Variations of Bacteroides fragilis with in vitro passage: presence of an outer membrane-associated glycan and loss of capsular antigen.

The amount of capsular antigen produced by Bacteroides fragilis appears to decrease significantly with in vitro passage on blood agar plates of strain 23745. This decrease in the quantity of isolable capsule is associated with the emergence of a small transparent colony type variant. The small colony type strain also has a glycogenlike material associated with its outer membrane. This glycogen is found intracellularly but is not associated with the outer membrane on the mucoid colony variant that predominates after passage in animals--the large colony type. No concomitant alteration in outer membrane proteins or lipopolysaccharides is associated with this colonial transformation. Electron micrographs, with either ruthenium red staining or indirect ferritin antibody staining of the capsular antigen, confirm the relative loss of capsule with in vitro passage. Thus, care must be taken during in vitro studies of the antigenic structure of B. fragilis to define the degree of passage of the strain.