Isolation and molecular identification of rare coagulase-negative Staphylococcus Aureus Variants Isolated from Bovine milk samples

Abstract

Staphylococci are broadly classified as coagulase-positive or-negative based on their ability to coagulate plasma. Besides the coagulase-positive Staphylococcus aureus, coagulase-negative staphylococci are also gaining significance in bovine mastitis. The precise grouping of staphylococci as coagulase-negative or-positive is an important preliminary step in establishing the aetiology of mastitis. While investigating the distribution of various species among 149 isolates obtained from bovine milk samples that were classified as coagulase-negative staphylococci based on the standard tube coagulase test, 19 isolates were identified as S. aureus by a species-specific polymerase chain reaction (PCR) for the presence of nuc, fib and 23S rRNA genes. Furthermore, characterisation revealed that all of them showed thermostable nuclease activity, but only 16 of them fermented mannitol. Only one of these was confirmed as belonging to S. aureus by phenotyping. None of the isolates except one possessed the antibiotic resistant genes mecA, aacA-Dor aph(3)-IIIa. Significantly, 14 of the isolates were positive by PCR for the coa gene. The study highlights the occurrence of rare coagulase-negative variants of S. aureus in bovine intramammary infections and the possibility of misidentification of these isolates as coagulase-negative staphylococci.