An Intestinal Symbiotic Bacterial Strain of Modulates Host Viability at Both Global and Post-Transcriptional Levels.

A rhabditid entomopathogenic nematode (EPN), , has a stable symbiotic relationship with the bacterial strain S1 harbored in its intestines and drastically reduced viability when associated with a non-native strain (186) of the same bacterial species. This nematode is thus a good model for understanding the molecular mechanisms and interactions involved between a nematode host and a member of its intestinal microbiome. Transcriptome analysis and RNA-seq data indicated that expression levels of the majority (8797, 87.59%) of mRNAs in the non-native combination of and 186 were downregulated compared with the native combination, including strain S1. Accordingly, 88.84% of the total uniq-sRNAs mapped in the transcriptome were specific between the two combinations. Six DEGs, including two transcription factors ( and ) and four kinases (, , , and ), as well as an up-regulated micro-RNA, oc-miR-71, were found to demonstrate the regulatory mechanisms underlying diminished host viability induced by a non-native bacterial strain. and play key roles in the viability regulation of by positively mediating the expression of to indirectly impact its longevity and stress tolerances and by negatively regulating the expression of to affect the olfactory chemotaxis and fecundity. In response to the stress of invasion by the non-native strain, the expression of oc-miR-71 in the non-native combination was upregulated to downregulate the expression of its targeting oc, which might improve the localization and activation of the transcription factor DAF-16 in the nucleus to induce longevity extension and stress resistance enhancement to some extent. Our findings provide novel insight into comprehension of how nematodes deal with the stress of encountering novel potential bacterial symbionts at the physiological and molecular genetic levels and contribute to improved understanding of host-symbiont relationships generally.