Genetic Determinants Enabling Medium-Dependent Adaptation to Nafcillin in Methicillin-Resistant Staphylococcus aureus

Area of Science:

• Microbiology
• Evolutionary Biology
• Antimicrobial Resistance

Background:

• Standard antimicrobial susceptibility testing uses cation-adjusted Mueller-Hinton broth (CA-MHB), which doesn't reflect the in vivo environment.
• Medium composition significantly influences antimicrobial activity and the ability of in vitro assays to predict in vivo efficacy.

Purpose of the Study:

• To investigate how different growth media affect the evolution of antibiotic resistance in MRSA.
• To elucidate medium-specific genetic adaptations and resistance mechanisms in response to nafcillin exposure.

Main Methods:

• Adaptive laboratory evolution of MRSA TCH1516 was performed in CA-MHB and RPMI media, with and without nafcillin.
• Genomic analysis was conducted to identify mutations conferring resistance and adaptation.

Main Results:

• MRSA adapted to RPMI showed increased growth rates due to mutations in apt and mntA, unlike in CA-MHB.
• Nafcillin resistance evolved through distinct, medium-specific mutations, with only the vraRST operon showing mutations in both environments.
• Medium-adapted strains did not exhibit reduced virulence.

Conclusions:

• The environment critically shapes MRSA's evolutionary strategies for antibiotic resistance.
• Adaptive laboratory evolution is a valuable tool for studying clinically relevant resistance mechanisms in a medium-specific context.