[Is the methicillin-resistant Staphylococcus aureus heteroresistant to vancomycin in Croatia?]

Staphylococcus (S.) aureus with reduced susceptibility to vancomycin has attracted much attention all over the world since the first report of Staphylococcus aureus isolate intermediarily resistant to vancomycin (VISA) in Japan 1997. Other authors from different parts of the world have also described VISA isolates in patients with treatment failures after prolonged vancomycin therapy. Most of the isolates were heterogeneously resistant (hVISA), i. e. only a part of the population showed resistance and the rest showed susceptibility to vancomycin.

Aim of the study was to determine the existence of methicillin-resistant S. aureus (MRSA) strains with reduced susceptibility to vancomycin in Croatia.

Abbreviated population analysis was used for detection of strains with reduced susceptibility to vancomycin. Forty-eight MRSA strains from three different hospitals in Croatia were tested on brain-heart infusion agar (BHIA) screen plate containing 4 mg/L vancomycin. Thirty-three (68.7%) strains that showed growth on a screen plate were inoculated on BHIA plates with rising vancomycin concentrations (1-20 mg/L). After subcultivation and growth on a vancomycin-free BHIA plate, minimal inhibitory concentrations (MICs) were determined for all strains.

Fourteen of 48 (29.1%) strains had vancomycin 8 mg/L and 1/48 (2.0%) strain had vancomycin 16 mg/L. In 3/48 (6.2%) MIC were stable after storage in liquid nitrogen for six months. Vancomycin MIC50 and MIC90 of all 33 strains grown on screen plate were 1 and 2 gm/L, respectively, when tested on Mueller-Hinton agar (MHA) before inoculation on BHIA with growing concentrations of vancomycin. Immediately thereafter, MIC were 4 and 8 mg/L, and after six months of storage, they were 4 and 4 mg/L, respectively.

The prevalence of hVISA in Croatia is low, but there are some strains with reduced susceptibility to vancomycin. Unfortunately, because of lack of clinical data neither clinical correlation with laboratory findings nor therapeutic failures can be discussed.