Methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin in Canada

Area of Science:

• Infectious Diseases
• Microbiology
• Pharmacology

Background:

• Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant healthcare challenge.
• Vancomycin remains a critical antibiotic for treating MRSA infections.
• Monitoring antimicrobial susceptibility is crucial for effective treatment strategies.

Observation:

• A patient in Canada developed MRSA isolates with increasing vancomycin Minimum Inhibitory Concentrations (MICs) over six months.
• The vancomycin MICs rose from less than or equal to 1 mg/L to 4 mg/L, with one isolate testing at 8 mg/L.
• Despite vancomycin therapy, MRSA persisted in the patient's wound, necessitating a switch to linezolid and rifampin.

Findings:

• All MRSA isolates were genetically identical by pulsed-field gel electrophoresis (PFGE).
• Transmission electron microscopy revealed a thicker cell wall in the more resistant MRSA isolates.
• Population analysis indicated vancomycin hetero-resistance in the resistant isolates.
• MICs of 4 mg/L were consistently detected using BD Phoenix, E-test, and broth microdilution methods.

Implications:

• This case represents the first documented instance of reduced vancomycin susceptibility in MRSA in Canada.
• The findings highlight the potential for MRSA to develop resistance during vancomycin therapy.
• Revised vancomycin susceptibility breakpoints may be necessary to accurately guide treatment decisions.