Abstract
Aquaculture is the fastest-growing food production sector in the world, and it supplies nearly 50% of the global food fish supply. However, disease outbreaks have become a major problem in the fish farming industry. The beneficial contribution of probiotic bacteria to aquatic animals’ health has been widely described, and they have been widely used in aquaculture for disease control and growth promotion. However, the action of probiotic bacterial components and mechanisms underlying protection against pathogens afforded by probiotic bacteria remain poorly understood. In the present study, we pre-colonized zebrafish larvae (before hatching) with 17 potential probiotic bacterial strains and screened for those possessing anti-infective effects against Aeromonas veronii. We found that Lactobacillus casei BL23 significantly increased the survival of zebrafish larvae upon A. veronii infection. Using a germ-free (GF) zebrafish model and gut microbiota transplant experiment, we showed that L. casei BL23 per se has anti-infective effects in zebrafish larvae, which does not involve microbiota. Furthermore, we identified an exopolysaccharide-protein complex (EPSP) extracted from L. casei BL23 cells, which consisted of a 40–45 KD size protein and an exopolysaccharide composed of α-Rha, α-Glc, β-GlcNAc, and β-GalNAc. EPSP significantly increased the survival rate of GF zebrafish at a dose of 10–20 μg/ml after A. veronii infection (P < 0.01). In addition, the EPSP induced a higher expression of TLR1 and TLR2, and modulated the expression profile of pro-inflammatory and anti-inflammatory cytokines in zebrafish liver (ZFL) cells. Our data indicated that the anti-infective effect of EPSP from L. casei BL23 was mediated by enhancement of immune responses in zebrafish, which might involve the TLR1/TLR2 signal pathway.
Highlights
Aquaculture is the fastest-growing food production sector in the world and a major contributor to global food production, contributing nearly 50% of the global food fish supply (Subasinghe et al, 2009)
In order to screen the probiotic strains that protect zebrafish larva from damage induced by A. veronii infection, we pre-colonized unhatched (3 dpf) conventional zebrafish larvae with 17 Gram-positive bacteria commonly often used as probiotics in aquaculture and elsewhere in the food industry (Table 1)
No significant difference in survival rate was observed between the larvae pre-colonized with L. casei BL23 at the dose of 1.0 × 105 CFU/ml and control larvae after A. veronii infection (Figures 1A,B)
Summary
Aquaculture is the fastest-growing food production sector in the world and a major contributor to global food production, contributing nearly 50% of the global food fish supply (Subasinghe et al, 2009). Aquaculture production of aquatic animals amounted to 73.8 million tons in 2014, with an estimated value of US $160.2 billion. It supplies 17% of animal protein in people’s diets worldwide. Disease outbreaks have become a major problem in the fish-farming industry due to the increasing intensification and commercialization of aquaculture practices (Bondad-Reantaso et al, 2005). Infectious diseases have been estimated to cost billions of dollars in the global aquaculture industry annually (Lafferty et al, 2015). Antibiotic resistance among bacterial pathogens and antibiotic residues in the animal products has piqued global interest in limiting the use of antibiotics in aquaculture (Chen et al, 2015; Huang et al, 2015; Pereira et al, 2015)
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