β-Glucanase Activity of the Oral Bacterium Tannerella forsythia Contributes to the Growth of a Partner Species, Fusobacterium nucleatum, in Cobiofilms

Abstract

Tannerella forsythia and Fusobacterium nucleatum are dental plaque bacteria implicated in the development of periodontitis. These two species have been shown to form synergistic biofilms and have been found to be closely associated in dental plaque biofilms. A number of genetic loci for TonB-dependent membrane receptors (TDR) for glycan acquisition, with many existing in association with genes coding for enzymes involved in the breakdown of complex glycans, have been identified in T. forsythia. In this study, we focused on a locus, BFO_0186-BFO_0188, that codes for a predicted TDR-SusD transporter along with a putative β-glucan hydrolyzing enzyme (BFO_0186). This operon is located immediately downstream of a 2-gene operon that codes for a putative stress-responsive extracytoplasmic function (ECF) sigma factor and an anti-sigma factor. Here, we show that BFO_0186 expresses a β-glucanase that cleaves glucans with β-1,6 and β-1,3 linkages. Furthermore, the BFO_0186-BFO_0188 locus is upregulated, with an induction of β-glucanase activity, in cobiofilms of T. forsythia and F. nucleatum. The β-glucanase activity in mixed biofilms in turn leads to an enhanced hydrolysis of β-glucans and release of glucose monomers and oligomers as nutrients for F. nucleatum. In summary, our study highlights the role of T. forsythia β-glucanase expressed by the asaccharolytic oral bacterium T. forsythia in the development of T. forsythia-F. nucleatum mixed species biofilms, and suggest that dietary β-glucans might contribute in plaque development and periodontal disease pathogenesis. IMPORTANCE The development of dental plaque biofilm is a complex process in which metabolic, chemical and physical interactions between bacteria take a central role. Previous studies have shown that the dental pathogens T. forsythia and F. nucleatum form synergistic biofilms and are closely associated in human dental plaque. In this study, we show that β-glucanase from the periodontal pathogen T. forsythia plays a role in the formation of T. forsythia-F. nucleatum cobiofilms by hydrolyzing β-glucans to glucose as a nutrient. We also unveiled that the expression of T. forsythia β-glucanase is induced in response to F. nucleatum sensing. This study highlights the involvement of β-glucanase activity in the development of T. forsythia-F. nucleatum biofilms and suggests that intake of dietary β-glucans might be a contributing risk factor in plaque development and periodontal disease pathogenesis.