Summary
Many reports have stated that flagellated protists in termite guts harbour ectosymbiotic spirochetes on their cell surface. In this study, we describe another bristle‐like ectosymbiont affiliated with the order
Bacteroidales
. The 16S rRNA phylotype Rs‐N74 predominates among
Bacteroidales
clones obtained from the gut of the termite
Reticulitermes speratus
. An Rs‐N74 phylotype‐specific probe was designed in this study and used for detection of the corresponding bacteria in the gut by fluorescence
in situ
hybridization (FISH) analysis. Surprisingly, the signals were detected specifically from the bristle‐like ‘appendages’ of various flagellate species belonging to the genus
Dinenympha
; these ‘appendages’ had been believed to be spirochetal ectosymbionts or structures of the protists. The Rs‐N74 bacteria attached to the cell surface of the protists by a tip and coexisted with the spirochetal ectosymbionts. An electron micrograph revealed their morphology to be similar to a typical
Bacteroidales
bacterium. This bacterium is proposed to represent a novel genus and species, ‘
Candidatus
Symbiothrix dinenymphae’, phylogenetically affiliated with a cluster consisting exclusively of uncultured strains from termite guts. A
Bacteroidales
‐specific probe for FISH further revealed that this type of symbiosis exists also in various other protists, including parabasalids and oxymonads, and is widespread in termite guts.