Prevalence of Methicillin, Vancomycin and Multidrug Resistance among Staphylococcus aureus 974-977
Dr. Dhanalakshmi T. A.
Department of Microbiology
Adichunchanagiri Institute of Medical Sciences
B.G.Nagara, Mandya district
Karnataka state, India - 571448.
Background and Objectives:
The treatment of Staphylococcus aureus infections is becoming increasingly more complicated due to the emergence of various types of antibiotic resistance. The present study was undertaken to know the prevalence of methicillin resistant Staphylococcus aureus (MRSA), vancomycin intermediate and vancomycin resistant Staphylococcus aureus (VISA and VRSA respectively) and multidrug resistant MRSA and to evaluate the phenotypic detection methods.
Materials and Methods:
A total of 250 non-duplicate isolates of Staphylococcus aureus which were isolated from various clinical samples were tested for methicillin resistance by using the oxacillin disc diffusion test (1μg), the cefoxitin disc diffusion test (30 μg) and the oxacillin agar screen method (Muller Hinton agar (MHA) with 4% NaCl+6 μg/ml oxacillin). Vancomycin screen agar (MHA containing 5 μg/ml vancomycin) was used for screening the VISA/VRSA isolates. The MIC of vancomycin was determined by using the agar dilution method and the E-test. The antibiogram of the isolates to other antibiotics was studied by the Kirby-Bauer disc diffusion method.
80 (32%) isolates were found to be methicillin resistant by the cefoxitin disc diffusion method, 78 (31.2%) were found to be methicillin resistant by the oxacillin agar screen test and 77 (30.8%) were found to be methicillin resistant by the oxacillin disc diffusion method. No VISA and VRSA isolate was detected by using the vancomycin screen agar test, agar dilution and the E-test. The vancomycin screen agar showed 100% specificity and 100% negative predictive value. Sixty seven (83.8%) of the 80 MRSA isolates and 26.8 % of the total 250 Staphylococcus aureus isolates tested were found to be multidrug resistant MRSA.
Where the facilities are limited, the cefoxitin disc diffusion test and the vancomycin screen agar test can be used for screening the MRSA and the VRSA isolates respectively. With the revised CLSI guidelines, the screening method for VISA with an MIC of 4μg/ml needs to be evaluated with further more studies.