Phylum Aquificae and phylum Thermotogae: the deeply branching bacteria

Phylum Aquificae and phylum Thermotogae: the deeply branching bacteria

These two phyla are regarded as the two deepest (= oldest) branches in the evolution of the Bacteria and both comprise highly thermophilic Gram-negative rods. They are the only members of the Bacteria that can compare with the Archaea in their ability to live at high temperatures (optimal growth >80 ^â—¦ C). The two phyla differ in their mode of nutrition: the Aquificae are autotrophs capable of oxidising hydrogen or sul-phur, while the Thermotogae are anaerobic heterotrophs, fermenting carbohydrates.

Members of the Thermotogae are characteristically surrounded, sometimes in a chain, by a proteinaceous sheath (or ‘toga’).

Representative genera: Aquifex (phylum Aquificae), Thermotoga (phylum Thermoto-gae)