Thermosipho africanus, a thermophilic bacterium from the order Thermotogales, was previously thought to be a non-carbohydrate fermenter, distinguishing it from related genera. This study demonstrates that T. africanus can ferment several carbohydrates including D-glucose, D-ribose, maltose, and starch, though D-galactose, fructose, and sucrose are poorly utilized. Fermentation produces acetate, hydrogen, and carbon dioxide as major end products. Significantly, the presence of thiosulfate as an electron acceptor greatly enhances growth and increases acetate production from carbohydrate substrates. Under thiosulfate conditions, ethanol and lactate production are eliminated. The stimulatory effect of thiosulfate is more pronounced than that of elemental sulfur. Based on these findings, the authors propose amending the genus and species descriptions of Thermosipho africanus to reflect its carbohydrate-fermenting capacity and ability to utilize thiosulfate as an electron acceptor. This study highlights the importance of including thiosulfate in culture media for accurate metabolic characterization of Thermotogales members.

Key findings

• Thermosipho africanus ferments D-glucose, D-ribose, maltose, and starch, contrary to previous descriptions of the genus as non-carbohydrate fermenting
• Thiosulfate as an electron acceptor dramatically improves growth and increases acetate production while eliminating ethanol and lactate synthesis
• The genus and species descriptions of Thermosipho africanus were emended to include carbohydrate fermentation and thiosulfate reduction as defining characteristics
• Thiosulfate stimulates growth more effectively than elemental sulfur in this and other Thermotogales species
• Culture media should include thiosulfate for accurate phenotypic and metabolic characterization of Thermotogales bacteria