Genetic diversification of methicillin-resistant Staphylococcus aureus as a function of prolonged geographic dissemination and as measured by binary typing and other genotyping methods.

The aim of the present study was to determine the extent of genome evolution among methicillin-resistant Staghylococcus aureus (MRSA) strains. Three different collections of strains were analysed, comprising locally, nationally and internationally disseminated genotypes. Various genotyping assays displaying different levels of resolution were used. Geographically and temporally diverse MRSA strains comprised the international group. MRSA strains recovered during an outbreak in a New York City hospital and Portuguese MRSA isolates, all resembling the so-called Iberian clone, were included in the local and national collections, respectively. Genotypes were determined by genome scanning typing techniques and procedures which analyse specific DNA elements only. The outbreak strains showed subclonal variation, whereas the Portuguese isolates displayed an increased number of genotypes. Among the epidemiologically unrelated MRSA strains, the different genotyping techniques revealed a wide heterogeneity of types. Different typing techniques appeared to show different levels of resolution, which could be correlated with the extent of geographic spread; the more pronounced the spread, the higher the degree of genome evolution. Binary typing and randomly amplified polymorphic DNA analysis are the typing methods of choice for determining (non)identity among strains that have a recent common ancestor and have undergone yet limited dissemination.