Effects on virulence of mutations in a locus essential for hyaluronic acid capsule expression in group A streptococci

Abstract

Mucoid or highly encapsulated strains of group A streptococci have been associated both with unusually severe infections and with acute rheumatic fever. Previously, we described an acapsular mutant, TX4, derived from a mucoid M-type 18 strain of a group A streptococcus by transposon mutagenesis (M. R. Wessels, A. E. Moses, J. B. Goldberg, and T. J. DiCesare, Proc. Natl. Acad. Sci. USA 88:8317-8321, 1991). We now report studies further characterizing strain TX4 as well as an additional acapsular mutant, TX72. Strain TX4 was found to contain a 9.5-kb deletion of chromosomal DNA adjacent to the site of transposon Tn916 insertion. Cloned chromosomal DNA from TX4 flanking the transposon insertion site was used as a probe to demonstrate the presence of homologous regions in 11 of 11 wild-type group A streptococcal strains of various M protein types. A second acapsular mutant, TX72, had a single transposon insertion and had no apparent deletion of chromosomal DNA. The Tn916 insertion in TX72 was mapped to the hasA locus (encoding hyaluronate synthase), which lies within the chromosomal region deleted in TX4. Strain TX72 was avirulent in mice and sensitive to phagocytic killing in vitro. Transduction of either the insertion-deletion mutation from TX4 or the simple insertion mutation from TX72 to a type 24 group A streptococcus strain also resulted in loss of capsule expression, demonstrating that a homologous region of the chromosome controls capsule expression in another serotype of group A streptococci. We conclude that the hyaluronic acid capsule plays an important role in virulence and that a region of the chromosome essential for capsular polysaccharide expression is conserved among diverse group A streptococcal strains.