J Clin Microbiol
2001 Feb;39(2):591-5
Department of Medicine, Siriraj Hospital, Mahidol University, Bangkok, and
Department of Medicine, Khon Kaen Hospital, Khon Kaen, Thailand.
To investigate whether there are methicillin-resistant Staphylococcus
aureus (MRSA) strains with reduced susceptibility to vancomycin in
Thailand, a total of 155 MRSA strains isolated from patients hospitalized
between 1988 and 1999 in university hospitals in Thailand were tested for
glycopeptide susceptibility. All the strains were classified as susceptible
to vancomycin and teicoplanin when judged by NCCLS criteria for
glycopeptide susceptibility using the agar dilution MIC determination.
Vancomycin MICs at which 50 and 90% of the isolates tested were inhibited
(MIC50 and MIC(90), respectively) were 0.5 and 1 microg/ml, respectively,
with a range of 0.25 to 2 microg/ml. For teicoplanin, MIC50 and MIC90 were
2 microg/ml, with a range of 0.5 to 4 microg/ml. However, one-point
population analysis identified three MRSA strains, MR135, MR187, and MR209,
which contained subpopulations of cells that could grow in 4 microg of
vancomycin per ml. The proportions of the subpopulations were 2 x 10(-4),
1.5 x 10(-6), and 4 x 10(-7), respectively. The subsequent performance of a
complete population analysis and testing for the emergence of mutants with
reduced susceptibility to vancomycin (MIC > or = 8 microg/ml) confirmed
that these strains were heterogeneously resistant to vancomycin. Two of
these strains caused infection that was refractory to vancomycin therapy.
Pulsed-field gel electrophoresis showed that the two strains had identical
SmaI macrorestriction patterns and that they were one of the common types
of MRSA isolated in the hospital. This is the first report of heterogeneous
resistance to vancomycin in Thailand and an early warning for the possible
emergence of vancomycin resistance in S. aureus in Southeast Asia