Antimicrobial resistance of Staphylococcus aureus isolates from dairy cows and genetic diversity of resistant isolates

Authors: ABDI, R - University Of Tennessee; GILLESPIE, B - University Of Tennessee; VAUGHN, J - University Of Tennessee; MERRILL, C - University Of Tennessee; HEADRICK, S - University Of Tennessee; ENSERMU, D - University Of Tennessee; D'SOUZA, D - University Of Tennessee; Agga, Getahun; ALMEIDA, R - University Of Tennessee; OLIVER, S - University Of Tennessee; KERRO, DEGO - University Of Tennessee

Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2017
Publication Date: 2/4/2018
Citation: Abdi, R.D., Gillespie, B.E., Vaughn, J., Merrill, C., Headrick, S.I., Ensermu, D.B., D'Souza, D.H., Agga, G.E., Almeida, R.A., Oliver, S.P., Kerro, D.O. 2018. Antimicrobial resistance of Staphylococcus aureus isolates from dairy cows and genetic diversity of resistant isolates. Foodborne Pathogens and Disease. https://doi.org/10.1089/fpd.2017.2362.
DOI: https://doi.org/10.1089/fpd.2017.2362

Interpretive Summary: Inflammation of the udder (commonly known as mastitis) is a common problem in dairy cows that can cause huge economic losses to the dairy industry. Staphylococcus aureus is a common bacterial cause of mastitis in dairy cows. Antibiotics are commonly used in dairy cattle production for the prevention and treatment of mastitis in cows. The widespread use of antibiotics in dairy cattle production system can select for antibiotic resistant bacteria that can also infect human beings. A collection of 239 Staphylococcus aureus isolates that were obtained from 33 dairy farms in the State of Tennessee between 2004 and 2016 were tested for their antibiotic resistance and genetic diversity. About one-third of the isolates were found to be resistant to at least one antibiotic. Twenty six farms were positive for antibiotic resistant Staphylococcus aureus isolate. Resistance to 3rd generation cephalosporin drug ceftiofur, which is analogue to ceftriaxone a critically important antibiotic for human medicine, was very low. Resistant strains showed 20 resistance patterns and nine genotypic types indicating high phenotypic and genetic diversity of the resistant isolates. This study indicates the presence of farm to farm variation in the occurrence of antibiotic resistant Staphylococcus aureus isolates and the genetic makeup of the isolates. Responsible use of antibiotics in dairy cattle production can reduce the spread of antibiotic resistant bacteria in the dairy cattle production environment.

Technical Abstract: Staphylococcus aureus is a frequent and major contagious mastitis bacterial pathogen. The antibiotic treatment cure rates vary considerably from 4% to 92%. Staphylococcus aureus readily becomes resistant to antibiotics, resulting in persistent noncurable intramammary infection that usually results in culling of infected animals. Because of its notorious ability to acquire resistance to the commonly used as well as last resort antimicrobials such as methicillin and vancomycin and the development of multidrug-resistant strains, antimicrobial resistance (AMR) in S. aureus is of paramount importance in human medicine. The objective of this study was to evaluate the prevalence of AMR and genetic diversity of S. aureus isolates from milk of dairy cattle. Staphylococcus aureus isolates (n = 239) from 33 dairy farms in Tennessee were tested against 10 antimicrobials by broth microdilution method using the Sensititer system. Genetic diversity of resistant isolates was evaluated by pulsed-field gel electrophoresis (PFGE). Overall, AMR of the S. aureus isolates varied from as low as 1.3% for ceftiofur to as high as 25% for sulfadimethoxine. Out of 239 S. aureus isolates, 82 (34.3%) of them were resistant to at least 1 of the 10 antimicrobials. The AMR isolates belonged to two major PFGE types, indicating the presence of dominant clonal patterns among the resistant isolates. In general, there was a variation of prevalence of AMR within and among farms over time, with an increasing trend in tetracycline resistance. Judicious use of antimicrobials in dairy cattle farms can reduce the development of antimicrobial-resistant S. aureus.