Detection of an archaic clone of Staphylococcus aureus with low-level resistance to methicillin in a pediatric hospital in Portugal and in international samples: Relics of a formerly widely disseminated strain?

Close to half of the 878 methicillin-resistant Staphylococcus aureus (MRSA) strains recovered between 1992 and 1997 from the pediatric hospital in Lis bon were bacteria in which antibiotic resistance was limited to beta-lactam antibiotics. The other half were multidrug resistant. The coexistence of M RSA with such unequal antibiotic resistance profiles prompted us to use mol ecular typing techniques for the characterization of the MRSA strains. Fift y-three strains chosen randomly were typed by a combination of genotypic me thods. Over 90% of the MRSA strains belonged to two clones: the most freque nt one, designated the "pediatric clone," was reminiscent of historically " early" MRSA: most isolates of this clone were only resistant to beta-lactam antimicrobials and remained susceptible to macrolides, quinolones, clindam ycin, spectinomycin, and tetracycline. They showed heterogeneous and low-le vel resistance to methicillin (MIC, 1.5 to 6 mu g/ml), carried the ClaI-mec A polymorph II, were free of the transposon Tn554, and showed macrorestrict ion pattern D (clonal type II::NH::D). The second major clone was the inter nationally spread and multiresistant "Iberian" MRSA with homogeneous and hi gh-level resistance to methicillin (MIC, >200 mu g/ml) and clonal type I::E ::A. Surprisingly, the multidrug-resistant and highly epidemic Iberian MRSA did not replace the much less resistant pediatric clone during the 6 years of surveillance. The pediatric clone was also identified among contemporar y MRSA isolates from Poland, Argentina. The United States, and Colombia, an d the overwhelming majority of these were also associated with pediatric se ttings. We propose that the pediatric MRSA strain represents a formerly wid ely spread archaic clone which survived in some epidemiological settings wi th relatively limited antimicrobial pressure.