Growth, hydrogen production and cellulose digestion by strain CD2 were considerably greater when the culture pH was maintained at 70 than when the pH was not controlled. Furthermore, if the pH of the growth medium was controlled the number of viable organisms was 6-fold greater after 3 d incubation and 100-fold greater after 6 d incubation compared with equivalent cultures in which the pH was not controlled. The differences were due to the combined effect of low pH values and acetic acid accumulation. The number of viable organisms was 2- to 3-fold lower after 12 h incubation in substrate-free medium containing 40 m-acetic acid at pH 55 than in the same medium at pH 70. Addition of 90 m-acetic acid during growth in a cellobiose-containing medium lowered the growth rate by 30% and the rate of hydrogen production by 40%. Exposure of to oxygen for up to 2 h did not affect viability measurements provided that the organisms were subsequently transferred to reduced media.


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