Contribution of Streptococcus mutans PAc to Bacteremia Occurrence

Objectives: Streptococcus mutans, known to be a major cariogenic bacterium, is occasionally isolated from the blood of patients with bacteremia and infective endocarditis. The purpose of the present study is to determine the surface protein of S. mutans associated with bacteremia occurrence and investigate the alteration of expression of the corresponding protein in clinical oral isolates. Methods: Glucosyltransferase (GTF) B-, C-, and D-, glucan-binding protein (Gbp) A-, B-, and C-, and protein antigen c (PAc)-defective mutant strains of S. mutans MT8148 were constructed by insertional inactivation of the respective gene with an antibiotics resistant cassette. Susceptibility to phagocytosis was analyzed by interactions of the tested bacteria with human polymorphonuclear leukocytes. Then, the expression of PAc in 100 clinical isolates taken from 100 Japanese children and adolescents was analyzed by Western blotting using anti-recombinant PAc antiserum. Results: The PAc-defective mutant strain showed the lowest rate of phgocytosis among all the tested major surface proteins of GTF and Gbp. Further, 4 of 100 strains showed no expression of PAc, each of which exhibited a significantly lower phagocytosis rate than that of 25 clinical strains randomly selected (P<0.01). In addition, 3 of the 100 strains, were shown to possess a lower molecular weight PAc and had a significantly lower rate of phagocytosis than the 25 reference strains (P<0.05). Conclusion: These results indicate that S. mutans PAc may be associated with the phagocytosis capabilities of human polymorphonuclear leukocytes. Further, approximately 7% of S. mutans clinical isolates with altered PAc expression may be possible risk factors for bacteremia occurrence. This study was a part of 21st Century COE entitled "Origination of Frontier BioDentistry" at Osaka University Graduate School of Dentistry supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.