Objective To analyze genotypic diversity of oral Streptococcus mutans（S.mutans） and find out horizon-tal transmission possibility of the microbe in day-nursery children. Methods The plaque samples were scratched with sterilized toothpicks from teeth of 32 day-nursery children aged between 3 and 4, then cultured on MSB plates. Clones with representative S.mutans-like were subcultured and identified to species level biochemically. AP-PCR fingerprinting was performed to distinguish genotypic diversity of those isolates. Then S.mutans isolated from different children with very similar amplicon profiles were examined by chromosomal DNA fingerprinting analysis. Results S.mutans were isolated in oral cavities of 78.1% children, 100% in caries and 69.6% in caries-free children. A total of 57 genotypes were identified by AP-PCR. More than one amplitypes were identified in 88% of the 25 children with S.mutans colonization. Two pair of children shared common genotypic S.mutans. Conclusion There is no evident relation between number of genotype detected and caries. The presence of matching genotypes of
S. mutans among day-nursery children suggests the horizontal transmission may exist.
