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°C). The two phyla differ in their mode of nutrition: the Aquificae are autotrophs capable of oxidising hydrogen or sulphur, while the Thermotogae are anaerobic heterotrophs, fermenting carbohydrates.
(a) Diaminopimelic acid (b) Ornithine
Figure 7.11 Members of the Deinococcus-Thermus group have an unusual form of pep-tidoglycan. The diaminopimelic acid at position three on the amino acid chain attached to N-acetylmuramic acid (see Figure 3.6) is replaced by ornithine
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)
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