We studied bacteriocinogenies and antibiotic resistances of naturally occurring strains of Streptococcus mutans isolated from different sources.It was found that naturally occurring strains from humans and rats were distinguished by their bacteriocin susceptibilities and production.We had a few incidences of tetracycline resistant strains only among those isolated from rats.It is quite interesting that none of the resistant strains we collected produced bacteriocins.Streptococcus mutans is well known for its cariogenicity ( 1,3,16). It is classified into seven serological types (12,19), and DNA analysis also indicated its heterogeneity (2), but little work has been reported on fresh isolates of this bacterium except for serotypings.We were planning to perform some genetic studies on the fresh isolates of this bacterium, and as the first step, we examined their non-physiological properties, such as bacteriocinogeny (10, 13), lysogeny (5) and antibiotic resistance. There are some reports on each of these properties (4,7,11,14,15,18), but all except Rogers (14) worked exclusively with established strains, and no comparative studies were carried out for all of these properties simultaneously.
MATERIALS AND METHODSStrains employed We isolated naturally occurring fresh strains from oral cavities of man and rats with gelatin agar medium containing 2% gelatin, 1% meat extract, 0.1% glucose, 30% sucrose and 2% agar (pH 7.2). Colonies which appeared on the plates after 48 hr of incubation were selected according to their morphological characteristics, purified on brain heart infusion (BHI) agar (Difco) after five sub-cultures, and identified by their cell-morphologies according to Gram stains, hemolytic activities, sugar fermenation, and adherence to the wall of a test tube containing sucrose-broth. These isolates were maintained on Todd-Hewitt agar (BBL) and were periodically checked for typical growth on mitis-salivarius agar (Difco). All incubations were carried out anaerobically using the BBL Gas Pak system at 37 C.