The Quorum Quenching Bacterium Bacillus licheniformis T-1 Protects Zebrafish against Aeromonas hydrophila Infection

Quorum sensing, bacterial cell-to-cell communication via small signaling molecules regulates virulence in many bacterial pathogens, and is a promising target for antivirulence therapy, which may inhibit virulence rather than cell growth and division. Herein, Bacillus strains capable of degrading QS molecules from freshwater environments were screened as potential aquaculture probiotics. A total of 34 Bacillus strains were isolated. Strain T-1 was selected with "H" streaking and double layer agar plate methods using Chromabacterium violaceum ATCC12472 as reporter, and eventually identified as Bacillus licheniformis based on biochemical and molecular identification. Quorum quenching by T-1 was confirmed using C. violaceum CV026. T-1 was non-hemolytic in vitro. In biocontrol experiments, T-1 reduced the pathogenicity of Aeromonas hydrophila cb15 in zebrafish co-injected intraperitoneally with both strains, achieving a relative percentage survival of 70%. Determination and analysis of the T-1 draft genome using the Illumina Hiseq 2500 platform identified the quorum quenching gene ytnP, encoding an acyl-homoserine lactone metallo-β-lactamase, as a potential QS quencher in T-1. In conclusion, B. licheniformis T-1 could be a safe and effective quorum quenching bacterium for protecting hosts against pathogenic bacterial infections in aquaculture.