Competition between Streptococcus mutans and Lactobacillus casei in mixed continuous culture

Abstract

In natural oral ecosystems, succession of Streptococcus mutans and Lactobacillus species occurs in bacterial communities which can be associated with the development of carious lesions. The objective of this study was to explore the interaction between 2 strains of S. mutans (SF2452) and Lactobacillus casei (RB1014) when grown together in continuous culture under conditions of varying environmental pH and in the presence of fluoride. When the strains, isolated from approximal carious lesions, were grown initially at a dilution rate of 0.1 h^-1 under a glucose limitation at pH 7.0, a stable mixed community developed (Days 5–10) with S. mutans as the dominant population. When the pH control was removed (Day 10), the binary culture lowered the pH to 5.1 causing a small reduction in the numbers of S. mutans and allowing L. casei to predominate. A more dramatic effect was seen when the environmental pH was rapidly lowered to 4.8 by the addition of acid. In this case, the S. mutans population was significantly reduced and the L. casei population rose to the level previously held by S. mutans. Re-adjustment of the pH to 7.0 resulted in a re-establishment of the population ratios prior to the addition of acid. The possibility that S. mutans was capable of aciduric adaptation during a biologically-generated pH reduction was examined by mixing cultures of both organisms after each had been grown to steady slate at pH 5.5 in separate chemostats. In this case S. mutans maintained its population and was dominant in the community at pH 5.5 indicating that adaptation by S. mutans had given it an ecological advantage over L. casei at this pH. The addition of fluoride (20 mM) to the mixed culture favored L. casei since the growth of S. mutans was greatly inhibited al this fluoride level. Nevertheless S. mutans was able to survive at low levels in the chemostat, presumably in surface films, for at least 5 days, since removal of fluoride from (he medium resulted in the re-establishment of S. mutans in a competitive position with L. casei. These experiments indicate that S. mutans possesses adaptive and survival characteristics which would be of considerable advantage in an ecological system such as dental plaque.