Epidemic community-associated methicillin-resistant Staphylococcus aureus: recent clonal expansion and...

Area of Science:

• Microbiology
• Evolutionary Biology
• Epidemiology

Background:

• Emerging infectious diseases, particularly from drug-resistant bacteria like community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), pose a global health threat.
• The rapid emergence of the CA-MRSA USA300 strain in the United States presents an epidemic, but its evolutionary origins remain unclear.
• Understanding whether USA300 arose from clonal expansion or independent evolutionary convergence is crucial for public health strategies.

Purpose of the Study:

• To investigate the evolutionary history and phylogenetic relationships of the CA-MRSA USA300 strain.
• To determine if the USA300 epidemic resulted from clonal emergence or convergent evolution towards hypervirulence.
• To assess the impact of genetic variation on the virulence of USA300 isolates.

Main Methods:

• Comparative whole-genome sequencing of 10 USA300 patient isolates from diverse US regions and infection types.
• Analysis of single nucleotide polymorphisms (SNPs) to infer phylogenetic relatedness and evolutionary pathways.
• Evaluation of isolate virulence using a mouse sepsis model to correlate genetic changes with mortality rates.

Main Results:

• Eight out of 10 analyzed USA300 isolates were closely related, exhibiting few SNPs, which strongly supports recent clonal diversification.
• Two clonal isolates demonstrated significantly reduced mortality in a mouse sepsis model compared to the reference strain, indicating minimal genetic changes can alter virulence.
• The findings indicate that the USA300 epidemic is primarily driven by clonal expansion and subsequent diversification, rather than independent evolutionary convergence.

Conclusions:

• The emergence and spread of the CA-MRSA USA300 strain are attributed to recent clonal expansion and diversification.
• Even minor genetic alterations within the bacterial genome can profoundly influence virulence, impacting disease severity.
• These evolutionary insights into USA300 may provide a model for understanding the historical spread of other resistant bacteria, such as phage-type 80/81 Staphylococcus aureus.