Rumen microbiome structure and metabolites activity in dairy cows with clinical and subclinical mastitis

Abstract Background: With high prevalence and complex etiology, bovine mastitis (BM) often causes serious damage to dairy cows’ health and milk quality. Although the shift in milk has been the main research interest on mastitis, recent studies have suggested that gastrointestinal microorganism has a crucial effect on the inflammation of the tissues outside the gut, such as mammary gland. However, studies related to the variation of rumen inner-environment during mastitis were limited. This study compared the differences of rumen microorganisms and metabolites under different udder health conditions, in order to investigate the correlation between udder health status and rumen microbiome structure and metabolites activity. Based on the somatic cell counts (SCC), California mastitis test (CMT) parameters and clinical symptoms of mastitis, 60 lactating Holstein dairy cows with similar body conditions (excepted for udder health condition) were divided into 3 groups (n = 20 per group): healthy (H) group, subclinical mastitis (SM) group and clinical mastitis (CM) group. Lactation and rumen fermentation parameters were detected, and rumen microbiota and metabolites were analyzed via 16S rRNA amplicon sequencing and untargeted metabolomics, respectively. Results: As the degree of mastitis increased, milk SCC and milk protein were significantly increased (P < 0.01). Milk yield, milk fat and lactose (P < 0.01), and rumen lactic acid (LA) (P < 0.01), acetate, propionate, butyrate, valerate (P < 0.001), total volatile fatty acids (TVFAs) concentrations (P < 0.01) and pH value (P = 0.024) were significantly decreased. In the rumen of CM cows, the significantly increased pathogens related to intestinal and oral inflammation, such as g__Lachnospiraceae_UCG-006 (P = 0.03), g__Moraxella and g__norank_f__Neisseriaceae (P < 0.01), etc., were accompanied by a significant increase in 12-oxo-20-dihydroxy-leukotriene B4 and 10beta-Hydroxy-6beta-isobutyrylfuranoeremophilane (P < 0.01). Meanwhile, in the rumen of SM cows, the g__Ruminiclostridium_9 (P = 0.04) and g_Enterorhabdus (P = 0.01) were increased with methenamine, 5-Hydroxymethyl-2-furancarboxaldehyde (5-HMF) and 6-Methoxymellein (P < 0.01). During mastitis, the decrease of short-chain fatty acids (SCFAs)-producing bacteria and intestinal probiotics in rumen, including g__Prevoterotoella_1 and g__Bifidobacterium (P = 0.03), etc., were significantly reduced, accompanied by a reduction in 2-Phenylbutyric acid (2-PBA) (P < 0.01).Conclusion: This study suggested that the microflora and metabolites associated with pathogen invasion, inflammatory response and rumen wall damage in the rumen of cows suffered mastitis were prominent.