UBIQUITOUS PRESENCE OF A MECA HOMOLOG IN NATURAL ISOLATES OF STAPHYLOCOCCUS-SCIURI

In an effort to explore the origin and/or reservoirs of the genetic de terminant(s) of methicillin resistance in Staphylococcus aureus, we ex amined over 200 strains representing 13 different species within the g enus Staphylococcus for the presence of the mecA gene, using a DNA pro be internal to this gene prepared from a methicillin-resistant strain of S. aureus, Occasional mecA- positive isolates were detected among s everal staphylococcal species, On the other hand, each one of the 134 isolates of Staphylococcus sciuri, a species considered taxonomically the most primitive among staphylococci and found primarily on rodents and primitive mammals, gave positive reaction with the DNA probe when tested under conditions of high stringency, About two thirds (99) of t hese isolates, all of which belonged to S. sciuri subspecies ''sciuri, '' as well as 9 of the 11 species carnaticum isolates, showed only mar ginal, if any, resistance to methicillin (minimal inhibitory concentra tion of 0.75-6.0 mu g/ml), while most of the remaining isolates that b elonged to the subspecies ''rodentius'' (13 isolates in all) expressed antibiotic resistance with a heterogeneous phenotype similar to those seen in many methicillin-resistance strains of S. aureus In SmaI dige sts of chromosomal DNA isolated from such ''methicillin-resistant S. a ureus-like'' strains, the mecA probe hybridized with DNA fragments in the range of 145-180 kb, while in subspecies ''sciuri'' and carnaticum isolates the mecA hybridizing fragment was located in the SmaI fragme nt with the highest molecular size (greater than or equal to 400 kb), A DNA probe comprising an internal sequence to the regulatory gene mec I from Staphylococcus epidermidis identified the presence of sequences with low degree of homology in isolates of the three S. sciuri subspe cies, The mecA-reacting sequences in these bacteria differed from mecA of S. aureus in several respects (e.g., by the absence of a ClaI rest riction site from mecA of subspecies ''sciuri'' and carnaticum, and in some isolates of subspecies ''rodentius.'' The uniform presence of me cA in each one of a large number of S. sciuri strains belonging to dis tinct ribotypes and macrorestriction patterns and recovered over a 20- year period from a wide variety of animal sources and geographic sites suggests that mecA may be a native genetic element with an as yet uni dentified physiologic function in this staphylococcal species.