Ectosymbiotic bacterial microbiota densely colonize the surface of thelastomatid nematodes in the gut of the wood‐feeding cockroach Panesthia angustipennis
&NA; Cockroaches generally harbor thelastomatid nematodes (pinworms) in their gut. In this study, we discovered that the surfaces of two undescribed thelastomatid species in the hindgut of the wood‐feeding cockroach Panesthia angustipennis were consistently and densely colonized by bacteria. Epifluorescence microscopy using 4′,6‐diamidino‐2‐phenylindole and transmission electron microscopy revealed that several distinct morphotypes of bacteria covered almost the entire body surface of the nematodes in single or multiple layers. Sequencing analysis of 16S rRNA amplicons of either entire nematodes or sections of nematode body surfaces indicated that the associated bacterial microbiota consisted of several dominant phylotypes belonging to either Dysgonomonadaceae (Bacteroidales termite cluster V), Rikennellaceae or Ruminococcaceae. These phylotypes formed clades with sequences previously obtained from cockroach and/or termite guts. Comparisons of the bacterial community structure of the entire cockroach hindgut microbiota vs the nematode‐associated microbiota suggested that these dominant bacterial phylotypes preferentially colonized the nematode surface. The two nematode species shared most of the dominant bacterial phylotypes, but the bacterial community structures differed significantly. Colonization by five predominant phylotypes was confirmed by fluorescence in situ hybridization analysis using phylotype‐specific probes. Our study provides fundamental information on this previously unknown ectosymbiosis between gut bacteria and thelastomatid pinworms. &NA; Graphical Abstract Figure. Discovery and ecology of a bacterial consortium attached to the surface of pinworms (nematodes) in guts of cockroaches.