Detection of different quorum-sensing signal molecules in a virulent Edwardsiella tarda strain LTB-4

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Abstract
Aims: The aim of this study was to elucidate the potential quorum‐sensing (QS) signal molecules of an emerging pathogen (Edwardsiella tarda strain LTB‐4) of cultured turbot (Scophthalmus maximus). Methods and Results: A sensitive and rapid double‐layer plate method using biosensor strain Agrobacterium tumefaciens KYC55 was developed to detect the N‐acylhomoserine lactone (AHL)‐related compounds in bacteria. LTB‐4 was found to have two QS systems, one was based on the AHLs and the other was based on the autoinducer‐2 (AI‐2). The AI‐2 activity produced by LTB‐4 was growth phase dependent and topped at OD600 of 1·0. The protocol to detect cholerae autoinducer 1 (CAI‐1) activity in bacteria was modified, lowering the background luminescence of biosensor strain Vibrio harveyi JAF375. CAI‐1 activity could not be detected in LTB‐4. Conclusion: Edwardsiella tarda LTB‐4 produced at least four kinds of AHLs during its whole growth phase. In comparison with the AHL‐inducing QS, AI‐2 may be the first predominant signal, functioning at early exponential phase. LTB‐4 did not produce any CAI‐1 activity. Significance and Impact of the Study: Different QS signal molecules of Edw. tarda LTB‐4 were clarified by improved bioassays. In contrast to earlier studies detecting two types of AHLs, strain LTB‐4 produced at least four kinds of AHLs, which seemed to be C4‐HSL, C6‐HSL, 3‐oxo‐C6‐HSL and an uncharacterized AHL molecule.

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