Mikaelyan, Aram

Summary Termite gut flagellates are typically colonized by specific bacterial symbionts. Here we describe the phylogeny, ultrastructure and subcellular location of ‘ Candidatus Adiutrix intracellularis’, an intracellular symbiont of Trichonympha collaris in the termite Zootermopsis nevadensis . It represents a novel, deep‐branching clade of uncultured Deltaproteobacteria widely distributed in intestinal tracts of termites and cockroaches. Fluorescence in situ hybridization and transmission electron microscopy localized the endosymbiont near hydrogenosomes in the posterior part and near the ectosymbiont ‘ Candidatus Desulfovibrio trichonymphae’ in the anterior part of the host cell. The draft genome of ‘ Ca . Adiutrix intracellularis’ obtained from a metagenomic library revealed the presence of a complete gene set encoding the Wood–Ljungdahl pathway, including two homologs of fdhF encoding hydrogenase‐linked formate dehydrogenases (FDH H ) and all other components of the recently described hydrogen‐dependent carbon dioxide reductase (HDCR) complex, which substantiates previous claims that the symbiont is capable of reductive acetogenesis from CO 2 and H 2 . The close phylogenetic relationship between the HDCR components and their homologs in homoacetogenic Firmicutes and Spirochaetes suggests that the deltaproteobacterium acquired the capacity for homoacetogenesis via lateral gene transfer. The presence of genes for nitrogen fixation and the biosynthesis of amino acids and cofactors indicate the nutritional nature of the symbiosis.

Summary The name Arthromitus has been applied collectively to conspicuous filamentous bacteria found in the hindguts of termites and other arthropods. First observed by Joseph Leidy in 1849, the identity of these filaments has remained contentious. While Margulis and colleagues declared them to be a life stage of Bacillus cereus , others have assumed them to belong to the same lineage as the segmented filamentous bacteria (SFB) from vertebrate guts, a group that has garnered much attention due to their unique ability to specifically modulate their host's immune response. Both SFB and Arthromitus filaments from arthropod guts were grouped under provisional name ‘ Candidatus Arthromitus’ by Snel and colleagues as they share a striking similarity in terms of their morphology and close contact to the host gut wall. While SFB form a distinct lineage within the family Clostridiaceae , the identity of the filaments from arthropod guts remains elusive. Using whole‐genome amplification of single filaments capillary picked from termite guts and fluorescence in situ hybridization of 16S rRNA with group‐specific oligonucleotide probes, we show that they represent a monophyletic lineage within the family Lachnospiraceae distinct from that of SFB. Therefore, ‘ Candidatus Arthromitus’ can no longer be used for both groups. Given the historic precedence, we propose to reserve this name for the filaments that were originally described by Leidy. For the SFB from vertebrate guts, we propose the provisional name ‘ Candidatus Savagella’ in honour of the American gut microbiologist Dwayne C. Savage, who was the first to describe that important bacterial group.