Streptococcus mutans and the mutans streptococci

The oral 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.

One group 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.

S. mutans 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 recognised then.

The two 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.