| Abstract |
Background: Vancomycin, a glycopeptide, is a drug
of choice for the treatment of methicillin-resistant
Staphylococcus aureus (MRSA) infections and is used
for an empirical treatment covered Gram-positive
organisms. Currently, there has been an upward trend
of vancomycin minimum inhibitory concentration (MIC)
for the MRSA isolates. According to Clinical and
Laboratory Standards Institute (CLSI) criteria, isolates
with vancomycin MICs of ≥ 16 μg/ml are defined as
vancomycin-resistant S. aureus (VRSA), those with
MICs of 4-8 μg/ml are vancomycin-intermediate S.
aureus (VISA), and those with MICs ≤ 2 μg/ml are
defined as vancomycin-susceptible S. aureus (VSSA).
In addition, S. aureus isolates, which have vancomycin
MICs within susceptible range but contain a resistant
population frequency of 1 in 106 or greater and are
able to grow in the presence of 4 μg/ml of vancomycin
or greater, are defined as heterogeneous vancomycinintermediate
S. aureus (hVISA). The aim of this study
was to investigate the change of vancomicin MICs for
the MRSA isolates when cultured in vancomycin
condition.
Methods: A total of 55 MRSA isolates from patients
in Srinagarind hospital were grown in media containing
2 μg/ml of vancomycin for 1-7 days. An isolate that
grew in the media was transferred to additional media
with 3, 4, 5, 6, 7 and 8 μg/ml of vancomycin until it was
unable to grow. Vancomycin MIC was determined by
an agar dilution method and vancomycin resistance was
screened by one point analysis (OPA) method.
Results: At the beginning, the 55 isolates had
vancomycin MICs of 1-2 μg/ml. After vancomycin
exposure, 20 (36.4%) could grow in the media
containing 3-8 μg/ml of vancomycin. Their
vancomycin MICs were 2-8 mg/ml. It was found
that 2, 2, 1, 8, 3 and 4 isolates had vancomycin MIC
increase in 8, 5, 4, 3, 2 and 0.5 times respectively.
The change of vancomycin susceptibility of these
isolates was generated between 1-7 days. Screening
for vancomycin resistance by OPA revealed that 10
isolates (50%) were VISA and 7 isolates (35%) were
hVISA. The hVISA isolates should be confirmed
by a population analysis profile with area under the
curve method (PAP-AUC) using S. aureus Mu3
strain as reference strain.
Conclusion: This in vitro study implied that isolate
with reduced susceptibility to vancomycin may occur
in patient with prolonged treatment with vancomycin. |
Publication
Journal Publication
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