Dynamics of the milk microbial community during subacute ruminal acidosis with or without intramammary lipopolysaccharide challenge in dairy cows

Lipopolysaccharides (LPS) from pathogenic Gram-negative bacteria play a key role in the pathophysiology of mastitis. Subacute ruminal acidosis (SARA) induces rumen dysbiosis, leading to LPS translocation and systemic immune activation. This study investigated the effects of a high-grain diet and intramammary LPS challenge on the milk microbiome of dairy cows. Cows were first fed a baseline control diet (day-7 to d-1; CON; 40% grain; n = 18). On d1, 12 cows were switched to a SARA diet (60% grain). On d30, six SARA (SARA_LPS) and the six CON cows (CON_LPS) were challenged intramammarily with LPS, while the other six SARA cows received a placebo (SARA_PLA). No CON_PLA group was enrolled. Milk samples were collected on d-2 (before feeding challenge), d30 (after feeding challenge; before LPS), and d32 (after LPS), and analysed using 16S rRNA gene amplicon sequencing and qPCR. During the feeding phases, more genera (70.1%) increased in CON than SARA cows, whereas more (65.3%) genera decreased their relative abundance in SARA compared to CON (p < 0.001). This decline persisted in SARA_PLA cows, with more genera (62.4%) decreasing their abundance (p < 0.001). However, LPS injection reversed the trend of the feeding effect, with more genera (79.3%) increasing in SARA_LPS cows in comparison to the other two groups, while more genera (85.5%) decreased in CON_LPS (p < 0.001) in comparison to the other two groups. Alpha diversity correlated positively with bacterial cell equivalents. Of all genera, 22.1% correlated negatively with milk amyloid A (MAA), which increased post-LPS injection, 21.7% positively with lactose, and 13.4% positively with milk urea. SCC showed significant differences in beta-diversity, but no distinct visual clustering nor many correlations. The microbial dynamics suggest that high-grain diet and the LPS injection influence the milk bacterial community. More taxa correlated with MAA than with SCC, suggesting that MAA may better reflect immune-microbial interactions in milk. A roughage-rich diet promoted higher microbial abundance, whereas high-grain feeding reduced abundance over the timespan of 30 days. Intramammary LPS challenge decreased absolute abundance in CON but increased it in SARA cows, suggesting a diet-dependent immune modulation of the mammary environment. These findings indicate that mammary gland epithelial integrity and immune mediators jointly shape the milk microbiome under metabolic and inflammatory stress.