Subacute ruminal acidosis (SARA) represents one of the most important digestive disorders in intensive dairy farms, and dairy cows are individually different in the severity of SARA risk. The objectives of the current study were to investigate differences in the ruminal bacterial community and metabolome in dairy cattle with different susceptibility to SARA. In the present study, 12 cows were initially enrolled in the experiment. Based on average ruminal pH, 4 cows with the lowest ruminal pH were assigned to the susceptible group (SUS, pH = 5.76, n = 4) and 4 cows with the highest ruminal pH assigned to the tolerant group (TOL, pH = 6.10, n = 4). Rumen contents from susceptible (SUS, n = 4) and tolerant (TOL, n = 4) dairy cows were collected through rumen fistula to systematically reveal the rumen microbial and metabolic alterations of dairy cows with different susceptibility to SARA using multi-omics approaches (16S and 18S rRNA gene sequencing and metabolome). The results showed that despite being fed the same diet, SUS cows had lower ruminal pH and higher concentrations of total volatile fatty acids (VFA) and propionate than TOL cows (P < 0.05). No significant differences were observed in dry matter intake, milk yield, and other milk compositions between the SUS and TOL groups (P > 0.05). The principal coordinates analysis based on the analysis of molecular variance indicated a significant difference in bacterial composition between the two groups (P = 0.01). More specifically, the relative abundance of starch-degrading bacteria (Prevotella spp.) was greater (P < 0.05), while the proportion of fiber-degrading bacteria (unclassified Ruminococcaceae spp., Ruminococcus spp., Papillibacter, and unclassified Family_XIII) was lower in the rumen of SUS cows compared with TOL cows (P < 0.05). Community analysis of protozoa showed that there were no significant differences in the diversity, richness, and community structure (P > 0.05). Metabolomics analysis revealed that the concentrations of organic acids (such as lactic acid), biogenic amines (such as histamine), and bacterial degradation products (such as hypoxanthine) were significantly higher in the SUS group compared to the TOL group (P < 0.05). These findings revealed that the higher proportion of starch-degrading bacteria/lower fiber-degrading bacteria in the rumen of SUS cows resulted in higher VFA-producing capacity, in particular propionate. This caused a disruption in metabolic homeostasis in the rumen which might be the reason for the higher susceptibility to SARA. Overall, these findings enhanced our understanding of the ruminal microbiome and metabolic changes in cows susceptible to SARA.

亚急性瘤胃酸中毒 (SARA) 是集约化奶牛场中最重要的消化系统疾病之一，奶牛在 SARA 风险的严重程度方面存在个体差异。本研究的目的是调查对 SARA 具有不同敏感性的奶牛瘤胃细菌群落和代谢组的差异。在本研究中，最初有 12 头奶牛参加了实验。根据平均瘤胃 pH 值，4 头瘤胃 pH 值最低的奶牛被分配到易感组（SUS，pH = 5.76，n  = 4），4 头瘤胃 pH 值最高的奶牛被分配到耐受组（TOL，pH = 6.10，n  = 4)。易感（SUS，n  = 4）和耐受（TOL，n = 4) 通过瘤胃瘘收集奶牛，使用多组学方法（16S 和 18S rRNA 基因测序和代谢组学）系统揭示对 SARA 不同易感性奶牛的瘤胃微生物和代谢变化。结果表明，尽管饲喂相同的日粮，SUS奶牛的瘤胃pH值较低，总挥发性脂肪酸（VFA）和丙酸盐浓度高于TOL奶牛（P  < 0.05）。SUS组和TOL组干物质采食量、产奶量及其他乳成分差异均无统计学意义（P  > 0.05）。基于分子方差分析的主坐标分析表明两组细菌组成存在显着差异（P = 0.01)。更具体地说，淀粉降解菌（普氏菌属）的相对丰度较大（P  < 0.05），而纤维降解菌（未分类的瘤胃球菌科、瘤胃球菌、乳头状杆菌和未分类的家族_XIII）的比例较低SUS 奶牛瘤胃与 TOL 奶牛比较 ( P  < 0.05)。原生动物群落分析表明，在多样性、丰富度和群落结构上没有显着差异（P > 0.05)。代谢组学分析显示，SUS组的有机酸（如乳酸）、生物胺（如组胺）和细菌降解产物（如次黄嘌呤）的浓度显着高于TOL组（P  < 0.05） . 这些发现表明，SUS 奶牛瘤胃中较高比例的淀粉降解细菌/低纤维降解细菌导致较高的 VFA 生产能力，特别是丙酸盐。这导致瘤胃中代谢稳态的破坏，这可能是对 SARA 更高易感性的原因。总体而言，这些发现增强了我们对 SARA 易感奶牛瘤胃微生物组和代谢变化的理解。