Absence of a cysteine protease effect on bacterial virulence in two murinemodels of human invasive group A streptococcal infection

The cysteine protease of group A streptococci has been suggested to contrib ute to the pathogenesis of invasive infection through degradation of host t issue, activation of the host inflammatory response, release of protective molecules from the bacterial cell surface, or other mechanisms. However, st udies of the effects on virulence of inactivating the cysteine protease gen e speB have yielded conflicting results. In some reports, a speB mutant was relatively avirulent in mouse models of invasive infection whereas little or no attenuation of virulence was observed in other studies of similar mut ant strains. Possible reasons for these discordant results include differen ces in the streptococcal strains from which the speB mutants were derived, differences in the infection models employed, or unintended effects on anot her virulence determinant(s) that arose during the derivation of a speB mut ant. We attempted to clarify these issues by characterizing the phenotypic properties and relative virulence in mice of two speB mutant strains, both derived from wild-type strain AM3: speB mutant AM3speB, which has been show n to be markedly attenuated in virulence in mice after intraperitoneal or s ubcutaneous challenge, and AM3speB Omega, a new mutant strain derived for t his investigation. Both mutant strains were negative for protease activity, as expected, and both produced wild-type amounts of type 3 M protein and s treptolysin O. However, AM3speB produced significantly less cell-associated hyaluronic acid capsule than did parent strain AM3 or strain AM3speB Omega . Compared to wild-type strain AM3, AM3speB was more sensitive to opsonopha gocytic killing in vitro and was significantly less virulent in mice after intraperitoneal challenge. By contrast, AM3speB Omega was fully resistant t o phagocytosis and did not differ significantly from the wild-type strain i n mouse virulence after an intraperitoneal or subcutaneous challenge. We co ncluded that previous reports attributing loss of virulence in strain AM3sp eB to inactivation of speB are in error. Within the limitations of the mode ls used, we found no effect of cysteine protease on invasive streptococcal infection.