Rumen Bacteria Abundance and Fermentation Profile during Subacute Ruminal Acidosis and Its Modulation by <i>Aspergillus oryzae</i> Culture in RUSITEC System

This study aimed at characterizing changes in rumen bacteria abundance and fermentation profiles by artificial saliva (AS) pH, and at evaluating the potential modulatory role of <i>Aspergillus oryzae</i> culture (AOC) in a rumen simulation technique (RUSITEC) system. The treatment included high AS pH (pH 6.8) or low AS pH (pH 5.5) according to the McDougall’s method, and low AS pH was sustained by changing the composition of the AS (NaHCO₃ from 9.8 to 1.96 g/L, Na₂HPO₄ from 9.3 to 1.86 g/L). In low AS pH condition, the diets contained either 0% AOC, 1.25% AOC, or 2.5% AOC. Therefore, there are four treatments: (1) high AS pH, 0% AOC (HASP); (2) low AS pH, 0% AOC (AOC0); (3) low AS pH, 1.25% AOC (AOC1); (4) low AS pH, 2.5% AOC (AOC2), respectively. The experimental diets were supplemented with 16 g basic diets with the forage to concentrate ratio of 40:60. The experiments were conducted two independent 13 days, with 9 days adaption periods and 4 days sample collection. The results showed that low AS pH decreased the degradabilites of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) (<i>p</i> < 0.05), which occurred due to a decreased abundance of fibrolytic <i>Ruminococcus albus</i> (<i>p</i> < 0.001). The total concentration of volatile fatty acid (VFA) and proportion of propionate were decreased in the low AS pH (<i>p</i> = 0.026) and tended to increase the molar proportion of butyrate (<i>p</i> = 0.086) and the ratio of acetate to propionate (<i>p</i> = 0.088). The abundances of phylum <i>Firmicutes</i> (<i>p</i> = 0.065) and <i>Proteobacteria</i> (<i>p</i> = 0.063) tended to be greater in low AS pH group than high AS pH group. Low AS pH increased the abundance of phylum <i>Actinobacteria</i> (<i>p</i> = 0.002) compared to the high AS pH and decreased the abundances of phylum <i>Spirochaetes</i> (<i>p</i> = 0.032). Compared with the high AS pH, low AS pH increased the abundances of <i>Prevotella</i> (<i>p</i> = 0.003), <i>Pseudoscardovia</i> (<i>p</i> = 0.001), <i>Mitsuokella</i> (<i>p</i> = 0.005), and <i>Dialister</i> (<i>p</i> = 0.047), and decreased the abundances of <i>Olivibacter</i> (<i>p</i> = 0.026), <i>Ruminobacter</i> (<i>p</i> = 0.025), <i>Treponema</i> (<i>p</i> = 0.037), and <i>Sphaerochaeta</i> (<i>p</i> = 0.027) at genus level. Under a severe SARA in RUSITEC, supplementation of 2.5% AOC increased OM degradability, the copy numbers of <i>Selenomonas ruminantium</i> and <i>Fibrobacter succinogenes</i>. These findings indicate that the reduction AS pH at 5.5 caused a strong shift in bacterial composition in rumen. In addition, the addition of AOC in diets increased the growth rate of certain rumen bacteria that digest fiber or utilize lactate under SARA condition in RUSITEC system.