Objective To investigate the genotypic diversity of Actinomyces naeslundii in aged people and the elationship between the genotypes of Actinomyces naeslundii and root caries. Methods According to the inclusion riteria, 20 aged people with root caries and 20 without root caries were chosen into two groups for this study. Two ites were chosen in subjects with root caries: One site was the exposed sound root surface, and the other site was he root caries. In subjects without root caries the sampling site of root surface was exposed. Bacteria were cultured nd then identified. Repetitive extragenic palindromic sequence -based polymerase chain reaction（REP -PCR） was sed to analyze the genotypic diversity of the Actinomyces naeslundii clinic isolates. Results 299 strains were isolated rom the groups, 156 strains were chosen to analyze, belonged to 61 different genotypes. At the site of sound oot surface in the subjects with root caries, there were 57 strains with 25 different patterns. At the site of root aries and of sound root surface in subjects without caries, there were 34 strains with 25 different patterns and 65 trains with 26 different patterns respectively. There was the genotypic diversity within Actinomyces naeslundii. here was significant difference in the genotypes in every individual site. Conclusion Many different genotypes of ctinomyces naeslundii concerned with occurrence of root caries.
