Comparative genomics of Fervidobacterium: a new phylogenomic landscape of these wide-spread thermophilic anaerobes.

Background: Fervidobacterium is a genus of thermophilic anaerobic Gram-negative rod-shaped bacteria belonging to the phylum Thermotogota. They can grow through fermentation on a wide range of sugars and protein-rich substrates. Some can also break down feather keratin, which has significant biotechnological potential. Fervidobacteria genomes have undergone several horizontal gene transfer events, sharing DNA with unrelated microbial taxa. Despite increasing biotechnological and evolutionary interest in this genus, only seven species have been described to date. Here, we present and describe six new and complete Fervidobacterium genomes, including the type strains Fervidobacterium gondwanense CBS-1 ^T, F. islandicum H-21 ^T and F. thailandense FC2004^T, one novel isolate from Georgia (strain GSH) and two strains (DSM 21710 and DSM 13770) that have not been previously described along with an evolutionary and phylogenomic analysis of the genus.

Results: The complete genomes were around 2 Mb with approximately 2,000 CDS identified and annotated in each of them and a G + C content ranging from 38.9 mol% to 45.8 mol%. Phylogenomic comparisons of all currently available Fervidobacterium genomes, including OrthoANI and TYGS analyses, as well as a phylogenetic analysis based on the 16S rRNA gene, identified six species and nine subspecies clusters across the genus, with a consistent topology and a distant and separately branching species, Fervidobacterium thailandense. F. thailandense harbored the highest number of transposases, CRISPR clusters, pseudo genes and horizontally transferred regions The pan genome of the genus showed that 44% of the genes belong to the cloud pangenome, with most of the singletons found also in F. thailandense.

Conclusions: The additional genome sequences described in this work and the comparison with all available Fervidobacterium genome sequences provided new insights into the evolutionary history of this genus and supported a phylogenetic reclassification. The phylogenomic results from OrthoANI and TYGS analyses revealed that F. riparium and F. gondwanense belong to the same genome species, and includes Fervidobacterium sp. 13770, while "F. pennivorans" strain DYC belongs to a separate genome species, whereas Fervidobacterium sp. 21710 and Fervidobacterium sp. GSH within the Fervidobacterium pennivorans clade represent two subspecies. F. changbaicum is reclassified as F. islandicum.