Alterations of gut microbiota and short-chain fatty acids induced by in the hindgut of Xenocyprinae fishes providing new insights into the relationship among protozoa, gut microbiota and host.

Introduction: Parasitic ciliates are protozoans with a global distribution. Along with the gut microbiota, they have formed a micro-ecosystem that affects the host's nutrition, metabolism, and immunity. The interactions and relationships among the three components of this microecosystem (protozoa, gut microbiota, and host) remain only partially understood. Xenocypris fish and the unique ciliate in its hindgut are good materials to study the interplay.

Methods: In this study, 16S rRNA gene amplicon sequencing and short-chain fatty acids (SCFAs) identification were used. Network was also constructed to understand their relationships.

Results: We found that the gut microbiota of -infected and had higher diversity, richness, and evenness than uninfected ones. could lead to an increase of and Chloroflexi in both and , while significantly increase the abundance of genera and in . Besides, could significantly increase the content of total SCFAs and acetic acid in and increase the concentrations of propionic, isobutyric and butanoic acids in . Furthermore, correlation analyses showed that may alter SCFAs by affecting key SCFAs-producing bacteria such as and .

Discussion: This study greatly expands our understanding of relationships among , gut microbiota and host Xenocypris fish, which sheds new insights into the mechanism of interaction among protozoa, gut microbiota and host.