Sporomusa aerivorans sp. nov., an oxygen-reducing homoacetogenic bacterium from the gut of a soil-feeding termite

Abstract

Previously undescribed, homoacetogenic bacteria were isolated from gut homogenates of the soil-feeding termite Thoracotermes macrothorax. The isolates were slightly curved, banana-shaped rods (0·6–0·7×1·3–7·0 μm) and were motile by one or more lateral flagella. In older cultures, cells formed club-like sporangia that developed into terminal, heat-resistant endospores. Cells stained Gram-positive but were Gram-negative in the KOH test. The isolates were mesophilic and grew homoacetogenically on H2/CO2 and l-lactate. Strain TmAO3T, which was characterized further, also grew homoacetogenically on pyruvate, citrate, l-alanine, d-mannitol, ethanol, formate and methanol. Succinate was decarboxylated to propionate; fumarate, l-malate and oxaloacetate were fermented to propionate and acetate. Hexoses were not used as substrates. Resting cells had a large capacity for hydrogen-dependent oxygen reduction [826 nmol min−1 (mg protein)−1], which enabled them to initiate growth in non-reduced basal medium that originally contained up to 1·5 kPa oxygen in the headspace, although growth commenced only after the medium had been rendered anoxic. Redox difference spectra of cell extracts indicated the presence of membrane-bound b-type cytochrome(s). Comparative 16S rRNA gene sequence analysis revealed that strain TmAO3T belongs to a subgroup of the phylum of Gram-positive bacteria that is characterized by low DNA G+C content and a Gram-negative cell wall. It is related most closely to representatives of the genus Sporomusa. Based on morphological and physiological properties and on 16S rRNA gene sequence similarity of 94–97 % to other Sporomusa species, the isolates are assigned to Sporomusa aerivorans sp. nov. (type strain, TmAO3T=DSM 13326T=ATCC BAA-625T).