Methicillin-Resistant Staphylococcus aureus: Global Epidemiology and Molecular Epidemiological Trends

Since its emergence in the 1960s, methicillin-resistant Staphylococcus aureus (MRSA) has been recognized worldwide as a major cause of healthcare-associated infections. Traditionally, healthcare-associated MRSA (HA-MRSA) has been characterized as multidrug-resistant strains carrying staphylococcal cassette chromosome (SCC) mec types I–III, whereas community-associated MRSA (CA-MRSA) has been characterized as strains carrying SCCmec types IV or V that remain relatively susceptible to non-β-lactam antimicrobial agents. However, in recent years, the distinction between these two categories has become increasingly blurred owing to the global replacement of predominant clones, with community-associated clones entering healthcare settings and healthcare-associated clones spreading into the community. This review summarizes the epidemiology of MRSA in North America, South America, Europe, Africa, the Middle East, Asia, Oceania, and Japan and highlights the characteristics of the major circulating clones and their antimicrobial resistance profiles. Although the proportion of MRSA among S. aureus isolates has declined in many high-income countries owing to improvements in infection prevention and control, high isolation rates continue to be reported in Asia, the Middle East, and Africa. From a molecular epidemiological perspective, classical healthcare-associated clones, including ST239, ST5-II, and ST228-I, are declining, whereas SCCmec type IV clones, such as EMRSA-15 (ST22-IV), USA300 (ST8-IV), the Asian-Pacific clone (ST59-IV), and Queensland CA-MRSA (ST93-IV), are becoming predominant worldwide. In addition, although CA-MRSA has traditionally been considered susceptible to non-β-lactam antimicrobial agents, several clones have acquired multidrug resistance or novel resistance mechanisms. Moreover, with the widespread adoption of whole-genome sequencing (WGS), it has become possible to investigate the evolution of circulating clones, the acquisition of virulence factors and antimicrobial resistance genes, and even international transmission pathways at high resolution. Continued monitoring of regional molecular epidemiological characteristics, together with WGS-based international surveillance, will contribute to improved control of MRSA infections, optimization of infection prevention and control strategies, more appropriate empirical antimicrobial therapy, and the development of novel diagnostic and therapeutic approaches.