Staphylococcus aureus isolates from humans and cattle on Vermont dairy farms making farmstead cheese belong to different clonal complexes
Chakrawarti, Ashma
Chakrawarti, Ashma
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Abstract
Staphylococcus aureus is a multi-host pathogen causing significant human and livestock diseases. Several reports describe spillover events where a S. aureus strain has switched host and developed a novel lineage. Some of these lineages demonstrate antimicrobial resistance. There is a need for studies on the population structure of antimicrobial-resistant (AMR) S. aureus at the human-animal interface on farms. The objective of this study is to assess genetic diversity and AMR patterns of S. aureus strains isolated from cattle milk and humans on farms that make and sell farmstead cheese. Cow quarter milk, bulk tank milk, human hand and nasal swab samples from 21 Vermont dairy farms were cultured on selective and non-selective media. Isolation and identification of S. aureus used conventional cultural and biochemical methods. Presumptive S. aureus isolates were confirmed by nuc gene presence using multiplex PCR assay that also screened for AMR genes blaZ and mecA. S. aureus strain types were determined by multilocus sequence typing and cataloged on PubMLST database. Antimicrobial susceptibility was determined for a panel of 19 antimicrobials using disk diffusion and broth microdilution methods following CLSI guidelines. SAS and R for data analysis and Geneious® software were used for sequence analysis. A total of 182 S. aureus isolates grouped into twenty defined sequence types (STs) and four novel STs. The most common strain type, ST151 was isolated from 10 different farms and the sources were either bulk tank milk or cow quarter milk. All ST398 strains were of human origin and erythromycin resistant. About 26% of isolates were resistant to beta-lactam antibiotics, were blaZ gene positive CC5 and CC8 strains. S. aureus isolated from cows or humans on the same farm belonged to different STs suggesting limited spread between host species. AMR strains were more frequently isolated from humans on these farms.
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Graduate
Date
2020-01-01