c N-Acylated homoserine lactone (AHL) lactonases are capable of degrading signal molecules involved in bacterial quorum sensing and therefore represent a new approach to control bacterial infection. Here a gene responsible for the AHL lactonase activity of Bacillus sp. strain AI96, 753 bp in length, was cloned and then expressed in Escherichia coli. The deduced amino acid sequence of Bacillus sp. AI96 AiiA (AiiA AI96 ) is most similar to those of other Bacillus sp. AHL lactonases (ϳ80% sequence identity) and was consequently categorized as a member of the metallo-␤-lactamase superfamily. AiiA AI96 maintains ϳ100% of its activity at 10°C to 40°C at pH 8.0, and it is very stable at 70°C at pH 8.0 for at least 1 h; no other Bacillus AHL lactonase has been found to be stable under these conditions. AiiA AI96 resists digestion by proteases and carp intestinal juice, and it has broad-spectrum substrate specificity. The supplementation of AiiA AI96 into fish feed by oral administration significantly attenuated Aeromonas hydrophila infection in zebrafish. This is the first report of the oral administration of an AHL lactonase for the efficient control of A. hydrophila.

In the current study, we assessed bacterial diversity in the gut content of pond-reared grass carp (Ctenopharyngodon idellus), in the associated habitat environments (pond water and sediment) and in the ingested food (commercial feed and the reed Phragmites australis) by analysing 16S rDNA sequences from clone libraries. The highest bacterial diversity was observed in the gut content and was determined by the total number of operational taxonomic units, Shannon diversity index (H), Shannon equitability index (E H ), Coverage (C good ) and rarefaction curves calculated from the 16S rDNA gene libraries. Our data indicated that allochthonous gut microbes of grass carp were distinctively di¡erent from the corresponding environmental microbes. The pairwise similarity coe⁄cient (C s ) for microbe communities between gut content and ingested food was higher than for those between the gut content and habitats, indicating that the allochthonous microbiota identi¢ed in the intestines of grass carp were phylogenetically closer to those in the ingested food than to those in the habitat. Based on our study and previous research, we suggest that the digesta of grass carp harbours a microbiota phylogenetic core of Proteobacteria and Firmicutes and this observation deserves further investigations with respect to a potential pool of probiotics to grass carp.