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?761?3-7?0 mm) 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 H 2 /CO 2 and L-lactate. Strain TmAO3 T , 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 TmAO3 T 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, TmAO3Reductive acetogenesis from H 2 and CO 2 in gut homogenates of wood-feeding termites was first reported by Breznak & Switzer (1986). In the following years, the presence of homoacetogenic activity has been demonstrated for a large number of termite species from all major feeding guilds, including representatives of wood-feeding, funguscultivating and soil-feeding termites (Brauman et al., 1992). Although reductive acetogenesis was always out-competed as a hydrogen sink by methanogenesis in gut homogenates of soil-feeding termites (Brauman et al., 1992), microinjection of H 14 CO 3 2 into intact hindguts of soil-feedingCubitermes spp. has identified a high potential for reductive acetogenesis (Tholen & Brune, 1999), indicating that the contribution of reductive acetogenesis to overall electron flow in the guts of soil-feeding termites may be larger than expected.In order to define the metabolic potential of termite gut homoacetogens and to identify specific adaptations to the gut environment, it is necessary to study these bacteria in pure culture . However, only five strains of homoacetogenic bacterium have so far been isolated from termite guts, four of them from wood-feeding species. They comprise Sporomusa termitida and Acetonema longum from Nasutitermes nigriceps and Pterotermes occidentis (Bre...

Cytochrome bd Oxidase, Oxidative Stress, and Dioxygen Tolerance of the Strictly Anaerobic Bacterium Moorella thermoacetica

“…Several bacteria classified as strict anaerobes, including some acetogens, have been shown to tolerate and consume dioxygen (7,8,10,11,26,27). M. thermoacetica cell suspensions showed significant consumption of O 2 in the absence of any exogenous electron donor (see footnote to Table 1), as has been observed in many other bacteria, including acetogens (7,10,26,27 · mg protein Ϫ1 ) of reducing agent were similar to those reported for other acetogenic bacteria (6).…”

“…This pathway conserves energy via chemiosmosis from a membrane-bound electron transport chain consisting of two b-type cytochromes (b 559 and b 554 ), and a menaquinone, MK7 (16,19). Although classified as strict anaerobes, several acetogenic bacteria, including M. thermoacetica, have been reported to tolerate traces of dioxygen (7,8,39,48). Karnholz et al showed that M. thermoacetica can survive exposure to and consume low levels (0.5 to 2%) of headspace dioxygen in liquid culture and also reported NADH oxidase activity in cell extracts (39).…”

Diversity of Formyltetrahydrofolate Synthetases in the Guts of the Wood-Feeding Cockroach Cryptocercus punctulatus and the Omnivorous Cockroach Periplaneta americana

“…Interestingly, methanogenic termites and cockroaches exhibit increased acetogenesis following addition of exogenous H 2 (3,29). This suggests that these insects are host to a robust population of bacteria that are capable of homoacetogenesis but may be primarily using alternative electron donors (and other substrates and pathways) in vivo.Acetogenic bacteria belonging to two bacterial phyla, Firmicutes and Spirochaetes, have been isolated from the guts of termites (1,4,11,12,14). Several surveys have targeted and used the gene for formyltetrahydrofolate synthetase (FTHFS), a key gene in the Wood-Ljungdahl pathway of acetogenesis (16), as a potential marker for the pathway (15, 18).…”

Agriculture-independent, sustainable, fail-safe and efficient food production by autotrophic single-cell protein

“…Adapations for the bioreactors are discussed in the respective section. [110] or Clostridium magnum [111], which do not grow but can quickly reduce pO 2 and continue acetate secretion even in the presence of O 2 . However, it is unclear to which extent the H 2 -dependent mechanism of O 2 consumption is energy-conserving.…”