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

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.