Iranian Journal of Medical Microbiology

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 Staphylococcus 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.

Conclusion: 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.