A Predominant Cariogenic Genotype of Streptococcus mutans in Schoolchildren of Mexico City: A Follow-Up Study

Streptococcus mutans is considered the primary bacterial species closely associated with the etiology of dental caries in humans. Recent studies suggest an association between caries and the genetic diversity of S. mutans. The aim of this study was to evaluate the genotypic diversity of S. mutans in schoolchildren, its stability in a one-year follow-up study, and its association with dental caries. We studied 25 schoolchildren. Representative S. mutans colonies were isolated from the dental plaque of each child, grown on mitis-salivarius-bacitracin agar, and inoculated in trypticase soy broth. We performed 16S rRNA gene PCR-RFLP analysis on S. mutans isolates. Dental caries in deciduous and permanent surfaces was scored according to the WHO criteria. After 12 months, the caries incidence, S. mutans count, and genotypic diversity were compared. We grouped samples to observe similarities using cluster analysis. A similarity coefficient of > 95% was considered for defining the genotypes. At baseline, we proved the genotypic diversity of S. mutans with five different genotypes. Caries scores were higher in children with genotype A (dmfs 5.4 vs. 3.4). The genotypic diversity of S. mutans in the schoolchildren decreased after one year, with the predominance of genotype A (52% to 92%) that was also associated with high bacterial counts (P = 0.0063). This study supports that there are changes in the genotype of S. mutans over time, and the more cariogenic genotype is more stable than the others.