Probiotic spore-based antigen delivery: a novel oral vaccine strategy against Vibrio infections in aquaculture

Vibriosis is a deadly illness caused by various species of the Vibrio genus. Due to its high incidence in aquaculture plants, vibriosis is responsible for significant economic losses. Currently, anti-vibriosis treatments rely on antibiotics. However, the global rise in antibiotic resistance necessitates the development of alternative approaches. Novel vaccines and effective probiotics have been proposed as potential alternative to antibiotics in fighting bacterial infections. Here we propose a combined vaccine/probiotic strategy based on the use of probiotic bacterial spores for the oral delivery of Vibrio antigens. Spores of various species of the Bacillus genus are widely used as probiotics and have been shown to efficiently display antigens in a non-recombinant way. Spores of various probiotic strains were analyzed to assess their effectiveness in displaying a heterologous model protein, and B. megaterium MV30 was identified as the most efficient strain. MV30 spores were then used to display two antigens of Vibrio harveyi , the entire Hsp33 protein of 33 kDa and a 239 amino acids fragment of OmpK (OmpK 21/260 ), identified as the most immunogenic part of the protein. While Hsp33 is a stable protein, OmpK 21/260 is unstable at conditions mimicking those encountered in an aquaculture plant and the interaction with MV30 spores reduced such instability. The protective ability of the combined probiotic/vaccination strategy was assayed on Medaka fish ( Oryzias latipes ), as a model. In a challenge experiment with a virulent strain of Vibrio harveyi , a protective effect was observed with MV30 spores alone and such effect was significantly increased when the same spores displayed either one of the two antigens. Our results support the use of probiotics and oral vaccines as a valid alternative to antibiotics and point to the application of probiotic spore-based antigen delivery as a novel strategy to fight pathogenic infections.