Bacteria variety causing clinical mastitis in Holstein-Friesian cows in Pelagonia region, North Macedonia

The main aim of this study are the bacteria that most often cause clinical mastitis (CM) and their impact on milk reduction in Holstein-Friesian cows in the Pelagonia – North Macedonia region. 36 milk samples were taken from Holstein-Friesian breed of cows with confirmed clinical mastitis by a veterinarian. The samples were taken for the period from January 2019 to December 2020 from 20 different smallholder farms situated in the monitored region. Two sterile tubes with 10 ml of milk in each of them were taken from the affected part of the udder of the cow. A total of 86 tubes with milk from 36 mastitis cows were taken. From each sample 300 µl drips were placed in petri dishes with different selective nutrient media: Mannitol Salt Agar, MacConkey Agar, Endo Agar and Edwards nutrient medium. The petri dishes were incubated at 35±2°C for 24-48 hours in Mannitol Salt Agar, at 30-35°C for 18 to 72 hours in MacConkey Agar, at 35±2°C for 18 to 24 hours in Endo Agar and at 35-37°C for 24-48 hours in Edwards nutrient medium. Morphology of colonies and cells were examined with a microscope. A total of 119 strains were obtained and the following physiological and biochemical studies were performed to determine the new isolates: oxidase reaction, catalysis activity, indol test, hydrolysis of the hyporate, acetoin formation (acetylmethylcarbinol, Voges-Proscauer reaction) and Methyl-Roth test (MR- test). The results obtained revealed that the most common bacterial species causing clinical mastitis in Holstein-Friesian cows in 2019 and 2020 were six species of bacteria, where E. coli and Staphylococcus spp. are dominant.