Genotypic heterogeneity of Streptococcus oralis and distinct aciduric subpopulations in human dental plaque

Abstract

The genotypic heterogeneity of Streptococcus oralis isolated from the oral cavity was investigated using repetitive extragenic palindromic PCR. Unrelated subjects harbored unique genotypes, with numerous genotypes being isolated from an individual. S. oralis is the predominant aciduric bacterium isolated from noncarious tooth sites. Genotypic comparison of the aciduric populations isolated at pH 5.2 with those isolated from mitis-salivarius agar (MSA) (pH 7.0) indicated that the aciduric populations were genotypically distinct in the majority of subjects (chi(2) = 13.09; P = 0.0031). Neither the aciduric nor the MSA-isolated strains were stable, with no strains isolated at baseline being isolated 4 or 12 weeks later in the majority of subjects. The basis of this instability is unknown but is similar to that reported for Streptococcus mitis. Examination of S. oralis strains isolated from cohabiting couples demonstrated that in three of five couples, genotypically identical strains were isolated from both partners and this was confirmed by using Salmonella enteritidis repetitive element PCR and enterobacterial PCR typing. These data provide further evidence of the physiological and genotypic heterogeneity of non-mutans streptococci. The demonstration of distinct aciduric populations of S. oralis implies that the role of these and other non-mutans streptococci in the caries process requires reevaluation.

FIG. 1

REP-PCR patterns of S. oralis strains isolated from interproximal plaque samples using (a) MSA and (b) MPN methods from two subjects. Lanes 1, 9, and 18 contain molecular size markers.

FIG. 2

Dendrogram illustrating genotypic relationships between S. oralis strains isolated from the saliva of a single subject. Strain identification is rendered time (0, 1, or 6 months) and strain number. Isolates 1, 2, 3, 7, and 11 from the 1-month sample were indistinguishable from isolates from the 6-month sample. Individual REP-PCR amplicons were marked, and the individual bands were analyzed by using the Dice coefficient and clustering using the UPGMA method.

FIG. 3

Representative dendrograms illustrating genotypic relationships between S. oralis strains isolated from cohabiting couples A and B with partners identified as 1 and 2. In each couple, the first number indicates the individual partner, MSA and ACID indicate the culture method used to isolate the strains, and the last number indicates the isolate number. In couple A, strain 2 ACID 4 was identical to strains from the other partner, and in couple B, strains 2 ACID 6, 10, and 13 were identical to strains from the other partner. For comparison, individual REP-PCR amplicons were marked, and the individual bands were analyzed by using the Dice coefficient and clustering using the UPGMA method.