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Variation in M protein production among Streptococcus pyogenes strains according to emm genotype

M protein is an important virulence determinant in Streptococcus pyogenes, but the amounts of M protein in various strains of the species remain to be elucidated. To assess the amount of M protein in strains of each emm genotype, dot blot analysis was performed on 141 clinically isolated strains. Among the cell membrane‐associated proteins, M protein was present in greater quantities in the emm1, 3, and 6 strains than in the other emm strains. In addition three strains, one each of the emm1, 3, and 6 types, showed prolific M protein production (M protein‐high producers). These three emm genotypes are frequently isolated in clinical practice. Sequencing of the csrRS gene, one of the two‐component signal transduction systems implicated in virulence, was performed on 25 strains bearing different amounts of M protein. CsrS mutations, in contrast to CsrR protein, were detected in 11 strains. The M protein‐high producer strain of emm1 type carried two amino acid substitutions, whereas the other three emm1 strains carried only one substitution each. The M protein‐high producer expressed its emm gene more strongly than the corresponding M protein‐low producer did according to TaqMan RT‐PCR. These observations suggest that the accumulation of amino acid substitutions in CsrS protein may contribute, at least in part, to the large amount of M protein production seen in several emm genotypes.