Adaptive Changes in a Strain of Streptococcus mutans during Colonisation of the Human Oral Cavity

When a bacterial strain undergoes a change in environment, its ability to colonise a new niche is optimised over time by the process of natural selection. In this study, a readily identifiable strain of Streptococcus mutans was recovered from supragingival dental plaque of human subjects 3 y after implantation. Fifty fresh isolates were compared to the original laboratory strain that was used to infect these subjects. It was found that the fresh isolates adhered less well than the laboratory strain to saliva-coated hydroxyapatite. The fresh isolates also produced more acetoin and less lactic acid, and had a greater cell yield during aerobic growth in glucose-containing medium. During growth in the presence of sucrose, the fresh isolates produced slightly greater amounts of plaque than the laboratory strain. No differences were observed between fresh and laboratory adapted strains with regard to bacteriocin production, antigenic composition, nutrient requirements, growth rates, or resistance to tetracycline. These results indicate that environmental factors operating in the oral cavity of human subjects select variants of S. mutans that are better adapted to proliferate in vivo. Identification of these factors is important to our general understanding of the ecology of S. mutans and to the development of therapeutic methods. KEY WORDS-StreptOCOCCUS mutans; Adaptation; Implantation