Initial acquisition and transmission of Streptococcus mutans from Japanese mothers to children

The purpose of this study was to determine the time-window of infection for Streptococcus mutans between mother and child in Japan and which factors affected colonization by this microorganism. After obtaining informed consent, saliva was collected from the mothers and their children. These samples were serially diluted and inoculated onto Mitis-Salivarius agar and Mitis-Salivarius bacitracin plates, and then cultured under anaerobic conditions at 37°C for 5 days. Isolated mutans streptococci were identified by conventional biochemical characteristics and species-specific polymerase chain reaction based on the dextranase gene. No mutans streptococci groups were isolated from 0–5-month-old children with no erupted teeth. No statistically significant difference was found in detection rates in S. mutans among mother's milk, bottled milk or mixture. Cross-sectional study revealed that detection rates of S. mutans in the saliva samples increased with number of erupted teeth. Pulse-field gel-electrophoresis patterns obtained from isolated S. mutans using SmaI from the mothers and their children indicated that colonization by S. mutans in some Japanese children resulted from vertical transmission. In strains in which vertical transmission was observed, the mutacin IV and Smb A and B genes were detected from 8 out of 12 isolates. These results suggest that the window of infection for S. mutans is at around 6–23 months, and that colonization by this microorganism is mainly affected by number of erupted teeth.