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Comparative Evaluation of Methicillin Resistant and Methicillin Sensitive Staphylococcus aureus of Livestock Origin for Antibiotic Sensitivity, Biofilm Formation and Virulence in Galleria mellonella

The objectives of the study were to isolate and identify livestock associated methicillin resistant Staphylococcus aureus (LA-MRSA) and methicillin sensitive S. aureus (LA-MSSA) from clinical mastitis cases and to compare their antibiotic susceptibility, biofilm formation and in vivo pathogenicity in Galleria mellonella larva model. A total of 60 milk samples were collected from cows suffering from mastitis and processed for isolation and identification of S. aureus using standard conventional methods. All the recovered S. aureas isolates were subjected for detection of MRSA and/or MSSA employing phenotypic (Cefoxitin disc assay) and genotypic (the mecA gene PCR) assays. Antibiotic susceptibility pattern of LA-MRSA and LA-MSSA test isolates was determined using disc diffusion method, biofilm formation by 96 well microtiter plate assay and pathogenicity testing in G. mellonella larvae. On microbiological, biochemical and PCR analyses, 14 S. aureus isolates were confirmed. Of these, 4 were tested as LA-MRSA and the remaining 10 isolates were LA-MSSA. Comparative evaluation suggested that MRSA isolates were resistant to different classes of antibiotics and were equally lethal to G. mellonella larvae. However, bioflim forming ability was significantly higher (p < 0.001) in the MSSA test isolates. An association of biofilm formation and pahogenicity testing was not observed between LA-MRSA and LA-MSSA test isolates. Further, LA-MRSA were resistant to different classes of antibiotic and were more lethal to G. mellonella larvae. These preliminary observations are of great concern as the LA-MRSA infections in the community have been documented and warrant in depth research for such pathogens.