The effects of pH on colonic bacteria grown in continuous culture.

A model of the proximal colon was used to investigate the effects of pH on fermentation by colonic bacteria in vitro. Twelve continuous anaerobic cultures of human faecal bacteria were maintained at constant pH in a medium simulating ileostomy effluent. Five cultures were maintained at pH 7, five at pH 6, and two at pH 5. The pH 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 pH exerted an important effect on bacterial metabolism without causing major changes in bacterial populations. Osmolality was lower in cultures run at a low pH. Total volatile fatty acid concentration was decreased at pH 5, and the production of propionic acid rather than acetic acid was favoured at pH 6. Changing the pH had no significant influence on the production of ammonia in these systems.