Increased vancomycin MICs for Staphylococcus aureus clinical isolates from a university hospital during a 5-year...

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Staphylococcus aureus is a leading cause of hospital-acquired infections.
• Rising rates of methicillin-resistant S. aureus (MRSA) and increasing vancomycin Minimum Inhibitory Concentrations (MICs) are global concerns.
• Monitoring antimicrobial susceptibility trends is crucial for effective treatment strategies.

Purpose of the Study:

• To analyze the prevalence of MRSA and trends in vancomycin MICs for S. aureus isolates from a university hospital.
• To evaluate changes in vancomycin susceptibility over a five-year period (2000-2004).

Main Methods:

• Analysis of 6,003 S. aureus isolates collected between January 2000 and December 2004.
• Utilized Clinical Laboratory Standards Institute (CLSI) reference broth microdilution MIC testing.
• Categorized isolates based on vancomycin MIC values and MRSA status.

Main Results:

• No vancomycin-resistant S. aureus isolates were detected.
• One MRSA isolate showed intermediate vancomycin resistance (MIC = 8 µg/ml).
• A significant shift in vancomycin MICs from ≤0.5 µg/ml to 1.0 µg/ml was observed in 6,002 isolates, with a notable increase in the percentage of isolates having an MIC of 1.0 µg/ml by 2004 (70.4% vs. 19.9% in 2000, P < 0.01).
• This trend towards higher vancomycin MICs was more pronounced in methicillin-susceptible S. aureus.

Conclusions:

• Routine susceptibility testing revealed a trend of decreasing vancomycin susceptibility in S. aureus clinical isolates over five years.
• The observed shift in vancomycin MICs warrants continued surveillance and may impact treatment guidelines.
• While vancomycin resistance remains rare, increasing MICs highlight the need for vigilance against evolving antimicrobial resistance patterns.