CLOSTRIDIUM-MAYOMBEI SP-NOV, AN H-2/CO-2 ACETOGENIC BACTERIUM FROM THE GUT OF THE AFRICAN SOIL-FEEDING TERMITE, CUBITERMES-SPECIOSUS

Clostridium mayombei sp. nov., a previously undescribed H-2-oxidizing CO2-reducing acetogenic bacterium, was isolated from gut contents of the African soil-feeding termite, Cubitermes speciosus. Cells were anaerobic, Gram positive, catalase and oxidase negative, endospore-forming motile rods which measured 1 x 2-6-mu-m and which had a DNA base composition of 25.6 mol% G + C (strain SFC-5). Optimum conditions for growth on H-2 + CO2 were at 33-degrees-C and pH 7.3, and under these conditions cells produced acetate according to the equation: 4 H-2 + 2 CO2 --> CH3COOH + 2 H2O. Other substrates supporting good growth included carbohydrates (e.g. glucose, xylose, starch), sugar alcohols, and organic and amino acids, and with these substrates acetate was almost always the principle fermentation product. Comparative analysis of 16S rRNA nucleotide sequences confirmed that C. mayombei was closely related to various members of the genus Clostridium. However, morphological and physiological differences between C. mayombei and other homoacetogenic clostridia were deemed significant enough to warrant creation of a new taxon. Results are discussed in light of the diversity of H-2/CO2 acetogens recently isolated from various termites, and in terms of the relative importance of H-2/CO2 acetogenesis to termite nutrition.