A coagulase-negative variant ofStaphylococcus aureusfrom bovine mastitis milk

Abstract

Bacteriological analysis of milk samples from quarters of a dairy cow suffering from subclinical mastitis yielded two isolates ofStaphylococcus aureuswhich gave a negative reaction in the standard coagulase test. Both isolates were also clumping factor and thermonuclease negative, and gave a negative reaction in the Staphaurex®test. The isolates were identified by using commercial biochemical systems, and by PCR analysis of different staphylococcal cell surface protein and exoprotein genes. Further molecular identification of the isolates, which included sequencing of the 16S rRNA gene and RT-PCR of coagulase (coa), clumping-factor (clfA) and thermonuclease (nuc) genes, was consistent with the diagnosis phenotypically ‘coagulase-negative variant ofStaph. aureus’. The fact that coagulase-negativeStaph. aureusvariants can occur in the context of intramammary infections in cattle may result in the incorrect diagnosis ‘coagulase-negative staphylococci (CNS)’ in routine mastitis diagnostic, at least in rare cases. To fully ensure correct species diagnosis, sequencing of the 16S rRNA gene and amplification of specific genes such ascoais necessary in these cases.