Biofilm formation in the life cycle of the cellulolytic actinomycete Thermobifida fusca

Actinomycetes have been used with enormous success in industrial processes; however, little is known about biofilm formation by these filamentous microbes, or community development on insoluble substrates such as cellulose. We hypothesized that biofilm formation is a general strategy used by actinomycetes in the degradation of cellulose, and that it may serve as a means for these microbes to secure nutrients and persist in their environments. The objective of this study was to examine biofilm production by Thermobifida fusca , an actinomycete that rapidly degrades cellulose by means of a well-characterized extracellular cellulase system. Thermobifida fusca cells grew as biofilms attached to both nutritive (e.g. dialysis tubing membrane) and non-nutritive surfaces. Dialysis tubing was colonized by T. fusca aleuriospores but not by mycelial pellets, except when mycelial pellets were disrupted by sonication. Microscopic examination of surface-attached growth revealed structures characteristic of biofilms, with cells embedded in fibrous material suggestive of an extracellular polymeric matrix. Concanavalin A bound to the extracellular polymeric substance of biofilms and mycelial pellets, indicating alpha-linked d -mannosyl and/or alpha-linked d -glucosyl residues. The carbohydrate content of both biofilms and mycelial pellets increased during growth. Also, DNase I inhibited biofilm production, suggesting a role for extracellular DNA in T. fusca biofilm development. Cellulose degradation and expression of celE (encoding endoglucanase E5) was similar for T. fusca biofilms and mycelial pellets. Results of this study indicate that, in the life cycle of this actinomycete, cellulose is specifically colonized by aleuriospores, which germinate, grow and degrade cellulose, ultimately developing into biofilms encased in a carbohydrate-containing exopolymeric matrix, a hallmark of biofilm production.