subtilis strains such as strain FT-3 (Morita et al., 1979).

Although specific roles for these polysaccharides have not been proposed, they are known to be comprised of glucose, galactose, fucose, glucuronic acid and O-acetyl groups in an approximate molar ratio of 2 : 2 : 1 : 1 : 1.5 (Morita et al., 1979). Information regarding the genes encoding the proteins that make these exopolysaccharides is also limited. yhxB is a gene related to the synthesis of an uncharacterized exopolysaccharide component of the B. subtilis biofilm matrix and putatively encodes an α-phosphoglucomutase and/or phosphomannomutase (Branda et al., 2004). In B. subtilis 3610, a deletion in yhxB is responsible for the production of a fragile surface pellicle when grown in a liquid culture and flat undifferentiated colonies when grown on CDK activity solid media. On the contrary, the B. subtilis wild-type strain shows a robust pellicle in liquid culture and colonies on

plates with web-like structures (i.e. bundled structures). Other genes important in matrix structure and biofilm architecture include the 16 genes of the eps operon (yveK-yvfF) involved in polysaccharide biosynthesis, modification and export (Branda ERK inhibitor et al., 2001). From sequence comparisons, two genes belonging to the eps operon, named epsG (yveQ) and epsH (yveR), may be involved in the synthesis of exopolysaccharides. epsG encodes a protein that is presumably involved in EPS polymerization, while epsH encodes a glycosyl-transferase (Branda et al., 2001). eps mutants in B. subtilis 3610 show a reduction in the carbohydrate content and complexity of biofilm pellicle (Branda et al., 2006). Blair et al. (2008) have pheromone recently demonstrated that another member of this eps operon,

the EpsE protein, is an inhibitor of cell motility. Despite the extensive study of the eps operon and its role, the structure and function of the polysaccharides resulting from the expression of these genes remain unknown. Characterization of this polysaccharide and its regulation awaits further investigations. The second category of EPS secreted by B. subtilis includes a polymer, which plays a role in the sorption of ions and/or charged molecules. Poly-γ-glutamate (γ-PGA) produced by B. subtilis strain IFO3336 is a well-characterized anionic, nontoxic and biodegradable viscous polymer of d- and l-monomers with a molecular mass of over 10 000 kDa. The γ-PGA of B. subtilis (natto) is composed 50–80% of d- and 20–50% of l-glutamate (Ashiuchi et al., 1999; Morikawa et al., 2006; Inbaraj et al., 2008). γ-PGA is synthesized by several Bacillus species, especially wild-type isolates, including B. subtilis strains IFO3336, IFO3335, TAM-4, F-2-01, B-1 (mucoid-type colonies), ZJU-7, B. subtilis (natto) and Bacillus anthracis (Kubota et al., 1993; Kunioka, 1995; Ito et al., 1996; Shi et al., 2006). The pgsBCA genes are responsible for the synthesis of γ-PGA.