Taxonomical and physiological comparisons of the three species of the genus Amphibacillus

Amphibacillus is a genus for Gram-positive, spore-forming, rod-shaped, facultatively anaerobic bacteria with low-G+C content of DNA, established by Niimura et al. in 1990. Amphibacillus xylanus, the type species of the genus, grows well under both strictly anaerobic and aerobic conditions in spite of lacking any isoprenoid quinones, cytochromes, and catalase. Amphibacillus fermentum and Amphibacillus tropicus were later proposed by Zhilina et al. in 2001 for the isolates from a soda lake. In this paper, we revealed the latter two species also lacked isoprenoid quinones, cytochrome and catalase, and that they grew well under strictly anaerobic and aerobic conditions. The consistent growth of A. xylanus under both conditions is due to the presence of anaerobic and aerobic pathways for glucose metabolism in the organism. Although A. fermentum and A. tropicus are supposed to have a side enzymatic pyruvate pathway to produce lactate under both conditions, the two species have two major pyruvate metabolic pathways as observed in A. xylanus. Analysis data indicated that NADH formed both by the aerobic pyruvate pathway and by the glycolytic pathway was re-oxidized by the NADH oxidase in A. fermentum and A. tropicus as well as A. xylanus, and furthermore that the NADH oxidase-Prx (AhpC) system, i.e., NADH oxidase scavenging hydrogen peroxide with Prx, also functions in A. tropicus as observed with A. xylanus. Not only the taxonomical character of the genus Amphibacillus but also the growth characterization based on the two metabolic pathways and unique oxygen metabolism are distinctive in those traits from other facultative anaerobes.