The present studies were performed for quantitative and qualitative characterization of oral mutans streptococci. For detection and isolation of mutans streptococci, saliva and/or plaque samples from 97 children, 146 adolescents and 159 adults were included. The subjects, participating in two cross-sectional studies and in two longitudinal studies, were also examined clinically for decayed, missing and filled teeth. Further, for qualitative analysis, bacterial isolates obtained from 64 individuals were subjected to genotyping and/or to phenotyping, in order to study the clonal diversity and site-specific colonization of intra-individual strains, and certain virulence factors of the organism. Phenotypical characterization included serotyping and determination of chlorhexidine (CHX) susceptibility, glucosyltransferase activity and bacteriocin (mutacin) activity of the isolates. In addition, the effects of periodic low-concentration CHX-NaF gel brushing in an instructed trial on salivary counts and transmission of mutans streptococci from mother to child was studied. Genotyping was performed by ribotyping 191 of the obtained isolates, and 598 isolates were fingerprinted by the AP-PCR method. Serotyping was performed for 661 isolates by Ouchterlony immunodiffusion using rabbit antisera. CHX susceptibility of 379 isolates was tested by the agar dilution method, and for testing of glucosyltransferase activity, using monoclonal antibodies in a semiquantitative cross-dot assay, 44 isolates were included. Mutacin production of 145 isolates was tested by the stab culture method, including 38 clinical mutans streptococcal isolates from our IDH (Institute of Dentistry, Helsinki) collection .
The main findings were as follows:
In cross-sectional studies of adolescents, when mutans streptococci are not detected by chair-side methods or by conventional agar plate culture methods of plaque and saliva, mutans streptococci may either be absent or present in numbers under the detection level. In this study, a longitudinal evaluation of adolescents with no mutans streptococci initially detected, revealed that only a few subjects remained culture-negative for the organism during follow-up.
The number of clonal types detected within a subject was from one to four, with saliva revealing 80% of the genotypes present. The isolates from related subjects were often similar, but from unrelated subjects they were, in almost all cases, distinguished by genotyping. The children with nursing caries harboured more types of mutans streptococci than caries-free children. The colonization of genotypes tended to be site-specific, implying that sampling for intra-individual mutans streptococcal genotypes should include sampling of multiple sites.
The CHX susceptibility of mutans streptococcal isolates was ribotype-specific, and the susceptibility exhibited stability at follow-up. S. sobrinus isolates were somewhat more resistant to CHX than S. mutans isolates. In the past, no CHX resistance in mutans streptococci has appeared. In this study, periodic low-concentration CHX-NaF gel brushing did not significantly affect salivary counts of mutans streptococci, CHX susceptibility of isolates or transmission of isolates from mother to child.
Glucosyltransferase activity and mutacin production by mutans streptococcal isolates were also ribotype-specific. The mutacin activity of isolates was fairly stable, and mutacin production of S. mutans isolates was shown to have an impact on the probability of strain transmission from mothers to their children.
These results support the original study hypothesis that mutans streptococcal strains may differ in their phenotypic properties; CHX susceptibility, glucosyltransferase activity and mutacin production were strain-specific. Moreover, the results imply that S. mutans strains that produce increased amounts of mutacin may be more easily transmitted from mothers to their young children. Therefore, the role of mutans streptococci in human dental caries may be characterized by not only the quantity of the bacteria, but also the quality of strain(s) colonizing.