Syntrophic Growth of Symbiobacterium in Association with Free-Living Bacteria

Abstract

Symbiobacterium thermophilum is a syntrophic thermophilic bacterium that grows in coculture with a cognate Geobacillus stearothermophilus. This chapter deals with the unique features of S. thermophilum in terms of taxonomy, ecology, physiology, and evolutionary history. Although S. thermophilum was first described as a gram-negative bacterium, molecular phylogenetic analyses revealed that it represents a unique taxon within Clostridia (Firmicutes, gram-positive bacteria). In contrast to the limited knowledge of bacterial systematics, an ecological survey aimed at detecting related DNA signatures revealed widespread occurrence of this bacterial group in the natural environment including the soil, animal intestines, and seawater, indicating that this group has remained unidentified due to the difficulty in its isolation. A long-term study has finally identified CO2 generated along with the precedent growth of G. stearothermophilus as a major growth-promoting factor. Another role played by the host bacterium is the creation of an anaerobic environment by consuming O-2. G. stearothermophilus also proves beneficial to S. thermophilum by inactivating the self-growth inhibitory activity of indolyl derivatives generated due to the tryptophanase activity of S. thermophilum. Therefore, the role of G. stearothermophilus is to establish a complex environment suitable for the growth of S. thermophilum, which is achieved by supplying and removing multiple factors. Genetic evidence from various model microorganisms has indicated that a high-CO2 atmosphere is required if the gene encoding carbonic anhydrase, a ubiquitous enzyme catalyzing interconversion between CO2 and bicarbonate, is knocked out. Surveys of whole genome sequences of Clostridia and Proteobacteria demonstrated the occurrence of gene loss for this enzyme in specific strains. Various observations demonstrate that high-CO2 atmosphere is fundamental to microbial phenotypes including primary as well as developmental growth. Thus, high CO2 may be an effective signal to elicit specific growth and functions in microbial community. Complex associations among microorganisms are fundamental to the constitution of an ecosystem. Deepening our knowledge regarding the interaction should lead to the better understanding of microbial ecosystem and its application.