PHASE-SHIFT OF POLYSACCHARIDE CAPSULE EXPRESSION IN GROUP-B STREPTOCOCCI, TYPE-III

The type-specific polysaccharide capsule is an important virulence det erminant in group B streptococci (GBS). The previously described inver se relationship between the buoyant density of a GBS-isolate and the c apsular thickness was used to assess the frequencies of polysaccharide capsular phase-shift in clinical GBS, type III strains. Shift from in termediate 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 lo w 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. Reve rsion frequencies were in some isolates as high as 10(-1). Phase-shift frequencies differed more than a thousand-fold between compared strai ns. Differences in phenotypic shift between strains were validated usi ng flow cytometry. Possible modulation of capsule expression by change s in culture conditions was assessed. Variation of temperature, oxygen -tension, and presence of human serum did not affect capsule expressio n. However, growth at pH below 5.5 decreased the amount of capsule bou nd native type III polysaccharide, probably through phenotypic modific ation rather than genetic shift. IS861, an insertion sequence which ha s been proposed a regulatory function on the GBS capsule expression, w as found in multiple copies in the isolates investigated. No differenc es in copy numberer location of IS861 between the differently encapsul ated phenotypes were found.