Regulation of CcpA on the growth and organic acid production characteristics of ruminal Streptococcus bovis at different pH

Background Catabolite control protein A (CcpA) regulates the transcription of lactate dehydrogenase and pyruvate formate-lyase in Streptococcus bovis, but knowledge of its role in response to different pH is still limited. In this study, a ccpA-knockout strain of S. bovis S1 was constructed and then used to examine the effects of ccpA gene deletion on the growth and fermentation characteristics of S. bovis S1 at pH 5.5 or 6.5. Results There was a significant interaction between strain and pH for the maximum specific growth rate (μmax) and growth lag period (λ), which caused a lowest μmax and a longest λ in ccpA-knockout strain at pH 5.5. Deletion of ccpA decreased the concentration and molar percentage of lactic acid, while increased those of formic acid. Strains at pH 5.5 had decreased concentrations of lactic acid and formic acid compared to pH 6.5. The significant interaction between strain and pH caused the highest production of total organic acids and acetic acid in ccpA-knockout strain at pH 6.5. The activities of α-amylase and lactate dehydrogenase decreased in ccpA-knockout strain compared to the wild-type strain, and increased at pH 5.5 compared to pH 6.5. There was a significant interaction between strain and pH for the activity of acetate kinase, which was the highest in the ccpA-knockout strain at pH 6.5. The expression of pyruvate formate-lyase and acetate kinase was higher in the ccpA-knockout strain compared to wild-type strain. The lower pH improved the relative expression of pyruvate formate-lyase, while had no effect on the relative expression of acetate kinase. The strain × pH interaction was significant for the relative expression of lactate dehydrogenase and α-amylase, both of which were highest in the wild-type strain at pH 5.5 and lowest in the ccpA-knockout strain at pH 6.5. Conclusions Overall, low pH inhibited the growth of S. bovis S1, but did not affect the fermentation pattern. CcpA regulated S. bovis S1 growth and organic acid fermentation pattern. Moreover, there seemed to be an interaction effect between pH and ccpA deletion on regulating the growth and organic acids production of S. bovis S1.