Potential resident bacterial microbiota in udder tissues of culled cows sampled in abattoir

Aseptic milking samples and microbiological analyses are used in routine for bovine mastitis diagnosis. Few papers treated about a resident microbiota in the ruminant healthy mammary gland tissues (Spuria et al., 2017), or about immunological consequences related with a such cohabitation (Rainard, 2017). In practice, it’s difficult to sample mammary gland tissues out of risks for cows health or milk production.
We thus design a study based on samples taken at abattoir. It aimed at identify, quantify, compare the cow milk and mammary gland tissues microbiota of macroscopically healthy mammary glands, by classical microbiological analyses and by amplicon sequencing.
We harvested thirteen couples of milk secretion and tissue samples, originated from the same quarter of reformed cows. Aseptic milking has been done just before culling and mammary gland tissues had been taken of the carcasses on slaughterline. Total and specific microbiological counting and metagenetic analysis were performed.
Metagenetic analyses showed one main bacterial genus, Corynebacterium, generally found in the milk in higher proportions than in tissues. When it dominates clearly other populations in milk secretions, it can be found in the same quarter tissues. In case of identification of pathogenic bacteria in milk samples, the same pathogen were detected in tissues from the same quarters but in very different proportions: higher for Streptococcus uberis, lower for Staphylococcus spp or Enterococcus faecium. In tissues, Flavobacterium and Atopostipes genera were statistically more abundant than in milk. Data show also that species evenness and beta diversity are greater in mammary glands than in milk secretions. In opposition, species richness is higher in milk samples.
These results show a potential resident microflora in mammary glands of culled cows in abattoirs. Metagenetic analysis of milk samples could be a good indicator of the udder microbiota and health in the future but our first results must be completed and confirmed on a larger number of samples. Hypothesis about nature of such a resident flora will have to be confirmed on producing cows before studying bacterial-host interactions.
Rainard, P. 2017. Mammary microbiota of dairy ruminants: fact or fiction? Vet.Res., , 48 (25), 1-10.
Spuria, L. et al. 2017. Microbial agents in macroscopically healthy mammary gland tissues of small ruminants. PeerJ, 5.