Atypical Staphylococcus Aureus as an Aetiological Agent of Mastitis in Cows

The aim of the study was to characterise the 493 tube-coagulase-negative Staphylococcus sp. strains isolated from the udder quarters of milking dairy cows in one herd with a high bulk milk somatic cell count (SCC). The isolates were examined phenotypically by cultural features, Gram stain, catalase, coagulase, and API biochemical tests, and genotypically by PCR for chosen genes. About 10% of the strains were examined by the disc diffusion method for their sensitivity to chosen antibiotics. All the analysed strains showed a α, β, or α+β zone of haemolysis on plates with 5% sheep blood agar, and they were Gram-positive cocci in clusters, catalase positive, API 20 Staph and slide coagulase test positive, but tube coagulase test negative. All the isolates from the 206 examined expressed the nuc gene, 97.4% of the 196 examined were positive for the coa gene, and 20.1% (from the 323 examined) had the blaZ gene. In addition, the 268 examined strains were negative for the mecA gene. The strains were highly susceptible in vitro to oxacillin and amoxicillin/clavulanic acid (100%), and penicillin, cloxacillin, cephalexin, cefoperazon, as well as bacitracine (more than 90%), and highly resistant to neomycin, streptomycin or tetracycline (more than 70%). The SCC in 21.4% of the premilk samples from the quarters infected with these atypical Staph. aureus was under 100,000/ml. In the remaining quarters, the SCC ranged from 101,000/ml to 2,000,000/ml, or even more. In conclusion, mastitis in cows can be caused by atypical coagulase tube negative strains of Staphylococcus aureus.