Edwardsiella tarda is a gram-negative pathogen with a broad host range that includes humans, animals, and fish. Recent studies have shown that the LuxS/autoinducer type 2 (AI-2) quorum sensing system is involved in the virulence of E. tarda. In the present study, it was found that the E. tarda LuxS mutants bearing deletions of the catalytic site (C site) and the tyrosine kinase phosphorylation site, respectively, are functionally inactive and that these dysfunctional mutants can interfere with the activity of the wild-type LuxS. Two small peptides, 5411 and 5906, which share sequence identities with the C site of LuxS, were identified. 5411 and 5906 proved to be inhibitors of AI-2 activity and could vitiate the infectivity of the pathogenic E. tarda strain TX1. The inhibitory effect of 5411 and 5906 on AI-2 activity is exerted on LuxS, with which these peptides specifically interact. The expression of 5411 and 5906 in TX1 has multiple effects (altering biofilm production and the expression of certain virulence-associated genes), which are similar to those caused by interruption of luxS expression. Further study found that it is very likely that 5411 and 5906 can be released from the strains expressing them and, should TX1 be in the vicinity, captured by TX1. Based on this observation, a constitutive 5411 producer (Pseudomonas sp. strain FP3/pT5411) was constructed in the form of a fish commensal isolate that expresses 5411 from a plasmid source. The presence of FP3/pT5411 in fish attenuates the virulence of TX1. Finally, it was demonstrated that fish expressing 5411 directly from tissues exhibit enhanced resistance against TX1 infection.

Aim:  To investigate the effect of copper on the virulence of Edwardsiella tarda. Methods and Results:  The pathogenic Edw. tarda strain TX5 was cultured under copper‐stressed conditions and examined for any potential alteration in capacities that are associated with pathogenicity. The results showed that compared to untreated TX5, Cu‐treated TX5 exhibits reduced planktonic and biofilm growth, an impaired ability to adhere to host mucus, modulation of host immune response, and dissemination in host blood and liver. Consistent with these observations, the overall bacterial virulence of Cu‐treated TX5 is significantly attenuated. SDS‐PAGE analyses of whole cell protein production showed that Cu‐treated TX5 differs from the untreated TX5 in its production of at least one protein. Quantitative real time reverse transcriptase PCR analyses showed that copper treatment decreased the expression of virulence‐associated genes encoding components of the type III and type VI secretion systems, the Eth haemolysin system, and the LuxS/AI‐2 quorum‐sensing system. Conclusions:  Prolonged exposure to copper has multiple effects on TX5 and results in significant attenuation of bacterial virulence. Significance and Impact of the Study:  The results of this study demonstrate that copper treatment has a broad and profound effect on the virulence‐associated capacities of TX5, which is exerted at least in part at the transcription level. These findings provide new insights to the antimicrobial mechanism of copper.