Improving tilapia (Oreochromis mossambicus) resistance to streptococcal disease by improving the gut biome through administration of the microorganisms Bacillus subtilis and Enterococcus faecalis

The destruction of microbial communities within the gut of farmed tilapia (Oreochromis mossambicus) by Streptococcus agalactiae infection can result in considerable impacts to the health of the host. It is necessary to find a more effective treatment to improve the gut biome of farmed tilapia. We applied a tilapia culture method incorporating microorganisms in the diet and tracked gut bacteria over a 15-week period. A model of S. agalactiae infected individuals was constructed by detecting blood parameters and actual observation. Good’s coverage index (> 0.98) and < 1 % of reads was indicated by rarefaction curves. Healthy fish with no inclusion of microorganisms in the diet as reference group (RG), we assessed the effects of microorganisms on gut bacterial profiles and described changes in microbial richness and diversity via a comparison of the gut microbiota profiles of infected tilapia with (PD) and without (CG) the addition of microorganisms. An abnormal gut bacterial profile was associated with higher prevalence of infection by S. agalactiae, the lower Vibrio in PDs compared to the CGs, Spongiimonas spp. was present in all PDs and in the 3- and 6-week CGs, Photobacterium spp. appeared in all CGs and in the PDs at 3 and 6 weeks, Synechococcus, Shewanella, Spirochaeta, Psychromonas, Psychrilyobacter, Kiloniella, Allofrancisella, Candidatus Hepatoplasma, and Aliivibrio genera were only observed in the PDs, but Carboxylicivirga and Arcobacter were only detected in the CGs. Infected fish cultured using the combined microorganisms displayed significantly greater gut microbial species richness and diversity (Chao1; p-value < 0.001. Shannon’s; p-value < 0.001). The survival ratio of PDs (> 60 %) were higher than CGs. The application of the combined microorganisms within the diet of cultured O. mossambicus may therefore benefit their health by promoting a gut microbiome of greater diversity and increasing their resistance to disease.