Detection of vancomycin susceptibility among clinical isolates of MRSA by using minimum inhibitory concentration method

P. Sreenivasulu Reddy, Maria Sindhura John, P. Vasundhara Devi, B. Siva Prasad Reddy


Background: Staphylococcus aureus is considered as a major pathogen causing a diversity of infections including bacteremia, pneumonia, skin and soft tissue including osteoarticular infections. Since 1961, Methicillin Resistant Staphylococci aureus (MRSA) emerged has one of the major and common cause of hospital acquired infection. However, due to wide spread usage of vancomycin for MRSA infections resulted in reduced susceptibility of S. aureus to vancomycin has been identified as a serious public health concern. The aim of the study is to identify the Methicillin Resistant Staphylococcus aureus (MRSA) from various clinical samples and to detect vancomycin susceptibility by Minimum Inhibitory Concentration (MIC) method.

Methods: This study was conducted over period of one year December 2013 to November 2014. Clinical samples like pus, blood, sputum, urine and cerebrospinal fluid were collected from various clinical departments in Narayana General Hospital for selective isolation of Staphylococcus aureus. A total of 100 Staphylococcal aureus isolates were isolatedby using standard laboratory procedures. MRSA were detected using Oxacillin Disc on Muller Hinton Agar with 4% NaCl. Sensitivity pattern for vancomycin (30 µg) disc and for other recommended antibiotics was determined by Kirby-Bauer’s disk diffusion method. Minimum Inhibitory Concentration (MIC) was done for vancomycin sensitive isolates by standard agar dilution method.

Results: Out of 100 S. aureus isolates, all were susceptible to vancomycin (30 µg) by disk diffusion method. But, 82 isolates of MRSA were susceptible to vancomycin at the concentration of 0.5-2 μg/ml of agar. 17 isolates showed intermediate sensitivity to vancomycin, in which 13 isolates with MIC 4μg/ml and 4 isolates with MIC 8 μg/ml and one isolate was resistant to vancomycin even with MIC of 16 μg/ml.

Conclusions: The present study reveals the emergence of Vancomycin Intermediate Sensitive Staphylococcus aureus (VISA) and Vancomycin Resistant Staphylococcus aureus (VRSA). Disc diffusion method should not be employed for detection of vancomycin sensitivity for MRSA stains. The major cause may be attributed to unawareness and irrational usage of broad spectrum antibiotics.




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