Caldicellulosiruptor bescii Adheres to Polysaccharides via a Type IV Pilin-Dependent Mechanism.

Biological hydrolysis of cellulose above 70°C involves microorganisms that secrete free enzymes and deploy separate protein systems to adhere to their substrate. Strongly cellulolytic is one such extreme thermophile, which deploys modular, multifunctional carbohydrate-acting enzymes to deconstruct plant biomass. Additionally, also encodes noncatalytic carbohydrate binding proteins, which likely evolved as a mechanism to compete against other heterotrophs in carbon-limited biotopes that these bacteria inhabit. Analysis of the pangenome identified a type IV pilus (T4P) locus encoded upstream of the tāpirins, that is encoded by all species. In this study, we sought to determine if the T4P plays a role in attachment to plant polysaccharides. The major pilin (CbPilA) was identified by the presence of pilin-like protein domains, paired with transcriptomics and proteomics data. Using immuno-dot blots, we determined that the plant polysaccharide xylan induced production of CbPilA 10- to 14-fold higher than glucomannan or xylose. Furthermore, we are able to demonstrate that recombinant CbPilA directly interacts with xylan and cellulose at elevated temperatures. Localization of CbPilA at the cell surface was confirmed by immunofluorescence microscopy. Lastly, a direct role for CbPilA in cell adhesion was demonstrated using recombinant CbPilA or anti-CbPilA antibodies to reduce cell adhesion to xylan and crystalline cellulose up to 4.5- and 2-fold, respectively. Based on these observations, we propose that CbPilA and, by extension, the T4P play a role in cell attachment to plant biomass. Most microorganisms are capable of attaching to surfaces in order to persist in their environment. Type IV (T4) pili produced by certain mesophilic promote adherence; however, a role for T4 pili encoded by thermophilic members of this phylum has yet to be demonstrated. Prior comparative genomics analyses identified a T4 pilus locus possessed by an extremely thermophilic genus within the Here, we demonstrate that attachment to plant biomass-related carbohydrates by strongly cellulolytic is mediated by T4 pilins. Surprisingly, xylan but not cellulose induced expression of the major T4 pilin. Regardless, the T4 pilin interacts with both polysaccharides at high temperatures and is located to the cell surface, where it is directly involved in attachment. Adherence to polysaccharides is likely key to survival in environments where carbon sources are limiting, allowing to compete against other plant-degrading microorganisms.