The evolutionary history of methicillin-resistant <i>Staphylococcus aureus</i> (MRSA)

TLDR

MRSA is a leading cause of hospital‑acquired infections, increasingly resistant to all current antibiotics, yet its evolutionary origins, nomenclature, and clone relationships remain poorly understood. This study aims to resolve these gaps by providing a comprehensive and precise analysis of MRSA clone evolution. Using multilocus sequence typing and a burst algorithm on 912 international MRSA and MSSA isolates, the authors inferred ancestral genotypes, descent patterns, and the horizontal transfer of methicillin resistance genes. They identified 11 major MRSA clones grouped into five related genotypes, showed that these clones repeatedly arose from epidemic MSSA strains, and noted that vancomycin‑decreased‑susceptibility isolates are emerging from some clones, underscoring escalating drug resistance.

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of hospital-acquired infections that are becoming increasingly difficult to combat because of emerging resistance to all current antibiotic classes. The evolutionary origins of MRSA are poorly understood, no rational nomenclature exists, and there is no consensus on the number of major MRSA clones or the relatedness of clones described from different countries. We resolve all of these issues and provide a more thorough and precise analysis of the evolution of MRSA clones than has previously been possible. Using multilocus sequence typing and an algorithm, burst , we analyzed an international collection of 912 MRSA and methicillin-susceptible S. aureus (MSSA) isolates. We identified 11 major MRSA clones within five groups of related genotypes. The putative ancestral genotype of each group and the most parsimonious patterns of descent of isolates from each ancestor were inferred by using burst , which, together with analysis of the methicillin resistance genes, established the likely evolutionary origins of each major MRSA clone, the genotype of the original MRSA clone and its MSSA progenitor, and the extent of acquisition and horizontal movement of the methicillin resistance genes. Major MRSA clones have arisen repeatedly from successful epidemic MSSA strains, and isolates with decreased susceptibility to vancomycin, the antibiotic of last resort, are arising from some of these major MRSA clones, highlighting a depressing progression of increasing drug resistance within a small number of ecologically successful S. aureus genotypes.