A model of the promixal colon was used to investigate the effects of H on fermentation by colonic bacteria . Twelve continuous anaerobic cultures of human faecal bacteria were maintained at constant H in a medium simulating ileostomy effluent. Five cultures were maintained at H 7, five at H 6, and two at H 5. The H of each of three further cultures was altered after they had reached steady state, either from 7 to 6 and then to 5, or from 5 to 6 to 7.

Both experimental designs showed that the H exerted an important effect on bacterial metabolism without causing major changes in bacterial populations. Osmolality was lower in cultures run at a low H. Total volatile fatty acid concentration was decreased at H 5, and the production of propionic acid rather than acetic acid was favoured at H 6. Changing the H had no significant influence on the production of ammonia in these systems.


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