SUMMARY: Bacterial cultures in media with a low (0.38%, w/v) initial glucose concentration showed much smaller pH changes during growth than cultures grown in media with excess (1.25%, w/v) glucose. Increasing the concentration of the phosphate or volatile fatty acid salt (VFA salt, i.e. acetate, propionate or butyrate) in the media had no beneficial effect on cultures of or which had only minor falls in pH during growth (final pH ≧ 6.0), but increased the dry weight yield (but not the molar growth yield) of cultures which had major pH falls (final pH ≦ 4.6) during growth. The improvements in dry weight yield could be correlated with increases in the pH buffering capacity of media caused by the increased phosphate or VFA salt concentration.

The response of to increases in the phosphate concentration of media was qualitatively similar to that of However, VFA salts (particularly acetate) always decreased both dry weight and molar growth yield. The effects of VFA salts on three other rumen bacteria, and were varied. The possible mechanisms and ecological implications of the effects of these compounds are discussed.


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