrolase CcsZ and solved the X-ray crystal structure of both apo- and product-bound CcsZ. Our results demonstrate that CcsZ is in fact an endo-acting cellulase belonging to glycoside hydrolase family 5 (GH-5). This is in contrast to the Gram-negative cellulose synthase, which instead encodes BcsZ, a structurally distinct GH-8. We further show CcsZ is capable of hydrolysis of the soluble mock substrate carboxymethylcellulose (CMC) with a pH optimum of 4.5. The data we present here serves as an entry point to an understanding of biofilm formation among class Clostridia and allowed us to predict a model for Clostridial cellulose synthesis.
Author summary Biofilms are communities of microorganisms that enmesh themselves in a protective matrix of elf-produced polysaccharide materials. Biofilms have demonstrated roles in both virulence and persistence among bacterial pathogens of global health importance. The class Clostridia are a polyphyletic grouping of primarily Gram-positive, anaerobic and spore-forming bacteria which contain the important and well-studied human pathogens Clostridioides difficile, Clostridium botulinum, and Clostridium perfringens, among others. Bacteria belonging to class Clostridia have been anecdotally reported to form biofilms made of cellulose, although the molecular mechanisms governing their production has not before been described. In this work, we identify a gene cluster, which we name ccsABZHI, for the Clostridial cellulose synthase, which bears remarkable similarity to molecular machinery required for the production of cellulose biofilms in other Gram-negative bacteria. We further biochemically characterize one of these enzymes, CcsZ, a predicted endoglucanase which we predicted from our model should cleave cellulose exopolysaccharides. We show that CcsZ is in fact capable of this activity and belongs to a broader family of glycoside hydrolases with unexpected taxonomic diversity. Our work represents an entry point towards an understanding of the molecular mechanisms governing cellulose biofilm formation in Gram-positive bacteria.
