Phenotypic and genotypic antimicrobial susceptibility of non-aureus staphylococci in dairy goats

Subclinical mastitis, most commonly due to non-aureus staphylococci (NAS), results in significant production losses within the dairy goat industry while also shortening the productive life of the animal due to the resultant chronic changes within the udder. The prevalence of this group of pathogens may be up to 50% in some dairies. Dry-off is a common production point to address these infections through the extra-label use of long-acting intramammary antimicrobial products which are approved for dairy cattle in the US. However, the impacts of this treatment on antimicrobial resistance development are unknown in goats. The objective of this project was to evaluate the impact of intramammary antimicrobial use at dry-off on antimicrobial susceptibility testing and antimicrobial resistance gene profiles of NAS isolates from commercial farms in the United States which were associated with persistent or new subclinical intramammary infection during the dry period in goats. The current study is a continuation of a previous study which evaluated the efficacy of intramammary antimicrobial treatment for curing subclinical mastitis caused by non-aureus staphylococci. That analysis included 260 does who provided the isolates for the current study. Antimicrobial sensitivity testing was performed using a commercially available broth microdilution plate designed for treatment of mastitis in dairy cattle on dry-off and post-kidding samples for isolates that failed to cure or developed new NAS infections after treatment with one of two β-lactam (cephaparin, CEPH; cloxacillin, CLOX) dry cow therapies or in non-treated control animals to evaluate the impact of targeted dry therapy on phenotypic and genotypic antimicrobial resistance development of NAS in dairy goats. The most prevalent NAS associated with non-cured infections for all three treatment groups was Staphylococcus epidermidis, while the most common NAS associated with new infections after kidding was S. equorum . There were small variations in MIC for multiple drugs when data was evaluated by treatment group and location, though these changes did not form a consistent pattern and are likely due to the small sample size and farm-level differences in pathogens. The minimum concentration of penicillin (surrogate test for β-lactamase producing strains) required to inhibit growth in 50% of isolates (MIC50) and 90% of isolates (MIC90) did not change for non-cured isolates after antimicrobial therapy across the entire study population. Whole genome sequencing was also used to identify antimicrobial resistance genes commonly associated with NAS species. Presence of the blaZ β-lactamase gene was highly correlated with increased penicillin MIC. Isolates with the highest MIC values (>4 μg/mL) were all positive for the gene, while 79% of isolates with penicillin MIC ≤0.12 μg/mL did not contain the blaZ gene. Overall, this study did not identify any significant changes in antimicrobial resistance gene profiles or MIC of isolates from treated animals compared to non-treated to indicate resistance gene selection for β-lactams in NAS after a single intramammary administration of CEPH or CLOX dry treatment in dairy goats.