| Summary: | <p>Abstract</p> <p>Background</p> <p>The C10 family of cysteine proteases includes enzymes that contribute to the virulence of bacterial pathogens, such as SpeB in <it>Streptococcus pyogenes</it>. The presence of homologues of cysteine protease genes in human commensal organisms has not been examined. <it>Bacteroides fragilis </it>is a member of the dominant <it>Bacteroidetes </it>phylum of the human intestinal microbiota, and is a significant opportunistic pathogen.</p> <p>Results</p> <p>Four homologues of the streptococcal virulence factor SpeB were identified in the <it>B. fragilis </it>genome. These four protease genes, two were directly contiguous to open reading frames predicted to encode staphostatin-like inhibitors, with which the protease genes were co-transcribed. Two of these protease genes are unique to <it>B. fragilis </it>638R and are associated with two large genomic insertions. Gene annotation indicated that one of these insertions was a conjugative Tn-like element and the other was a prophage-like element, which was shown to be capable of excision. Homologues of the <it>B. fragilis </it>C10 protease genes were present in a panel of clinical isolates, and in DNA extracted from normal human faecal microbiota.</p> <p>Conclusions</p> <p>This study suggests a mechanism for the evolution and dissemination of an important class of protease in major members of the normal human microbiota.</p>
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