Abstract The innate immune response of Charybdis japonica treated with Aeromonas hydrophila was explored using bioinformatics. Metabolomics data were integrated with a gut microbial 16S rRNA dataset, together with information on corresponding enzyme activity. The results of the study showed that after being infected with A. hydrophila , some beneficial genera of bacteria in the gut of C. japonica , such as Photobacterium, Rhodobacter, Polaribacter, Psychrilyobacter, Mesoflavibacter, Fusibacter and Phormidium, could directly inhibit Vibrio or produce extracellular polysaccharides with highly effective antibacterial properties. The intestinal probiotics of C. japonica such as Mesoflavibacter have a mutually reinforcing relationship with Phaeobacter, Colwellia, Bacillus, Psychrobacter and Cohaesibacter. Conditional pathogenic bacteria in the gut of healthy crabs may also have such a symbiotic relationship with intestinal probiotics, promoting their growth and reproduction. For example, Phormidium has a mutualistic relationship with Aeromonas and Azopira. Metabolites in the gut of C. japonica infected with A. hydrophila , including beta-alanine metabolism, nitrogen metabolism, inositol phosphate metabolism, galactose metabolism, histidine metabolism, ascorbate and arginine and proline metabolism were increased, with alanine metabolism being the most abundant. The activity of metabolite related enzymes such as lipid peroxidase, phenoloxidase, superoxide dismutase, nitric oxide synthase, glutathione transferase and mid-glutathione decreased and NO levels also decreased. The positive correlation with the probiotic flora suggests that metabolites increase with bacterial abundance and that microbial metabolites or co-metabolites can, in turn, achieve many pleiotropic effects to resist invasion by A. hydrophila . These results may contribute to further research in the resistance of C. japonica to invading pathogens. Importance With the rapid development of the C. japonica farming industry, investors, in pursuit of economic benefits, have encountered problems such as frequent outbreaks of various diseases, resulting in high mortality and huge economic losses. The open water circulation system can give rise to several crab bacterial diseases. Among these, A. hydrophila is a pathogenic bacterium affecting fish and crustaceans, which leads to huge economic losses. Our results suggest that metabolites increased with the abundance of bacteria. It is possible that the autoimmune system and the entry of A. hydrophila into the intestinal tissues of C. japonica react immunologically and that the organism is producing certain metabolites which may be metabolised by various bacteria and absorbed into the circulation. In addition, some of these metabolites are modified or bound in the hepatopancreas to produce microbiota-host co-metabolites. These microbial metabolites or co-metabolites can resist invasion by A. hydrophila .

Effect of Aeromonas hydrophila infected on metabolomic response and gut microbial 16S rRna of Charybdis japonica