The rapid accumulation of organic acids, particularly lactate, has been suggested as the main cause of ruminal acidosis (RA) for ruminants fed high-concentrate diets. Previous research has shown that a gradual shift from low-to high-concentrate diets within 4 to 5 weeks effectively reduces the risk for RA. However, the mechanisms remain unknown. In this study, 20 goats were randomly allocated into four groups (n = 5) and fed with a diet containing a weekly increasing concentrate portion of 20%, 40%, 60%, and 80% over 28 d. At d 7, 14, 21, and 28, one group (named C20, C40, C60, and C80 according to the last concentrate level that they received) was killed and the ruminal microbiome was collected. Ruminal acidosis was not detected in any of the goats during the experiment. Nonetheless, ruminal pH dropped sharply from 6.2 to 5.7 (P < 0.05) when dietary concentrate increased from 40% to 60%. A combined metagenome and metatranscriptome sequencing approach identified that this was linked to a sharp decrease in the abundance and expression of genes encoding nicotinamide adenine dinucleotide (NAD)-dependent lactate dehydrogenase (nLDH), catalyzing the enzymatic conversion of pyruvate to lactate (P < 0.01), whereas the expression of two genes encoding NAD-independent lactate dehydrogenase (iLDH), catalyzing lactate oxidation to pyruvate, showed no significant concomitant change. Abundance and expression alterations for nLDH- and iLDH-encoding genes were attributable to bacteria from Clostridiales and Bacteroidales, respectively. By analyzing the gene profiles of 9 metagenome bins (MAG) with nLDH-encoding genes and 5 MAG with iLDH-encoding genes, we identified primary and secondary active transporters as being the major types of sugar transporter for lactate-producing bacteria (LPB) and lactate-utilizing bacteria (LUB), respectively. Furthermore, more adenosine triphosphate was required for the phosphorylation of sugars to initiate their catabolic pathways in LPB compared to LUB. Thus, the low dependence of sugar transport systems and catabolic pathways on primary energy sources supports the acid tolerance of LUB from Bacteroidales. It favors ruminal lactate utilization during the adaptation of goats to a high-concentrate diet. This finding has valuable implications for the development of measures to prevent RA.

有机酸（尤其是乳酸）的快速积累被认为是饲喂高浓度饲料的反刍动物瘤胃酸中毒（RA）的主要原因。先前的研究表明，在 4 至 5 周内从低浓度饮食逐渐转向高浓度饮食可有效降低患 RA 的风险。然而，其机制仍然未知。在这项研究中，20 只山羊被随机分为四组（n = 5) 并在 28 天内饲喂每周增加 20%、40%、60% 和 80% 浓缩部分的饮食。在第 7、14、21 和 28 天，杀死一组（根据他们收到的最后浓缩物水平命名为 C20、C40、C60 和 C80）并收集瘤胃微生物组。实验期间没有在任何山羊中检测到瘤胃酸中毒。 尽管如此，当日粮浓度从 40% 增加到 60% 时，瘤胃 pH 值从 6.2 急剧下降到 5.7（P < 0.05）。宏基因组和宏转录组测序相结合的方法发现，这与编码烟酰胺腺嘌呤二核苷酸 (NAD) 依赖性乳酸脱氢酶 (nLDH) 的基因丰度和表达急剧下降有关，催化丙酮酸酶促转化为乳酸 ( P < 0.01），而编码不依赖于 NAD 的乳酸脱氢酶（iLDH）（催化乳酸氧化为丙酮酸）的两个基因的表达没有显示出显着的伴随变化。nLDH 和 iLDH 编码基因的丰度和表达变化分别归因于梭菌目和拟杆菌目细菌。通过分析 9 个具有 nLDH 编码基因的宏基因组箱 (MAG) 和 5 个具有 iLDH 编码基因的 MAG 的基因谱，我们确定了初级和次级活性转运蛋白是产乳酸菌 (LPB) 糖转运蛋白的主要类型，并且分别是乳酸利用细菌（LUB）。此外，与 LUB 相比，LPB 中糖的磷酸化需要更多的三磷酸腺苷来启动其分解代谢途径。因此，糖转运系统和分解代谢途径对主要能源的低依赖性支持拟杆菌目 LUB 的耐酸性。它有利于山羊适应高浓度饮食期间瘤胃乳酸的利用。这一发现对于制定预防 RA 的措施具有重要意义。