IDENTIFICATION AND GENETIC-CHARACTERIZATION OF MELIBIOSE-NEGATIVE ISOLATES OF STREPTOCOCCUS-MUTANS

Streptococcus mutans is frequently identified on the basis of phenotyp ic characteristics such as the ability to ferment carbohydrates. The u sefulness of some of these identification tests may be limited in the case of isolates which are atypical with regard to their fermentation properties. We previously identified isolates of S. mutans which were unable to ferment melibiose, a characteristic which is included in som e typing schemes. In all of these isolates there was a large chromosom al deletion which included the multiple sugar metabolism (msm) operon which encodes several genes involved in the uptake and metabolism of a number of sugars including melibiose. In the present study, sugar fer mentation tests, ribotyping, colony hybridisation with DNA probes and polymerase chain reaction (PCR) were used to investigate the relatedne ss of these atypical isolates. The PCR and colony hybridisation proced ures were based on amplification and detection of two genes: the wapA gene which encodes a surface protein found in all S, mutans strains an d the gtfA gene which lies within the msm operon. The colony hybridisa tion and PCR results confirmed loss of the gtfA gene in the melibiose- negative isolates. Three new melibiose-negative isolates were also ide ntified, but in only 2 of these was the gtfA gene absent, the third di d not appear to have lost this region of the chromosome. Biotyping, as well as ribotyping based on an EcoRI digest of chromosomal DNA, revea led that the melibiose-negative isolates fell into a number of distinc t groups. The identification of an isolate which is unable to ferment melibiose but does not appear to have lost the msm operon indicates th at the melibiose-negative phenotype can arise from more than one type of genetic event.