Distribution of S-layers on the surface of Bacillus cereus strains: phylogenetic origin and ecological pressure

Bacillus anthracis, Bacillus cereus and Bacillus thuringiensis have been de scribed as members of the Bacillus cereus group but are, in fact, one speci es. B. anthracis is a mammal pathogen, B. thuringtensis an entomopathogen a nd B. cereus a ubiquitous soil bacterium and an occasional human pathogen. In two clinical isolates of B. cereus, in some B. thuringiensis strains and in B. anthracis, an S-layer has been described. We investigated how the S- layer is distributed in B, cereus, and whether phylogeny or ecology could e xplain its presence on the surface of some but not all strains. We first de veloped a simple biochemical assay to test for the presence of the S-layer. We then used the assay with 51 strains of known genetic relationship: 26 g enetically diverse B. cereus and 25 non-B. anthracis of the B. anthracis cl uster. When present, the genetic organization of the S-layer locus was anal ysed further. It was identical in B. cereus and B. anthracis. Nineteen stra ins harboured an S-layer, 16 of which belonged to the B. anthracis cluster. All 19 were B. cereus clinical isolates or B. thuringiensis, except for on e soil and one dairy strain. These findings suggest a common phylogenetic o rigin for the S-layer at the surface of B. cereus strains and, presumably, ecological pressure on its maintenance.