Selection of heterogeneous vancomycin-intermediate Staphylococcus aureus by imipenem

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• Vancomycin-intermediate Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA) are critical challenges in treating S. aureus infections.
• These resistant strains are thought to emerge from vancomycin-susceptible S. aureus (VSSA) through spontaneous mutations during vancomycin exposure.
• Previous work identified a laboratory mutant (H14) with hVISA phenotype after imipenem exposure, showing vraSR overexpression.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the conversion of VSSA to hVISA.
• To investigate the role of the vraSR two-component system and specific mutations in this conversion process.
• To determine if beta-lactam antibiotics, in addition to vancomycin, can select for reduced glycopeptide susceptibility in S. aureus.

Main Methods:

• Whole-genome sequencing of the laboratory mutant H14.
• Phenotypic characterization including morphology, cell wall synthesis activity, and gene expression analysis.
• Genetic manipulation by replacing the vraS gene in VSSA strain Delta IP with mutated or native vraS genes from different strains.

Main Results:

• Genome sequencing identified a novel mutation (S(329)-->L) in the vraS gene of H14, distinct from mutations in clinical hVISA strains.
• This S(329)-->L mutation in vraS was sufficient to induce vraSR overexpression and confer an hVISA phenotype, including heterogeneous vancomycin resistance and enhanced cell wall synthesis.
• Exposure of VSSA to vancomycin or imipenem yielded hVISA isolates, with vraSR mutations found only in the H14 mutant, suggesting alternative evolutionary pathways.

Conclusions:

• The S(329)-->L substitution in vraS is a key driver for the conversion of VSSA to hVISA.
• The vraSR two-component system plays a crucial role in the emergence of reduced glycopeptide susceptibility in S. aureus.
• Both vancomycin and beta-lactam antibiotics can act as selective agents, promoting the evolution of hVISA strains.