Phenotypical Identification of Atypical Staphylococcus Aureus Strains Isolated from Milk of Cows from One Herd

The aim of the study was to characterise the free coagulase-negative Staphylococcus sp. strains isolated from normal (9.4%) and inflamed quarter milk samples of dairy cows in one herd localised in southern Poland. The isolates were examined phenotypically by cultural features, tube coagulase test, clumping factor (CF), and commercial agglutination test. Total number of 427 strains were cultivated on different media: Congo red agar, DNase medium, mannitol salt agar, and crystal violet agar. About 25% of the strains were examined by the disc diffusion method for their sensitivity to chosen antibiotics. All of the analysed strains showed α, β, α+β, or lh type of haemolysis on plate with 5% sheep blood agar, and had positive reaction in CF and agglutination tests, but were negative in tube coagulase test. These strains showed DNase activity and ability to mannitol fermentation; however, none of the strains produced slime. There were variations in growth on crystal violet agar: examined strains belonged to three types of growth (A, E, and C). The strains were highly susceptible in vitro to oxacillin (99.2%), cefoperazone (95.2%), and amoxicillin with clavulanic acid (93.6%), and highly resistant to neomycin (27.2%), tetracycline (21.6%), and streptomycin (20.8%). In conclusion, it can be stated that intramammary infections in cows can be caused by atypical free coagulase-negative strains of Staphylococcus aureus.