Distribution of oral streptococci highly resistant to amoxicillin in dental plaque specimens from Japanese children and adolescents

Oral streptococci are major pathogens of infective endocarditis. Prophylactic antibiotics are commonly given to subjects with certain kinds of heart disorders when invasive dental treatments are performed, with amoxicillin (AMPC) being widely used for this purpose. However, there is little information regarding AMPC-resistant oral streptococci. Here, a total of 344 dental plaque specimens collected from 253 healthy Japanese children, adolescents and young adults (aged 2–22 years) were diluted and streaked onto culture medium containing high-dose AMPC. The MICs for the isolated strains were evaluated using a macrodilution broth method described by the Clinical and Laboratory Standards Institute. Bacterial DNA was extracted from each strain and the entire sequences of the 16S rRNA gene were compared with those in GenBank to identify the species. The results showed that strains with AMPC MICs >16 µg ml−1 were isolated from 18 specimens from 14 patients. Analyses of the 16S rRNA gene sequences of these strains identified them as major oral streptococcal species, including Streptococcus oralis and Streptococcus mitis. These findings indicate that oral streptococci with elevated MICs for AMPC exist in certain small populations of healthy children, and highlight the need for further studies to determine risk factors that lead to the appearance of such strains.