Next-generation sequencing reveals significant variations in bacterial compositions across the gastrointestinal tracts of the Indian major carps, rohu (Labeo rohita), catla (Catla catla) and mrigal (Cirrhinus cirrhosis).

Bacterial communities strongly influence the digestion, health and immune status of fish. This study investigates the microbial distribution of the anterior, middle and distal gut sections of three economically important carp species in Bangladesh, rohu, catla and mrigal (commonly known as Indian major carps), using 16S rRNA-based Illumina sequencing technology. The alpha-diversity measurement with one-way ANOVA indicated high species richness, Shannon and Simpson indices in the middle and distal gut, while the anterior gut of IMCs had the lowest diversity. At the phylum level, there was high abundance of Proteobacteria in the GITs of rohu and mrigal, whereas Fusobacteria was dominant in the anterior and middle guts of catla. At the genus level, diverse microbial communities were identified across the three GIT sections, with six indicator genera found in rohu, catla and mrigal, as revealed by linear discriminant analysis (LDA) at a 0·05 level of significance. Of the 218 genera identified, only 33 were common across the anterior, middle and distal guts of all three species. Bacterial diversity was significantly higher (P < 0·05) in mrigal, followed by catla and rohu, respectively. Alongside the common bacteria Aeromonas, Enterobacter and Serratia, the overwhelming abundance of Cetobacterium, Shewanella and Plesiomonas warrants further investigation. SIGNIFICANCE AND IMPACT OF THE STUDY: This study investigates the microbial communities of the gastrointestinal tracts (GITs) of three Indian major carp (IMC) species-rohu, catla and mrigal, obtained from a polyculture pond under the same feeding regime. Diverse microbial communities were found, with significantly different relative abundances and diversities of phyla and genera. The results provide valuable information on GIT microbial communities that may be useful for nutrition and health management in IMCs.