Abstract
The growth rates of Ruminococcus albus, Ruminococcus flavefaciens, and Fibrobacter succinogenes, which are known to be sensitive to low pH, were decreased to less than half of the maximum rates at extracellular pH (pHe) 6.0, and their growth rates were extremely low at pHe 5.6. Their intracellular pH(pHi) was decreased linearly as pHe was lowered from 6.8 to 6.0, irrespective of the presence or absence of cellobiose. However, pHi was not decreased markedly as pHe was lowered from 6.0 to 5.6 when cellobiose was given, resulting in the increased pH gradient across the cell membrane (ΔpH). On the other hand, when cellobiose was not supplied, pHi was still linearly decreased with the decrease of pHe to 5.6. These results suggest that much energy is needed to form ΔpH at pHe 5.6. Membrane potential (Δψ) and proton motive force (PMF) were not significantly affected by the change in pHe. Addition of 3% ethanol markedly decreased the ΔpH of these bacteria at pHe 5.6. In R. flavefaciens and F. succinogenes, Δψ was notably decreased by 3% ethanol, while in R. albus Δψ was slightly decreased by ethanol. Compared with Megasphiaera elsdenii, which is relatively tolerant to low pH, these cellulolytic bacteria appear to have low capacity to maintain their pHi against the decrease in pHe.
