The phylum thermotogae

Abstract

The phylum Thermotogae is comprised of anaerobic, thermophilic, as well as mesophilic bacteria that are surrounded by an outer sheathlike envelope referred to as a toga. The species from this deep-branching group exhibit Gram-negative staining, but due to the absence of an archetypal outer cell membrane, they are considered monoderm (or atypical diderm) bacteria. This phylum presently contains 10 genera harboring 41 validated species which are all part of a single family, Thermotogaceae, within the order Thermotogales. Comparative analyses of genome sequences have led to the discovery of numerous conserved signature indels (CSIs), in proteins, which are specific for different monophyletic clades of Thermotogae. These molecular markers provide reliable means for demarcation of different clades of the Thermotogae and for the taxonomical organization of this phylum. Based upon their phylogenetic branching and the discovered molecular markers, it has been proposed that the class Thermotogae be divided into three orders. These include an emended order Thermotogales containing the families Thermotogaceae (emended) and Fervidobacteriaceae fam. nov. And two new orders, Petrotogales and Kosmotogales. The identified CSIs also suggest that although the Thermotogae species have undergone gene exchanges with other prokaryotes, particularly with thermophilic organisms, the extent of such gene transfers is limited and they do not significantly affect the monophyly or distinctness of species from this phylum. Thermotogae species, due to their ability to efficiently metabolize numerous organic substrates, producing H2 gas as a by-product, in conjunction with the thermostability of their enzymes, have also become an important focal point for different biotechnological applications.