Sm. Colby et al., IDENTIFICATION AND GENETIC-CHARACTERIZATION OF MELIBIOSE-NEGATIVE ISOLATES OF STREPTOCOCCUS-MUTANS, Caries research, 29(5), 1995, pp. 407-412
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.