Physiological interactions between a mesophilic cellulolyticClostridiumand a non-cellulolytic bacterium

Abstract

A mesophilic cellulolytic bacterium (Clostridium strain C7) capable of N2 fixation and a non-cellulolytic bacterium (Klebsiella strain W1), both isolated from freshwater environments rich in decaying plant material, were co-cultured in a chemically defined, vitamin-deficient medium containing cellulose as the carbon and energy source. In the co-culture, an extracellular cellulase complex produced by the Clostridium hydrolyzed cellulose to soluble sugars that served as fermentable substrates for the Klebsiella. In turn, the Klebsiella excreted growth factors, identified as biotin and p-aminobenzoic acid, which were required by the Clostridium. Furthermore, demonstration of NH4+-repressible acetylene reduction by co-cultures growing in medium lacking combined nitrogen showed that the Clostridium fixed N2, thus allowing growth of the Klebsiella, which was not a nitrogen fixer. The mutualistic relationships observed in the co-cultures may be representative of interactions that take place in natural environments in which cellulose-containing plant materials are biodegraded.