Effects of oral delivery of two antimicrobial peptides CATHPb1 and As-CATH4 by bait microalga [Tetraselmis subcordiformis (Wille) Butcher] on resistance to bacterial infection in turbot (Scophthalmus maximus L.)

To find a safer alternative to antibiotics for the treatment of bacterial infection in aquaculture, attempts were made to express exogenous antimicrobial peptides (AMPs) in the bait microalga Tetraselmis subcordiformis and feed the obtained alga to turbot ( Scophthalmus maximus L.) to evaluate its antimicrobial effects. Our study revealed the synergistic antimicrobial effects of two previously identified AMPs CATHPb1 and As-CATH4. Subsequently, the chloroplast expression vectors were assembled in tandem and introduced into T. subcordiformis by particle bombardment. The results of in vitro experiments showed that the T. subcordiformis transformant (CAD) exhibited significant inhibitory effects on both Gram-positive and Gram-negative bacteria, exceeding 2.8 times higher than that of wild type (WT). The effects of oral administration of CAD on turbot were examined by mixing CAD with commercial feed (CF), using the turbot fed with the mixture of WT T. subcordiformis and CF as negative control and the turbot fed with only CF as blank control. The results demonstrated that oral administration of CAD significantly increased the survival rate of turbot by 32 % and reduced the colonization rate of Vibrio parahaemolyticus in the gut of turbot by more than 90 %. Furthermore, the incorporation of CATHPb1 and As-CATH4 altered the microbial community and exerted an anti-inflammatory effect with immunological activity on the gut of turbot. Our study suggested the significant potential of successful expression of AMPs in T. subcordiformis as a viable alternative to antibiotics for resistance of bacterial infections in aquaculture. • CATHPb1/As-CATH4 showed synergistic effects on V. parahaemolyticus and S. aureus. • Co-expression of CATHPb1/As-CATH4 in microalga showed strong antibacterial activity. • Feeding T. subcordiformis transformant reduced V. parahaemolyticus colonization. • CATHPb1/As-CATH4 acted as antibacterial and anti-inflammatory agents in turbot.