Methicillin-resistant Staphylococcus aureus (MRSA) Prevalence in Mastitic Dairy Cows from San Bernardino County
Authors:Laura Banks, Anisah Mohd Zin
Mentor:Shelton Murinda, Associate Professor – Animal & Veterinary Sciences/Food Safety, California State Polytechnic University, Pomona
Staphylococcus aureus (Staph. aureus) is a major pathogen associated with mastitis, an intramammary infection that affects dairy cows. Mastitis can result in major economic losses to the dairy industry, and is a potential health risk to humans who consume raw milk and products manufactured with raw milk. Although Staph. aureus is a common cause of mastitis in dairy cows, few studies have been conducted on the prevalence of methicillin resistant Staph. aureus (MRSA) in the United States. The objective of this study was to determine the prevalence of MRSA in dairy cows in Chino and Ontario, CA. A total of 318 milk samples were collected from individual cows and were used for isolation of bacteria on blood agar (tryptic soy agar; TSA with 5%/10% sheep blood). The isolates were purified on TSA and further characterized by gram staining and morphology. There were 71 (22%) presumptive Staph. aureus isolates. For confirmation of isolates as Staph. aureus or MRSA, PCR that targets amplification of the mecA (methicillin resistance), nuc (thermostable nuclease), coa (coagulase), mup (mupirocin resistance) and Staphylococcus genus specific 16s rRNA was used. Specificity and sensitivity of tests were conducted using bacterial strains that are positive (n=5) and negative (n=15) for the gene sequences. There were no false positive PCR tests, however, there was one false negative test. Preliminary results suggest a moderately low prevalence of Staph. aureus. Current studies are testing mecA positive isolates for antibiotic sensitivity to cefoxitin and oxacillin (which define MRSA ), and mupirocin, to identify and establish the prevalence of MRSA.