Exploring associations between the teat apex metagenome and Staphylococcus aureus intramammary infection risk in primiparous cows under organic directives

The primary objective of this study was to identify associations between teat apex microbiome and Staphylococcus aureus intramammary infection (IMI) risk in primiparous cows during the first 5 weeks after calving. We performed a case-control study using shotgun metagenomics of the teat apex and culture-based milk data collected longitudinally from 710 primiparous cows on 5 organic dairy farms. We observed a strong association between S. aureus DNA in the metagenomic teat apex data prior to parturition and the odds of S. aureus IMI after parturition (OR = 38.9, 95% CI: 14.84-102.21).  Differential abundance analysis confirmed this association, with cases having a 23.8 higher log fold change (LFC) in abundance of S. aureus in their samples compared to controls. Of the most prevalent microorganisms in controls, those associated with a lower risk of post-calving S. aureus IMI included Microbacterium phage Min 1 (OR = 0.37, 95% CI: 0.25-0.53), Corynebacterium efficiens (OR = 0.53, 95% CI: 0.30-0.94), Kocuria polaris (OR = 0.54, 95% CI: 0.35-0.82), Micrococcus terreus (OR = 0.64, 95% CI: 0.44-0.93) and Dietzia alimentaria (OR = 0.45, 95% CI: 0.26-0.75). Microcin B17 was the most prevalent antibacterial peptide on the teat apex of cases and controls (99.7% in both groups). The predicted abundance of Microcin B17 was also higher in cases compared to controls (LFC 0.26). Cow and farm random effects often explained a large proportion of the observed variability in the teat apex microbiome, suggesting that our results need to be interpreted within the context of the random effects.