Methicillin-resistant Staphylococcus aureus (MRSA) evolution: the multiple facets of an old pathogen

Infections due to methicillin-resistant Staphylococcus aureus (MRSA) have shown a dramatic global increase over the last few decades. Hospital-associated MRSA (HA-MRSA) strains compete in different environments, such as medical wards, which are largely devoid of selective antibiotic pressure and therefore the selection for factors that contribute to ecological fitness may outweigh the need for multiple resistance determinants. Recently, infections due to virulent community-acquired MRSA (CA-MRSA) strains, susceptible to many non-β- lactam antibiotics, have rapidly emerged. Today, there is even more concern about MRSA infections because these micro-organisms are becoming less easily treated by currently recommended antibiotic agents. This article describes the evolutionary processes undergone by Staphylococcus aureus, the dynamics of the genotypic changes in both HA- and CA-MRSA strains due to variations in staphylococcal cassette chromosome mec (SCCmec) and strongly linked to their antibiotic-resistance shifts and the changing trends in MRSA epidemiology