Isolation and characterization of new potential probiotic bacteria based on quorum-sensing system

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Abstract
This work was aimed at identifying strains which can degrade quorum-sensing (QS) molecules from fish gut, with properties suitable for use as probiotic in aquaculture. A total of 200 strains were obtained from the intestine gut of Carassius auratus gibelio after enrichment in KG medium contained 500 μg l(-1) of C6-HSL as the sole source of carbon and nitrogen; one strain named QS inhibitor (QSI)-1 was identified as the genus Bacillus spp. by morphological phenotypes, and the strain also possessed an aiiA homologue gene using PCR amplification. In vitro, QSI-1 strongly interfered with violacein production by Chromobacterium violaceum. Coculture of QSI-1 with fish pathogen effectively reduced the amount of acyl-homoserine lactones (AHLs) and the extracellular proteases activity of Aeromonas hydrophila YJ-1. The oral LD50 of QSI-1 to fish was more than 10(11) CFU shown that it was avirulent to fish. Fish fed diet supplemented with QSI-1 had good survival, suggesting that QSI-1 showed protection against Aer. hydrophila infection. The results indicate that the isolate QSI-1 might have the potential possibility to be used as a probiotic in aquaculture. This is the first report to describe a bacterium isolated from the intestine gut of C. auratus gibelio which can degrade AHLs and has the probiotic characteristics for its use in aquaculture.

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