EFFECTS OF EXTRACELLULAR PH ON THE INTRACELLULAR PH AND MEMBRANE-POTENTIAL OF CELLULOLYTIC RUMINAL BACTERIA, RUMINOCOCCUS-ALBUS, RUMINOCOCCUS-FLAVEFACIENS, AND FIBROBACTER-SUCCINOGENES

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