Summary
Rs‐N31, a 16S rRNA phylotype affiliated with the genus
Desulfovibrio
, has frequently been detected from the gut of the wood‐feeding termite
Reticulitermes speratus
. In this study, we designed a probe specifically targeting phylotype Rs‐N31 and performed fluorescence
in situ
hybridization to identify the corresponding cells. The signals were detected exclusively inside the cells of the flagellate
Trichonympha agilis
, which simultaneously harbours another intracellular bacterium belonging to the candidate phylum Termite Group 1 (TG1). The detected cells were coccoid or short rods and specifically localized in the cortical layer of mainly, the anterior part of the flagellate cell. Approximately 1800 cells were contained in a single host cell, accounting for, in total, 2% of the whole prokaryotic gut microbiota. The genes
dsrAB
and
apsA
for sulfate reduction and a gene‐encoding H
2
‐uptake hydrogenase, both possessing a high sequence identity with those of known desulfovibrios, were obtained by polymerase chain reaction (PCR) from the host cells isolated using a micromanipulator, and their expression was verified by reverse‐transcription PCR. Thus, we suggest that this endosymbiont acts as a sink for the hydrogen generated by both the flagellates and possibly TG1 symbionts. For this uncultured bacterium, we propose a novel species, ‘
Candidatus
Desulfovibrio trichonymphae’.