Investigation of antibiotic resistance genes in staphylococcal isolates from bovine milk

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Modern medical and veterinary practice relies on effective antibiotics to prevent and treat infections in humans and animals. The prevalence of antibiotic resistance in bacteria is one of the most intractable challenges in 21st century public health. Approximately 80% of the total antibiotic consumption are used in food animals. Thus, a better understanding of the impacts of antibiotics used on farms would produce a more robust platform for public policy. The most common reason for antibiotic use on dairy farms is to treat and prevent mastitis. Mastitis is a disease of major economic importance (approximately $350 million loss per year for the Canadian dairy industry). Mastitis can be caused by both Gram-positive and Gram-negative bacteria. Most antimicrobial therapies for mastitis have been targeted against Gram-positive bacteria. The most common Gram-positive bacteria from bovine mammary glands are staphylococci, including Staphylococcus aureus and coagulase-negative staphylococcus (CNS). Most milk S. aureus and CNS isolates are multi-drug resistant. A particular type of antimicrobial resistance among staphylococci, methicillin resistance (MR), has been a recent focus of mastitis researchers, since members of the -lactam class of antibiotics are the most commonly used drugs on dairy farms. My long-term research objective is to understand the host-pathogen interaction during mastitis. We have been working on antibiotic resistance of milk S. aureus and CNS isolates for the last several years. My short-term objectives for the next 5 years are (1) to reveal organizational and structural characteristics of these isolates, especially for antibiotic resistant genes and to understand the phylogenetic relationship between milk staphylococcal isolates and published staphylococcal strains from other animals and humans; (2) to determine whether and how regulatory RNAs (called small RNAs for bacteria) regulate expression of antibiotic resistant genes; (3) to study the horizontal gene transfer of antibiotic resistance genes in vitro and in vivo between S. aureus and CNS. The proposed experiments are original and innovative. Novel and potentially transformative findings are expected for preventing or reducing the transfer of antibiotic resistance. This proposed research will have an impact on management and treatment of mastitis, in terms of choice of antibiotics based on resistance patterns and development of new treatment strategies. By reducing occurrence of mastitis, it will have a positive impact on animal welfare. In addition, understanding antibiotic resistant genes in S. aureus and CNS isolates from milk is relevant to human health because MRSA and MRCNS from milk potentially pose a human health hazard, both through horizontal transfer of resistance genes between staphylococcal species and through direct transmission of resistant pathogens between humans and animals.