Evaluation of vancomycin MIC creep in Staphylococcus aureus

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Pharmacology

Background:

• Vancomycin is a critical antibiotic for treating methicillin-resistant Staphylococcus aureus (MRSA) infections.
• Emerging vancomycin resistance, indicated by 'MIC creep', poses a significant clinical challenge.
• Monitoring local susceptibility patterns is crucial for effective MRSA management.

Purpose of the Study:

• To analyze the vancomycin MIC distribution for S. aureus isolates over four years.
• To compare vancomycin MICs determined by different susceptibility testing methods.
• To assess the presence of vancomycin MIC creep in a Portuguese hospital.

Main Methods:

• S. aureus isolates were tested for vancomycin MIC using the VITEK®2 automated system.
• Broth microdilution testing was employed as a comparative susceptibility testing method.
• Statistical analysis was performed to identify significant differences between methods.

Main Results:

• Significant differences in vancomycin MIC values were observed between the VITEK®2 system and broth microdilution (P=0.021).
• The study did not detect evidence of vancomycin MIC creep during the four-year study period.
• Results indicate variability in MIC determination based on the testing methodology.

Conclusions:

• Vancomycin MIC creep is a localized phenomenon requiring regional surveillance.
• Accurate, local antibiogram data derived from MIC assays are essential for guiding MRSA treatment.
• Methodological choices in susceptibility testing can influence observed vancomycin MICs.