| Background and Aim: Vancomycin is one of the last drugs used by human for
treatment of infections caused by Methicillin resistant Staphylococcus aureus (MRSA)
isolates. Since s.` aureus with heterogeneous, intermediate and complete resistance to
vancomycin are in relation to high mortality rate and are increasing in the world, so
investigating the incidence of these isolates is necessary.
Materials and Methods: By using several phenotypic tests such as Gram stain,
catalase, coagulase, DNase and mannitol fermentations and by screening resistance
to methicillin, 100 MRSA isolates were identified. For identifying resistance to
vancomycin, phenotypic tests with the disc diffusion method, determining MIC by
E-test method and screening test in BHIA (Brain Heart Infusion Agar) containing
vancomycin and molecular PCR test were carried out in order to identify femB, mecA
and vanA genes.
Results: Resistance to methicillin was 59%. Ranges of vancomycin MICs were
between 0.75 μg/ml to 4 μg/ml. By E-test method 92% of isolates were sensitive
and 8% showed heterogeneous resistance to vancomycin. However, screening tests
in BHIA, containing 3 μg/ml and 4 μg/ml vancomycin, detected 18% heterogeneous
resistance and 4% intermediate resistance to vancomycin, respectively. In screening
tests using BHIA, with 6 μg/ml vancomycin no isolate was able to grow. Although all
of the isolates contained femB and mecA genes, but none of them contained vanA gene.
Conclusions: E-test alone was not able to identify all S.aureus isolates of
heterogeneous resistance and intermediate resistance to vancomycin. As a result, the
screening tests also should be used along with MIC tests. |