Staphylococcus aureus isolates from five large teaching hospitals and one medium-size community hospital located in geographically distant parts of Brazil, in the south and southeast (Rio de Janeiro, Niteroi, Sao Paulo, Porto Alegre) and in the north (Manaus), were tested for their antibiotic resistance patterns and genetic backgrounds. Eighty-five of the 152 isolates were identified as methicillin-resistant S. aureus (MRSA) by using a combination of an agar dilution screen and a mecA gene-specific DNA probe. All MRSA isolates were resistant to penicillin, erythromycin, gentamicin, oxacillin, and cephalothin, and the majority of isolates (74%) were also resistant to chloramphenicol, sulfamethoxazole-trimethoprim, ciprofloxacin, and clindamycin as well and were susceptible only to vancomycin. Isolates obtained from hospitals in Sao Paulo, Rio de Janeiro, Niteroi, and Porto Alegre (1,600 km from one another) and Manaus (3,700 km from Rio de Janeiro) were examined by a variety of molecular fingerprinting techniques: the nature of the mecA polymorph and Tn554 attachment sites and restriction fragment length polymorphism of genomic DNAs after SmaI restriction and separation of the digested DNA by pulsed-field gel electrophoresis. The overwhelming majority of the isolates shared a common pulsed-field gel electrophoresis pattern and carried mecA polymorph III in combination with Tn554 pattern B, indicating the presence of a single, epidemic MRSA clone spread over large geographic distances of Brazil.

Methicillin-resistant Staphylococcus aureus (MRSA) are important nosocomial pathogens. Diseases caused by these resistant bacteria frequently are serious and there is a need to control the spread of epidemic MRSA clones in hospitals. However, detection is complicated by the fact that expression of the resistance is variable and, commonly, heterogeneous within strains. The reliability of several tests recommended to discriminate heterogeneous MRSA isolates from borderline-resistant and susceptible strains was evaluated. Screening for growth on agar with methicillin 25 mg/L was the only method that detected all MRSA strains tested, but screening on agar with methicillin 10 mg/L or oxacillin 6 mg/L detected all but one of 10 heterogeneously resistant strains tested. None of the borderline-resistant nor any truly susceptible staphylococci tested grew on any of these screening plates.

No abstract