Physiological interactions between a mesophilic cellulolytic Clostridium and 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.