TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The first gene endogenous to the organism has been inactivated by genetic engineering, the first gene encoding a protein required for the generation of formate, where the inactivation of the first gene results in a decrease in formate production in the organism. The first gene and a second gene have been inactivated by genetic engineering, the first gene encoding a protein required for the generation of formate, the second gene encoding a post-translational modifying protein capable of facilitating maturation of hydrogenase in the organism. The post-translational modifying protein is capable of facilitating maturation of at least one hydrogenase synthesized by the organism in its native state. The conditions comprises temperature of at least 50 degrees C. In the method, one or more genes in addition to the first gene is inactivated in the bacterium, the one or more genes encoding one or more proteins comprising lactate dehydrogenase, acetate kinase and/or phosphotransacetylase. The second gene encodes a protein i.e. required by the organism to produce lactic acid as a fermentation product. Preferred Components: The first gene has at least 90% sequence identity with a polynucleotide sequence comprising SEQ ID NO. 1 (comprising one of fully defined 1062 base pair sequences given in specification), SEQ ID NO. 2 (comprising one of fully defined 1203 base pair sequences given in specification), SEQ ID NO. 3 (comprising one of fully defined 1379 base pair sequences given in specification). The organism is: a bacterium; or a thermophilic, anaerobic bacterium. In the organism, a second gene encoding a protein other than a post-translational modifying protein is inactivated. The second gene encodes a lactate dehydrogenase. The second gene encodes a protein comprises acetate kinase and phosphotransacetylase. The organism and bacterium are a member of the Clostridiumgenus, preferably Clostridium thermocellum. . The reaction mixture further comprises a cellulolytic material. The cellulolytic material comprises a cellulase. The first gene has at least 90% sequence identity with SEQ ID NOs. 1-3. The organism is aClostridium thermocellumstrain deposited under Patent Deposit Designation No. PTA-11763 or PTA-11764. The first gene has at least 90% sequence identity with a polynucleotide sequence comprising SEQ ID No. 4 (comprising one of fully defined 2231 base pair sequences given in specification), SEQ ID NO. 5 (comprising one of fully defined 717 base pair sequences given in specification). In the organism, a third gene is inactivated, where the third gene encoding a protein required for the generation of lactate in the organism. The first gene is the pyruvate formate-lyase gene (pfl gene), the third gene is the ldh gene, and the second gene encode a posttranslational modifying protein comprises HydE, HydF and HydG proteins. The acetate kinase and the phosphotransacetylase genes have been inactivated.    TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preparation: No preparation method is given. EXAMPLE - No suitable example given.