FEMS Microbiology Letters 123 (1994) 281-288 © 1994 Federation of European Microbiological Societies 0378-1097/94/$07.00 Published by Elsevier 281 FEMSLE 06234 Methylotrophic growth of a mutant strain of the acetogenic bacterium Eubacterium limosum that uses acetate as co-substrate in the absence of CO2 Pascal Loubiere * and Nicholas D. Lindley Centre de Bioing~nierie Gilbert Durand, (CNRS-URA 544, Lab. Ass. INRA), Institut National des Sciences Appliqu~es, Complexe Scientifique de Rangueil, F-31077 Toulouse Cedex, France (Received 28 July 1994; accepted 31 August 1994) Abstract: A mutant strain (the CO 2 strain) of the acetogenic bacterium Eubacterium limosum was isolated by nutritional selection on a medium containing methanol and acetate but lacking CO 2. The ability of the CO~ strain to grow on methanol/acetate without CO 2 or other one-carbon compounds was stable through several transfers in the presence of CO 2. The phenotypic difference between the parent and the mutant strain was the ability to support sustained growth in the absence of CO 2 by consumption of acetate throughout the growth period. The maximal specific rate of acetate consumption (5 mmol C.g-i h-1) was similar to that previously observed for a brief period early in the exponential growth phase of the parent strain but was maintained for a prolonged duration. The methanol consumption rate was considerably lower than in the presence of CO 2 and was similar to the rate of acetate consumption, hence maintaining equilibrated reducing equivalent pools. The maximal growth rate (0.08 h-l) was lower than that of the parent strain and the optimal pH for growth was 6.5 instead of 7.4 in the presence of CO 2. These acidic conditions enhanced the toxic effect of butyric acid, resulting in a more pronounced inhibition of growth, as seen by the decrease in YATP proportional to the accumulated butyric acid. Key words: Eubacterium limosum; Methanol; Acetate; Acetogenesis Introduction The catabolic reduction of CO z to acetic acid was first reported for Clostridium aceticum [1]. During the next 40 years, homoacetogenic CO 2 fixation was studied in the context of sugar metabolism using Clostridium thermoaceticum [2], * Corresponding author. Tel: (+ 33) 61 55 99 86; Fax: (+ 33) 61 55 99 65. Butyribacterium rettgeri [3], Clostridium formi- coaceticum [4] and Acetobacterium woodii [5]. Only in 1980 was the growth of the anaerobic bacterium Buteribacterium methylotrophicum de- scribed with methanol as carbon substrate [6] though many homoacetogens are now known to be able to ferment methanol in the presence of CO 2 as essential co-substrate. Almost all the known acetogens ferment single-carbon com- pounds to acetate (homoacetogens), but during methylotrophie acetogenesis, B. methylotroph- SSD1 0378-1097(94)00386-6 Downloaded from https://academic.oup.com/femsle/article-abstract/123/3/281/650502 by guest on 21 May 2020