Genotypes and phenotypes of staphylococci on selected dairy farms in Vermont

The genus Staphylococcus contains at least 47 species and 23 subspecies. Bacteria in this genus are ubiquitous; many are commensals on human and animal skin and can be opportunistic pathogens. In dairy cattle, staphylococci are the leading cause of intramammary infections (IMI) and mastitis. Mastitis is the inflammation of the mammary gland, and is one of the leading infectious diseases causing production losses in the dairy industry. Staphylococcus species other than Staphylococcus aureus are referred to as non-aureus staphylococci (NAS). In the dairy industry, S. aureus is the most common mastitis causative agent and is considered a major mastitis pathogen compared to the NAS, which as a group have been described as minor mastitis pathogens. The NAS species are increasingly recognized as an important cause of bovine mastitis, although the relative role of some species is still uncertain. Our understanding of the local and global epidemiology of NAS mastitis is improving with application of more accurate DNA sequence-based species identification methods and techniques to discriminate between strains within species.

The major goals of this project, part of my PhD dissertation, were to describe Staphylococcus mastitis epidemiology, and to identify phenotypes that may contribute to persistence in various niches on selected dairy farms in Vermont. We conducted 2 field studies on 2 groups of farms in Vermont. In the first study, we collected S. aureus isolates from bulk tank milk of

44 certified organic dairy farms. In the second field study, we completed quarter milk, cow skin, and environmental sampling of 5 herds that make farmstead cheeses. In both studies, we used non-selective and selective agar media to isolate staphylococci from the farm sources. From these studies, we collected 1,853 Staphylococcus spp. isolates. We used PCR-amplicon sequence-based species identification to describe Staphylococcus species diversity on these selected Vermont dairy farms. S. aureus isolates were strain-typed using an established Multilocus Sequence Typing (MLST) scheme. A novel MLST scheme was developed to investigate the molecular epidemiology of S. chromogenes, one of the leading NAS species causing bovine mastitis. We also evaluated antibiotic resistance and biofilm formation phenotypes and genotypes of staphylococci to test the hypothesis that these phenotypes may be associated with strain types.
In the study of organic dairy farms, 20 S. aureus strain types (STs) were identified, including ten novel STs. The majority of STs belonged to clonal complexes (CCs) previously identified as cattle adapted (e.g. CC97 and CC151). Associations between ST and carriage of beta-lactam resistance and biofilm forming capacity were identified among the S. aureus isolates from these farms. In the 5-herd study, 27 different staphylococci species were identified from various niches including humans, but only five species; S. chromogenes, S. aureus, S. haemolyticus, S. simulans, and S. xylosus were commonly identified to cause IMI. S. aureus and S. chromogenes strain types were niche specific.

Robert Mugabi was born and raised in Uganda. He holds a Bachelor of Veterinary Medicine (BVM) degree from Makerere University, Uganda, a Master of Science (MS) degree from North Dakota State University, USA, and a PhD from the University of Vermont, USA under Dr. John Barlow’s mentorship. Robert’s research interests include; Molecular Epidemiology, Food Safety Microbiology, Antimicrobial Resistance, and Bacterial Genomics. Currently, he is an Associate/Postdoctoral Scientist at Kansas State University, College of Veterinary Medicine, working with Dr. Victoriya Volkova. In his free time, Robert enjoys playing football and watching the English premiership league football games.

Text and picture: Robert Mugabi