gut metagenome Metagenome

Background: The rapid accumulation of organic acid, particularly lactate, in excess of the rumen buffering capacity has been proposed as the main cause of ruminal acidosis (RA) for the ruminant animals fed with the high-concentrate diet, resulting in huge economic losses to the global feedlot. The practices showed that a gradual shift from low- to high-concentrate diets within 4-5 weeks could effectively reduce RA risk, and however, the mechanism has not been known yet. Results: Ruminal microbiome was collected from goats fed with a diet containing the weekly increased concentrate from 20% to 80%. RA was not detected for all the goats during the experiment. By applying the metagenome and metatranscriptome sequencing approaches, we found that the abundance and expression of the NAD-dependent lactate dehydrogenase genes (nLDHs), encoded the enzymes catalyzing the conversion of pyruvate to lactate and mainly identified in the bacteria from the order Clostridiales here, showed a sharp decrease when the dietary concentrate increased from 40% to 60% and ruminal pH dropped from 6.2 to 5.7, while the abundance and expression of two NAD-independent lactate dehydrogenase genes (iLDHs), encoded the enzymes catalyzing the oxidation of lactate to pyruvate and mainly identified in the bacteria from the order Bacteroidales here, had a significant decrease when the dietary concentrate increased from 60% to 80% and the rumen pH stayed at 5.7 - 5.8. By analyzing the gene profiles of 9 metagenome bins with nLDHs and 5 metagenome bins with iLDHs, we found that the primary active transporters and secondary active transporters were the major types of sugar transporters for the lactate-producing microbes (LPMs) and lactate-utilizing microbes (LUMs), respectively. Furthermore, more ATPs were required for the phosphorylation of sugars in the initial of their catabolism pathways for LPMs than that of LUMs. Conclusions: The acid tolerance of the LUMs from Bacteroidales, due to the low dependence of sugar transporter systems and catabolism pathways on the primary energy sources, i.e., ATPs and phosphoenolpyruvates, were suggested to contribute to the rumen lactate homeostasis during the goats adapted to a high-concentrate diet. This finding has significant significance for the development of economic measures to prevent RA in feedlots.