The organism, which the authors say is responsible for as many as 80 percent of all Bacteroides infections in humans, comprises a single circular chromosome containing around 5.2 million base pairs, which are predicted to encode 4,274 genes and a single plasmid.
Although B. fragilis accounts for between 4 percent and 13 percent of all microbes that exist in normal human feces, the organism is responsible for between 63 percent and 80 percent of all Bacteroides-related infections, according to the team, which was led by Julian Parkhill at the Sanger Institute.
By comparison, B. thetaiotaomicron, a related microbe, accounts for between 15 percent and 29 percent of all bacteria living in normal human feces, but is responsible for between 13 percent and 17 percent of Bacteroides infections, the authors note.
Writing in the March 4 issue of Science, the researchers report that "a single strain of B. fragilis may reversibly produce three different encapsulating surface structures: the large capsule and the small capsule, both visible by light microscopy, and an electron-dense layer visible by electron microscopy. In addition, reversible within-strain antigenic variation of multiple antigenically distinct high molecular mass polysaccharides and other components is evident."
"Before the advent of the genome sequencing program, the potential mechanisms generating this variation were unknown," the researchers write.
The team sequenced the non-enterotoxin-producing DNA homology group I B. fragilis, strain NCTC 9343. A paper published last year in Proceedings of the National Academies of Science reported the genome sequence of another B. fragilis strain.