Alternative Evolutionary Pathways for Drug-Resistant Small Colony Variant Mutants in Staphylococcus aureus

Area of Science:

• Microbiology
• Evolutionary Biology
• Genetics

Background:

• Small colony variants (SCVs) of Staphylococcus aureus are linked to persistent infections and aminoglycoside resistance.
• SCVs exhibit unstable phenotypes, often losing resistance upon regaining growth.
• Mechanisms for growth compensation without resistance loss remain poorly understood.

Purpose of the Study:

• To investigate mechanisms of growth compensation in SCVs that avoid loss of antibiotic resistance.
• To characterize extragenic suppressor mutations compensating for SCV slow growth.

Main Methods:

• Serial passaging under selective pressure to isolate compensatory mutants.
• Characterization of suppressor mutations, including translational suppressors and SrrAB pathway activators.
• RNA sequencing to analyze gene expression changes.

Main Results:

• Identified two distinct bypass mechanisms for SCV growth compensation: translational suppression and SrrAB activation.
• Translational suppression restored growth but reduced antibiotic susceptibility.
• SrrAB activation increased growth rate without loss of aminoglycoside resistance, via upregulation of peptide transport and pyruvate fermentation.
• Discovered four distinct pathways for SCV growth compensation, with one preserving antibiotic resistance.

Conclusions:

• Compensatory evolution via SrrAB activation allows SCVs to increase growth rate without losing antibiotic resistance.
• This pathway represents a significant threat to effective antibiotic treatment of Staphylococcus aureus infections.
• Understanding these evolutionary pathways is crucial for developing strategies against persistent, drug-resistant bacterial infections.