Phenotypic characterization of Staphylococcus aureus isolated from skin and soft tissue infections.

Thesis Report.

ABSTRACT: Staphylococcus aureus, being the major cause of mortality and morbidity have been growing burden on health care systems. This study was designed to determine prevalence of S. aureus including methicillin resistant S. aureus (MRSA) isolates in skin and soft tissue infections (SSTIs) along with their phenotypic characterization. S. aureus were isolated and identified following standard microbiological method. Antibacterial susceptibility test was performed by disk diffusion, for oxacillin and vancomycin minimum inhibitory concentration determined by broth micro dilution method and for vancomycin by E-test strips. β lactamse was detected by three different methods viz. clover leaf technique, iodometric and acidometric test in all isolates. Clover leaf technique, microbiological method was considered the gold standard, then the sensitivity, specificity, positive and negative predictive values of the iodometric and acidometric tests were calculated. A total of 108 S. aureus isolates were analyzed in between April and September, 2014. Of these isolates, 40.7% were MRSA which include 22.73% of homogeneous MRSA and 77.27% of heterogeneous MRSA. Of the 44 MRSA isolates, 11 were non-multiresistant oxacillin resistant Staphylococcus aureus (NORSA), defined as being resistant to two or fewer antibiotics other than beta-lactams. Thirty-three were multiresistant oxacillin resistant Staphylococcus aureus (MORSA) strains. D test showed that inducible macrolide-lincosamide-steptogramin-B (MLSB) resistance was 14.8%. Resistance to clindamycin and erythromycin were not associated with MRSA (p>0.05). Iodometric method was found to be more sensitive (92.75%) and had high negative predictive value (86.79%), whereas, acidometric test was more specific (87.18%) and had high positive predictive value (92.19%). MRSA infections are still one of the most threatening infections in the hospitals. Therefore, regular surveillance of MRSA must be carried out in all hospital settings. D tests should be made mandatory in all S. aureus isolates as inducible MLSB resistance cannot be detected in routine susceptibility test unless erythromycin and clindamycin are placed 15-26 mm apart. These findings suggest the need of β lactamase test prior to the antibiotic prescription. Key words: - Staphylococcus aureus, Methicillin resistant S. aureus, Non-multi resistant Oxacillin Resistant S. aureus, Multi Resistant Oxacillin Resistant S. aureus, β lactamase, Inducible Macrolide Lincosamide Streptogram B