| Current definition of subacute ruminal acidosis (SARA) is based on low rumen pH typically generated on high starch or low coarse fiber diets. However, some researchers believe rumen pH has a low correlation with SARA symptoms, and that other indicators such as free rumen lipopolysaccharide (LPS) must be included in the definition of SARA. In this research, we developed two animal models of SARA, one based on wheat-barley pellets and the other based on alfalfa pellets, to determine if different nutritional models result in a similar increase in free LPS in the rumen fluid and inflammatory markers in peripheral blood, and if this inflammatory response is due to the LPS. Additionally, the rumen microbial community dynamics were compared in grain- versus alfalfa pellet-induced SARA.;Based on adopted threshold of SARA of at least 180 min/d of rumen pH < 5.6, SARA was successfully induced with both nutritional models. Rumen free LPS concentration increased during both grain- (28,184 vs. 107,152 EU/mL) and alfalfa pellet-induced SARA (38,019 vs. 165,959 EU/mL). In grain-induced SARA, this increase was accompanied by an increase in LPS and inflammatory indicators including serum amyloid-A, haptoglobin, and LPS-binding protein in the peripheral blood. However, LPS translocation or immune activation did not occur in alfalfa pellet-induced SARA. Rumen bacterial community analysis indicated that the proportion of phylum Bacteroidetes, which are likely the largest contributor to free rumen LPS pool, declined from 35% in alfalfa pellet- to 16% in grain-induced group. Moreover, the severity of SARA and inflammatory response were associated with an increase in Escherichia coli and not free LPS in the rumen. E. coli populations differed between the grain- and alfalfa pellet-induced SARA. E. coli isolates from control and alfalfa pellet-induced SARA fell into one cluster, while isolates from grain-induced SARA grouped into a separate cluster. These results suggest that inflammatory responses observed with grain-induced SARA might be due to a sub-population of E. coli from grain based diet that could possess virulence features. Alternately, grain-induced SARA could have modified the hindgut microbiota by increasing the amount of starch in the intestine, and thus, LPS translocation might have occurred in the intestine rather than the rumen. |
