Capsule of Streptococcus pyogenes is essential for delayed death of mice in a model of streptococcal toxic shock syndrome

We have previously reported a mouse model of severe group A streptococcal infection (Microbiol. Immunol. 45: 777-786, 2001). When we injected Streptococcus pyogenes strains intramuscularly, the mice suffered from acute phase of infection for a few days but recovered from the illness and gained body weight. These mice, however, began to die after 3 weeks of infection, which we called 'delayed death.' Bacterial strains isolated from organs of the dead mice showed thick capsules. We, therefore, constructed a hyaluronic acid capsule gene, hasA, knockout mutant by homologous recombination and the effect of capsule on the death was observed. hasA knockout strain did not cause delayed death, though it caused acute death at high doses of infection. According to this result, the capsule is a critical pathogenic factor for causing the delayed death in our mouse model.