In vitroantimicrobial activity of non-aureus StaphylococciandMammaliicocciagainstStaphylococcus aureusandStreptococcus uberisand their relationship with the presence of intramammary infections in organic dairy cows

Abstract

ABSTRACTThis study aimed to: (1) Assess thein vitroantimicrobial activity of non-aureus StaphylococciandMammaliicocci(NASM) isolates from organic cows teat apices with and without intramammary infections (IMI) byStaphylococcus aureus(SAU) orStreptococcusspp. andStreptococcus-like organisms (SSLO); (2) Investigate the relationship between IMI status and NASM counts on teat apex; (3) Explore the relationship between NASM species assigned taxonomy and the inhibitory activity against SAU andStreptococcus uberis(SUB). A total of 114 cows from 2 farms were sampled and followed from 8 weeks before to 5 weeks after calving. Weekly milk samples were cultured and used to design a case control study. Cases were defined as cows that had an IMI by SAU or SSLO, and controls were randomly chosen from cows without an IMI by these microorganisms. One teat apex sample from the week prior to IMI diagnosis was aerobically cultured. Taxonomy of recovered isolates was confirmed using mass spectrometry and whole genome sequencing. The inhibitory activity against SAU and SUB was investigated using agar dilution methods. Isolates were ranked based on in vitro inhibitory activity against SAU and SUB and categorized as “top 10” with the lowest minimum inhibitory concentration or not. The presence of NASM isolates classified as “top 10” against SAU was highest in controls (19.4%) compared to cases (5.8%). No difference in NASM counts on teat apex was observed between cases and controls. The inhibitory activity varied across NASM, withStaphylococcus succinusshowing the highest inhibitory activity against both SAU and SUB.