The isolates of Staphylococcus aureus strains were examined phenotypically by cultural features, tube coagulase test and clumping factor (CF), and genotypically by conventional PCR. The strains had positive reaction in CF test, but were negative in tube coagulase test. The analysed strains from the same cows in each year expressed also nuc and coa genes. About 25% of the strains were examined by the disc diffusion method for their sensitivity to antibiotics. During three years, the strains were highly susceptible in vitro to amoxicillin with clavulanic acid, oxacillin, bacitracin, and cefoperazone (more than 90%), and highly resistant to tetracycline, neomycin, and streptomycin. Forty randomly chosen strains, and eight strains from the same cows in each year were analysed for minimal inhibitory concentration of penicillin G using microdilution method. An increasing resistance to the penicillin was noted. Moreover, eight strains, the same in each year, were also examined for β-lactamase production and methicillin resistance. No β-lactamase producers and no methicillin resistant strains were found using phenotypic and genotypic methods. In conclusion, it can be stated that antimicrobial susceptibility can change from one year to another.
