# Discovery and genomics of H2-oxidizing/O2-reducing Deferribacterota ectosymbiotic with protists in the guts of termites and a Cryptocercus cockroach

## TL;DR

Scientists discovered a new type of bacteria living on protists in the guts of termites and a wood-eating cockroach, which may help these protists digest wood.

## Contribution

The discovery of a novel ectosymbiotic Deferribacterota clade associated with gut protists in termites and cockroaches is reported.

## Key findings

- Ectosymbiotic Deferribacterota were found attached to spirotrichonymphid and trichonymphid protists in termite and cockroach guts.
- Genome analysis suggests these bacteria ferment sugars and use H2 for respiration, possibly aiding protist digestion.
- A new genus and family, Termitispirillum and Termitispirillaceae, are proposed for these bacteria under SeqCode.

## Abstract

Members of the phylum Deferribacterota inhabit diverse environments, but their symbiosis with protists has never been reported. We discovered an ectosymbiotic clade of Deferribacterota specifically associated with spirotrichonymphid protists in the guts of the termites Reticulitermes speratus and Hodotermopsis sjostedti and trichonymphid protists in the gut of the wood-feeding cockroach Cryptocercus punctulatus. The ectosymbiotic Deferribacterota were spiral shaped and attached to 16%–91% of the host protist cells. These formed a monophyletic cluster within an uncultured insect gut-associated family-level clade, which is sister to the vertebrate gut-associated family Mucispirillaceae. The complete genome of an ectosymbiotic Deferribacterota was obtained from a Trichonympha acuta cell in a C. punctulatus gut and analyzed together with a single-cell amplified genome of another ectosymbiotic Deferribacterota associated with Holomastigotes sp. in the gut of R. speratus. Genome analyses suggest that these Deferribacterota ferment monosaccharides and conduct fumarate and oxidative respiration with H2 as an electron donor. They thus possibly contribute to the removal of hydrogen and oxygen to protect the fermentative activity of the protist hosts. The ectosymbionts possess reduced signal transduction gene repertoires, implying that the association has provided a relatively stable environment for these bacteria. The ectosymbionts likely possess flagella with an unusually expanded number of flagellin variants up to 40, which may reflect an adaptation to their ectosymbiotic lifestyle. We propose a novel genus, Termitispirillum, for these ectosymbionts and a novel family, Termitispirillaceae, for the insect-gut clade, under SeqCode. Our findings provide new insights into the ecology and evolution of Deferribacterota.

Graphical Abstract