Date of Award

January 2013

Degree Type

Open Access Thesis

Document Type

Master Thesis

Degree Name

Master of Science (MS)


Biological Sciences

First Advisor

Marcia M. Pierce

Department Affiliation

Biological Sciences


Bovine mastitis is most significant disease seen in dairy farms worldwide, resulting in the largest profit loss of any other disease affecting dairy cows. The aim of this thesis was to determine the predominant species responsible for bovine mastitis in a subset of ten Kentucky dairy herds, and to assess the presence of antibiotic resistance in these pathogens. In this study, 308 milk samples were obtained from cow's selected based on their recent somatic cell count. Samples positive for growth were identified using the gram stain and various biochemical tests. After identification, resistance to 11 antimicrobial agents was assessed using the Kirby-Bauer test. Staphylococcus aureus was found to be the most common species causing bovine mastitis, which was identified in 13% of milk samples. Coagulase negative Staphylococci (11%) and streptococci species (10%) were also found to be major causes of mastitis in Kentucky. Only one isolate of Streptococcus agalactiae was identified, indicating that this species is not prevalent in this state. S. aureus isolates were highly susceptible to all antibiotics used in the laboratory, with the only minor resistance seen in penicillin (7%), ampicillin (5%), oxacillin (2%), and cephalothin (2%). Coagulase negative Staphylococci species showed their highest resistance to oxacillin (31%), pirlimycin (23%), tetracycline (17%), and ampicillin (14%). Streptococci species were the least susceptible group of all the major pathogens identified, with many of these species resistance to kanamycin (69%), tetracycline (59%), and oxacillin (50%). Overall, the major pathogens recovered in this study were largely susceptible to cephalosporins, indicating that this group of antibiotics may be effective in the treatment of Kentucky's common bovine mastitis infections.