Evolution of methicillin-resistant Staphylococcus aureus towards increasing resistance

Area of Science:

• Microbiology
• Evolutionary Biology
• Genetics

Background:

• Staphylococcus aureus clonal complex (CC) 8 includes globally significant epidemic lineages.
• Hospital-associated and community-associated methicillin-resistant S. aureus (MRSA), such as USA300, are part of CC8.

Purpose of the Study:

• To reconstruct the evolutionary history of Staphylococcus aureus CC8.
• To understand the acquisition and distribution of antimicrobial resistance and mobile genetic elements within CC8 lineages.

Main Methods:

• Phylogenetic reconstruction of 174 S. aureus CC8 isolates using 112 genetic loci.
• Analysis of antimicrobial resistance traits and mobile genetic elements across different clades.
• Dating evolutionary events, including methicillin resistance acquisition.

Main Results:

• Identified nine distinct phylogenetic clades within CC8.
• Documented at least eight independent methicillin resistance acquisition events.
• Dated the origin of a methicillin-resistant USA300 progenitor to the mid-1970s.
• Observed increasing antibiotic resistance trends in evolving lineages, including USA300.
• Found widespread carriage of plasmidic rep genes, with varying stability.

Conclusions:

• Proposed an evolutionary model for S. aureus CC8 involving lineage diversification and mobile genetic element dynamics.
• Highlighted the concerning evolution of MRSA USA300 towards multi-drug resistance.
• Emphasized the role of mobile genetic elements in spreading virulence and resistance traits.