Intramammary lipopolysaccharide infusion impacts the hindgut microbial composition and fermentation in dairy cows challenged with subacute ruminal acidosis.
Subacute ruminal acidosis (SARA) is a prevalent metabolic disorder which occurs due to high grain feeding. We recently observed that dairy cows experiencing SARA are more responsive, compared to healthy cows, when they are infected by exogenous lipopolysaccharide (LPS). However, the effect of translocated LPS on the microbial composition and metabolism in the hindgut of dairy cows has not yet been evaluated. The aim of this study was to determine the relationship between the hindgut microbial community and fermentation products in the hindgut of dairy cows experiencing SARA and effect of preconditioning on the response of individual animals and subjected to an exogenous LPS challenge. A total of 18 early-lactating Simmental cows were randomly divided into two different feeding groups: Control (CON, n = 6) and SARA (n = 12). The CON group received a total mixed ration (TMR) with 40% concentrate throughout the trial, whereas the SARA group was fed in an interrupted feeding program to induce a SARA challenge. The feeding program started with 60% concentrate in a TMR for 8 d, followed by 40% concentrate for  7 d, and then again 60% concentrate for the remaining 17 d of the trial. On d 30, 6 SARA (SARA-LPS) and 6 CON (CON-LPS) cows were intramammarily challenged with a single dose of 50 µg LPS from E. coli (O26:B6), diluted in 10 mL of sterile NaCl solution, while the remaining 6 SARA (SARA-PLA) received 10 ml of sterile NaCl solution as placebo (PLA). The interrupted feeding program resulted in a successful SARA challenge (previous paper?) and subsequent decreased fecal pH, increased total short chain fatty acids (SCFA) and changes in the proportions of acetate, butyrate and propionate in the  feces. The average abundance Firmicutes (82.45%, 10.81-92.33%), Proteobacteria (11.41%, 2.18-85.74%), Verrucomicrobia (1.33%, 0.02-5.57%), Euryarchaeota (0.45%, 0.08-1.40%), Lentisphaerae (0.08%, 0-0.36%), Elusimicrobia (0.04%, 0-0.32%), TM7 (0.005%, 0-0.019%), and Synergistetes (0.001%, 0-0.006%) was significantly different between feeding programs. The LPS challengedid not impact fecal pH or SCFA but did impact the relative abundance of Verrucomicrobia (1.13%), Actinobacteria (0.19%), and Spirochaetes (0.002%) . Overall, the shifting of microbial community was correlated with the fecal fermentation properties.