Preemption of Streptococcus mutans 10449S colonization by its mutant 805

Oral infection of rats with Streptococcus mutans mutant 805 was used to prevent the establishment of its highly virulent wild-type progenitor NCTC 10449S. The dose of wild-type cells required to colonize 100% of the specific pathogen-free Osborne-Mendel rats (21 to 43 days old) consuming caries test diet 2000 was greater than 4 x 10(5) but less than 4 x 10(6) cells. Therefore, the latter dose was used to challenge rats which had already been colonized by an oral dose of about 6 x 10(8) cells of mutant 805. This prior infection by 805 either completely protected rats from subsequent colonization by wild-type cells or greatly delayed and diminished their emergence. Rats in which wild-type cells became established showed much lower percentages of wild-type cells in their total recoverable floras than did rats that were not first infected by the mutant. Large doses of mutant 805, however, did not displace wild-type cells from rats once it became established. There was no evidence of reversion of the mutant, which is defective in intracellular polysaccharide synthesis and hence is less virulent than wild-type cells. The data indicate that the S. mutans cell which first colonizes rats gains the strongest ecological position and is difficult to displace. Also, they suggest the possible prophylactic utility of infection by this mutant of S. mutans.