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Application of multifunctional probiotics is considered to be a practical strategy for the control of infections caused by Gram-negative bacteria. Consequently, the present study was designed to investigate the possible inhibitory effects of quorum quenching (QQ) probiotics on several N-acyl-homoserine lactone (AHL)-associated virulence factors in Vibrio alginolyticus and V. harveyi in vitro, and to evaluate the impacts of their administration in the diet on non-specific immune responses, antioxidant capacity and resistance to V. alginolyticus in Asian seabass. For the in vitro assay, the effect of exogenous AHL was assayed on the regulation of several virulence factors including lytic enzymes (amylase, gelatinase, hemolysin, lecithinase, lipase and protease productions), swarming and swimming motilities, and biofilm formation in V. alginolyticus and V. harveyi. Exogenous 3-oxo-C10-HSL positively regulated the production of amylase, gelatinase, protease and biofilm in V. alginolyticus. Exogenous 3-oh-C4-HSL positively regulated swarming, swimming and biofilm formation in V. harveyi. To determine the virulence-inhibitory effects of QQ probiotics on the AHL-dependent virulence factors, cell-free lysates were obtained from Bacillus thuringiensis QQ1 and B. cereus QQ2. Both probionts could reduce the production of amylase, gelatinase and protease in V. alginolyticus. Bacillus thuringiensis QQ1 was also able to decrease the biofilm formed by V. alginolyticus. Bacillus thuringiensis QQ1 and B. cereus QQ2 decreased swarming and swimming motilities, and biofilm formation in V. harveyi. In order to assay the effectiveness of the QQ probiotics in vivo, Asian seabass were fed with basal diet (control) and basal diet supplemented with 109 CFU/g feed of B. thuringiensis QQ1 or B. cereus QQ2 for 8 weeks. At the end of pre-infection trial (day 42), catalase (CAT), superoxide dismutase (SOD), lysozyme, antiprotease, myeloperoxidase (MPO), respiratory burst (RB) and bactericidal activities were significantly (p < 0.05) higher in the probiotic-treated fish than un-treated fish. After challenge with V. alginolyticus, significantly (p < 0.05) higher levels of glutathione (GSH) in the QQ2 treatment (day 49) and lower levels of malondialdehyde (MDA) in both probiotic groups were observed compared to the control group. Moreover, CAT, MPO, RB, alternative pathway of complement (APC), lysozyme, antiprotease and bactericidal activities were significantly (p < 0.05) higher in the probiotic treatments compared to the control group after 7 and 14 days post-infection. Both probionts significantly (p < 0.05) improved the cumulative survival in Asian seabass against V. alginolyticus. Our results indicate that QQ probiotics can modulate immune and antioxidant responses, and provide high protection against Vibrio infection in Asian seabass.

The use of probiotics is a promising and increasingly common strategy to maintain a healthy farming environment in an eco-friendly and sustainable manner. The goal of this study was to investigate the possible effects of dietary supplementation with autochthonous quorum quenching (QQ) probiotics on growth, digestive enzymes activity, haemato-biochemical responses, gut microbial communities and resistance against Vibrio harveyi in Asian seabass. Asian seabass (75 ± 0.6 g and 15.5 ± 0.3 cm) were fed with basal diet (control groups) and basal diet containing 1 × 109 CFU per g Bacillus thuringiensis QQ1 or Bacillus cereus QQ2 for 6 weeks. Feeding with diets supplemented with QQ probiotics for 35 days (pre-infection period) resulted in a significant (p