[Methicillin resistance and vancomycin susceptibility pattern among blood isolates of Staphylococcus aureus]

Staphylococcus aureus is capable of acquiring resistance against all antimicrobial agents. Vancomycin has been the cornerstone therapy for serious methicillin-resistant S. aureus infections. However, vancomycin treatment failures have been reported.

From March to August 2010, S. aureus blood isolates were included to determine methicillin-resistance and the vancomycin susceptibility by using a standard microdilution method. To detect methicillin-resistance, a Mueller-Hinton agar plate added with oxacillin 4 µg/mL and 2 % NaCl, and an agglutination test were used. Growth of S. aureus on the agar plate and/or reactive agglutination defined a methicillin-resistant organism. Vancomycin susceptibility was assessed by determining the minimal inhibitory concentration (MIC) in Muelller-Hinton agar plates prepared with dilutions ranging from 16 to 0.5 µg/mL.

A total of 25 blood-isolates of S. aureus were included. A 60 % was methicillin-resistant. All isolates were vancomycin-susceptible (MIC ≤ 2 µg/mL) showing the next MICs distribution: 48 % ≤ 0.5 µg/mL; 44 % 1 µg/mL, and 8 % 2 µg/mL.

The high proportion of methicillin-resistance among S. aureus and the presence of vancomycin susceptible phenotypes (MIC of 2 µg/mL) not only claim for an enforcement of standard precautions and antimicrobial control, but also for a regular surveillance of vancomycin susceptibility pattern using a reference method.