cavity is home to many different species of streptococci and it is not
surprising, considering they share the same habitat, that they have
many features in common. This can pose problems in identification and
in sorting out the relationship between the various species. The application
of a wide range of biochemical tests (particularly for sugar fermentation
and glycosidase enzymes) and, more recently, the analysis of ribosomal
RNA sequences has led to general agreement about the species boundaries
and 19 distinct species are recognised. Nevertheless, much remains to
be resolved as new discoveries are made about the exchange of genetic
material between bacteria, which leads to mosaic chromosomes.
of oral streptococci is closely related to S. mutans and is referred
to as the 'mutans group' or the 'mutans streptococci'. Note that the
species name is written in italics while the group name is not.
is carried by virtually everyone and the only other species common in
man is S. sobrinus, carried by between 8 and 35% of people in
different countries. Although S. mutans and S. sobrinus
can be distinguished by appropriate laboratory tests, these are expensive
and time-consuming so it is not always practicable to identify down
to the species level in large-scale epidemiological studies. Nor has
anyone managed to invent a selective medium that would allow us to look
for the presence of a single species, in saliva samples for example.
As a consequence, most work on the relationship of bacteria to caries
has lumped the two species together as the mutans streptococci (MS).
Because of its greater prevalence, most of the isolates will in fact
be S. mutans and some authors erroneously use the single name
S. mutans even though they could not tell if is S. sobrinus
was also present. Of course, all older papers published before the mid-80s
refer to S. mutans because S. sobrinus was not officially
selective media that are widely used for isolating caries-related streptococci
are based on Mitis-Salivarius agar and TYC agar to which the antibiotic
Bacitracin is added (TYCSB). This suppresses the growth of most species
but allows S. mutans and S. sobrinus to grow. The inclusion
of sucrose leads to the formation of glucans and distinctive colony
appearance that aids identification. Diagnostic kits designed for use
in the dental clinic are also based on similar selective media so note
that they are measuring total MS, not just S. mutans.