Combination of Host-Associated Rummeliibacillus sp. and Microbacterium sp. Positively Modulated the Growth, Feed Utilization, and Intestinal Microbial Population of Olive Flounder (Paralichthys olivaceus)

SIMPLE SUMMARY: Global concerns have been raised about the negative effects of antibiotic usage on aquaculture. Synthetic antibiotics have biological alternatives, namely, probiotics, which have the potential to improve fish growth, immunity, intestinal microbial population, and disease resistance. Host-associated probiotics are more adaptable and functional in the known intestinal environment, and two novel strains of Rummeliibacillus sp. and Microbacterium sp. were identified from the flounder (Paralichthys olivaceus) intestine. It is scientifically established that the effect of multiprobiotics was better than that of individual probiotics; as such, these two strains were administered in 1 × 10(8) CFU/g feed at a 50:50 ratio. At the end of the feeding experiment, the growth and feed utilization of the probiotic group (Pro) were higher than those of the control fish. Although serum biochemical parameters did not change, the immune parameter myeloperoxidase increased in the combined probiotic-fed group. These two strains also increased the beneficial microbial abundance in the intestine of the Pro group compared with that in the control fish. Therefore, the identification of the two novel probiotic strains from the flounder intestine has provided a basis for conducting further research, and their combination can be supplemented in commercial flounder cultures in future applications. ABSTRACT: Two novel strains of Rummeliibacillus sp. and Microbacterium sp. were identified from the intestine of olive flounder (Paralichthys olivaceus) and characterized in vitro as potential probiotics. Feeds without probiotic and with a 50:50 mixture of these two strains (1 × 10(8) CFU/g feed) were denoted as the control and Pro diets, respectively. Three randomly selected tanks (20 flounders/tank, ~11.4 g each) were used for each diet replication. After 8 weeks of feeding, the growth and feed utilization of the flounder in the Pro group improved (p < 0.05) compared to the control. Among four immune parameters, only myeloperoxidase activity was elevated in the Pro group. Serum biochemistry, intestinal microbial richness (Chao1), and diversity (Shannon index) remained unchanged (p ≥ 0.05), but phylogenetic diversity was enriched in the Pro fish intestine. Significantly lower Firmicutes and higher Proteobacteria were found in the Pro diet; the genus abundance in the control and Pro was as follows: Staphylococcus > Lactobacillus > Corynebacterium and Lactobacillus > Staphylococcus > Corynebacterium, respectively. Microbial linear discriminant scores and a cladogram analysis showed significant modulation. Therefore, the combination of two host-associated probiotics improved the growth and intestinal microbial population of flounder and could be supplemented in the Korean flounder industry.