Abstract
To clarify the relative importance of pH and substrate starch level in fermentation characteristics and regulatory mechanism of Streptococcus bovis S1 in rumen acidosis, an in vitro fermentation of three levels of soluble starch (1, 3 and 9 g/L) was established with pH in the media were maintained constant at 5.5 or 6.5. The results showed that the dominant product of S. bovis S1 was lactate at both pH, the production depended on the starch level, and more lactate was produced at pH 6.5 than that at pH 5.5 (P < 0.001). At pH 5.5, the activity of lactate dehydrogenase (LDH) and α-amylase (α-AMY), their abundances, the relative expressions of LDH, PFL (gene encoding pyruvate formate-lyase), CCPA (gene encoding global catabolite control protein A) and α-AMY genes were higher than those at pH 6.5 (P < 0.05), whereas the concentration of fructose-1,6-diphosphate (FDP) was lower. The activity of LDH, α-AMY and FDP, and the relative expressions of LDH, PFL, CCPA and α-AMY genes were, in general, positively related to the starch level. The canonical regression analysis indicated that the pH had more profound effect compared with the starch level, in terms of the acid productions, enzyme activity and gene expressions. It was concluded that the fermentation of S. bovis was regulated at the transcription level in response to both pH and substrate starch concentration, but more sensitive to pH changes.
Highlights
Rumen acidosis is a common digestive disorder in ruminants fed high concentrate diets, and typically characterized with low rumen pH that results in changes in rumen fermentation and microbial profiles, disturbed intake and reduced productive performances (Tajima et al 2001; Rotger et al 2006)
Our previous work with dairy cows demonstrated that rumen acidosis was associated with an initially over growth of Streptococcus bovis (S. bovis) and the inability of lactate utilizing bacteria like Megasphaera elsdenii and Selenomonas ruminantium in the rumen (Wang et al 2015)
There was an interaction between pH and starch level on the formate production: decreasing at pH 6.5 but increasing at pH 5.5, with the starch levels (P < 0.05)
Summary
Rumen acidosis is a common digestive disorder in ruminants fed high concentrate diets, and typically characterized with low rumen pH that results in changes in rumen fermentation and microbial profiles, disturbed intake and reduced productive performances (Tajima et al 2001; Rotger et al 2006). The pH of the rumen fluid is about 6.5 (Wang et al 2015). When a large amount of starch is applied to the diet, ruminal pH decreases below 5.5 with an accumulation of volatile fatty acids (VFA) and lactate, leading to an occurrence of rumen acidosis (Kleen and Cannizzo 2012). These two events are usually simultaneous, and. The fermentative capacity of S. bovis has been studied and suggested that the pH in the rumen fluid was an important indicator in
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