Genetics of streptomycin resistance in methicillin-sensitive multiply-resistant Staphylococcus aureus

Streptomycin resistance was detected in 17 of 20 multiply-resistant Staphylococcus aureus isolates from a hospital in a southeastern Nigerian town. The isolates were uniformly sensitive to methicillin, erythromycin, gentamicin, mupirocin, ciprofloxacin and vancomycin but produced beta-lactamase and were resistant to other antimicrobial agents and harbored different plasmids which ranged in size and number from 1.0 to c, 40 kb and one to six respectively. Curing and transfer experiments demonstrated that streptomycin resistance (Smr) was located on plasmids in 15 of the 17 isolates. 16 Smr plasmids were isolated and characterized. They belonged to three distinct groups based on size and resistance phenotypes. Eight 4.4 kb plasmids encoded Smr only, three 4.7 kb plasmids encoded resistance to streptomycin and chloramphenicol (SmCm) and five 23.8 kb plasmids encoded resistance to streptomycin, kanamycin, neomycin, cadmium and beta-lactamase production (CPKNS). Restriction endonuclease analysis demonstrated that the 4.4 kb Smr plasmids were similar to one another and indistinguishable from pUB109, an incompatibility group 5 Smr plasmid, suggesting that they may also belong to incompatibility group 5. The SmCm and the CPKNS plasmid groups also gave identical restriction patterns with single and double enzyme digests. Further transfer experiments with one of the SmCm plasmids led to the isolation of a 4.4 kb Smr plasmid which was indistinguishable from the other 4.4 kb plasmids, suggesting that the SmCm plasmids are natural recombinants between a streptomycin and chloramphenicol resistance plasmid. The results demonstrate a multiple origin of streptomycin resistance in the S. aureus population studied.