Phase-shift of polysaccharide capsule expression in group B streptococci, type III.

The type-specific polysaccharide capsule is an important virulence determinant in group B streptococci (GBS). The previously described inverse relationship between the buoyant density of a GBS-isolate and the capsular thickness was used to assess the frequencies of polysaccharide capsular phase-shift in clinical GBS, type III strains. Shift from intermediate density (ID) of parental strains, to high density (HD), i.e. shift from intermediate capsule thickness to poor encapsulation, was found to range from 1.2 x 10(-3) to 4.8 x 10(-6). Shift from ID to low density (LD), i.e. shift to abundant encapsulation, ranged from 1.9 x 10(-4) to 1.1 x 10(-7). Shifts were reversible in all cases, either directly (HD-->LD or vice versa) or through intermediate forms. Reversion frequencies were in some isolates as high as 10(-1). Phase-shift frequencies differed more than a thousand-fold between compared strains. Differences in phenotypic shift between strains were validated using flow cytometry. Possible modulation of capsule expression by changes in culture conditions was assessed. Variation of temperature, oxygen-tension, and presence of human serum did not affect capsule expression. However, growth at pH below 5.5 decreased the amount of capsule bound native type III polysaccharide, probably through phenotypic modification rather than genetic shift. IS861, an insertion sequence which has been proposed a regulatory function on the GBS capsule expression, was found in multiple copies in the isolates investigated. No differences in copy number or location of IS861 between the differently encapsulated phenotypes were found.