Contribution of cell surface protein antigen PAc of Streptococcus mutans to bacteremia

Streptococcus mutans, a major cariogenic bacterium, is occasionally isolated from the blood of patients with bacteremia and infective endocarditis. Mutant strains of S. mutans MT8148, defective in the major surface proteins glucosyltransferase (GTF) B-, C-, and D-, and protein antigen c (PAc), were constructed by insertional inactivation of each respective gene with an antibiotic resistant cassette. Susceptibility to phagocytosis was determined by analyses of interactions of the bacteria with human polymorphonuclear leukocytes, and the PAc-defective mutant strain (PD) showed the lowest rate of phagocytosis. Further, when PD and MT8148 were separately injected into the jugular veins of Sprague–Dawley rats, PD was recovered in significantly larger numbers and for a longer duration, and caused more severe systemic inflammation than MT8148, indicating that S. mutans PAc is associated with its systemic virulence in blood. Next, 100 S. mutans clinical isolates from 100 Japanese children and adolescents were analyzed by Western blotting using antisera raised against recombinant PAc, generated based on the pac sequence of MT8148. Four of the 100 strains showed no positive band and each exhibited a significantly lower phagocytosis rate than that of 25 randomly selected clinical strains (P < 0.01). In addition, three of the 100 strains possessed a lower molecular weight PAc and a significantly lower rate of phagocytosis than the 25 reference strains (P < 0.05). These results suggest that S. mutans PAc may be associated with phagocytosis susceptibility to human polymorphonuclear leukocytes, with approximately 7% of S. mutans clinical isolates possible high-risk strains for the development of bacteremia.