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

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 center dot 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 C-4-HSL, C-6-HSL, 3-oxo-C-6-HSL and an uncharacterized AHL molecule.