Emergence and Spread of Gentamicin-susceptible Strains of Methicillin-resistant Staphylococcus Aureus in Belgian Hospitals

The aim of this study was to follow the evolution of the clonal distribution and antimicrobial susceptibility of clinical strains of methicillin-resistant Staphylococcus aureus (MRSA) recovered from Belgian hospitals between 1995 and 1997-1998. MRSA strains were genotyped by inter-IS256 spacer length polymorphism PCR and SmaI macrorestriction analysis. MICs of 18 antimicrobials were determined by the agar dilution method. MRSA strains from the 1997-1998 survey were further tested by vancomycin screen agar, E-test, broth microdilution methods, and population analysis. Between 1995 and 1997-1998, epidemic group A strains decreased in proportion from 73% to 44%, whereas MRSA Group B and C strains increased from 17% to 38% and from 5% to 8%. The proportion of strains susceptible to gentamicin increased between the surveys from 22% to 48%. This was associated with a higher proportion of group B and C strains in the last survey. Heterogeneous vancomycin-intermediate S. aureus (hetero-VISA) strains were found in 2% isolates from 1997 to 1998. These hetero-VISA isolates were genotypically related to the MRSA group A strains and were resistant to gentamicin. In conclusion, two emerging epidemic MRSA genotypes, susceptible to gentamicin, have spread among Belgian hospitals during the 1990s. Hetero-VISA were present at low frequency among MRSA strains belonging to a widespread endemic genotype.