The B. fragilis group constitute the most common opportunistic pathogens of the genus Bacteroides. These slim, pale-staining, capsulate, Gram-negative rods form colonies overnight on blood agar medium. The implication of fragility in the name is misleading, because they are actually among the hardier and more easily grown anaerobes. Most strains produce superoxide dismutase and are relatively tolerant to atmospheric oxygen. B. fragilis has surface pili and a capsule composed of a polymer of two polysaccharides. The LPS endotoxin in the B. fragilis outer membrane is less toxic than that of most other Gram-negative bacteria, possibly due to modification or absence of the lipid A portion.
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