DOI 10.1515/pjvs-2016-0105 Original article Effects of udder infections with Staphylococcus xylosus and Staphylococcus warneri on the composition and physicochemical changes in cows milk M. Vasil 1 , E. Pecka-Kielb 2 , J. Elec ˇ ko 1 , A. Zachwieja 3 , W. Zawadzki 2 , F. Zigo 1 , J. Illek 4 , Z. Farkas ˇ ova ´ 1 1 Department of Animal Breeding, University of Veterinary Medicine and Pharmacy, Komenskeho 73, Kosice 041 81, Slovak Republic, 2 Wroclaw University of Environmental and Life Sciences, Department of Biostructure and Animal Physiology, Norwida 31, 50-375 Wroclaw, Poland 3 Wroclaw University of Environmental and Life Sciences, Department of Cattle Breeding and Milk Production, Chełmońskiego 38c, 51-630 Wrocław, Poland 4 Clinical Laboratory for Large Animals, Faculty of Veterinary Medicine, University of Veterinary and Pharma- ceutical Sciences, Brno, Czech Republic Abstract The aim of this study was an evaluation of the effects of two species of coagulase negative staphylococci (CNS), Staphylococcus xylosus and Staphylococcus warneri, on the changes in techno- logical parameters of cows’ milk. The study was conducted in a herd of Slovak Pied cattle breed (with share of HF blood). Based on the performance results from three subsequent months, cows in the 2 nd and 3 rd lactation with SCC up to 200 thousand/ml (8 heads, 32 quarters), and above 800 thousand/ml (8 heads, 32 quarters), after the 4 th month of lactation, were selected. The samples were subjected to microbiological analysis, total bacteria count, somatic cell count; basic milk composition and physicochemical properties were also examined. The research has found the impact of bacterial infection on the increasing (p<0.05) of the number of somatic cells and a decrease (p<0.01) in protein levels in milk due to both types of staphylococci. There was a significant (p<0.05) reduction in C8: 0, C10: 0 and the total amount of saturated fatty acids in the milk of the infected cows in comparison to the healthy ones. The research also revealed higher (p<0.01) levels of C14: 0 and C20: 1 in milk from the healthy cows. Key words: cows, milk, Staphylococcus xylosus, Staphylococcus warneri, physicochemical properties Correspondence to: M. Vassil, e-mail: milan.vasil@uvlf.sk and E. Pecka-Kiełb, e-mail: ewa.pecka@up.wroc.pl Polish Journal of Veterinary Sciences Vol. 19, No. 4 (2016), 841–848