Isolation and characterization of type III group B streptococcal mutants defective in biosynthesis of the type-specific antigen

Four classes of mutants of type III group B streptococcus were isolated by serial subculture of the wild-type strain in the presence of type III-specific rabbit antiserum. Class I mutants no longer synthesized sialic acid but still elaborated the core antigen. Class II mutants maintained the ability to synthesize sialic acid but could not attach it to the core antigen. Class III mutants did not produce the core antigen but still synthesized intracellular sialic acid. Class IV mutants synthesized the complete antigen; however, only ~4% of the antigen synthesized was found associated with the cell wall peptidoglycan (in the wild-type strain > 85% of the antigen synthesized is covalently attached to the cell wall peptidoglycan), whereas >90% of the antigen was secreted into the growth medium. Production of other components (CAMP factor, group B antigen, β-hemolysin, neuraminidase) by these mutants appeared similar to those of the wild-type strain. Mouse lethality studies of these strains indicated that all four classes have >3 log₁₀-higher 50% lethal dose values than that of the wild-type strain. To understand the basis for this variation, the invasive ability of the wild-type strain and the sialic acid-deficient mutant strain M-10 (class I) was examined. Mice received 10⁵ CFU of each organism; they were then sacrificed at various times postinoculation, and viable group B streptococci from different organs were enumerated. Mice were able to clear M-10 more efficiently, with >80% of M-10 cells being phagocytized by macrophages within 1 h, whereas the wild-type strain was able to evade phagocytic killing and disseminate to other tissues. These data, therefore, strongly indicate that the sialic acid moiety greatly enhances the virulence of the type III antigen. In addition, the level of cell-associated type-specific antigen appears to contribute significantly to the pathogenicity of the organism.