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Background  Delivery of antigens by live bacterial carriers can elicit effective humoral and cellular responses and may be an attractive strategy for live bacterial vaccine production through introduction of a vector that expresses an exogenous protective antigen. To overcome the instability and metabolic burden associated with plasmid introduction, alternative strategies, such as the use of  in vivo- inducible promoters, have been proposed. However, screening an ideal  in vivo -activated promoter with high efficiency and low leak expression in a particular strain poses great challenges to many researchers.    Results  In this work, we constructed an  in vivo  antigen-expressing vector suitable for  Edwardsiella tarda , an enteric Gram-negative invasive intracellular pathogen of both animals and humans. By combining quorum sensing genes from  Vibrio fischeri  with iron uptake regulons, a synthetic binary regulation system (ironQS) for  E. tarda  was designed.  In vitro  expression assay demonstrated that the ironQS system is only initiated in the absence of Fe 2+  in the medium when the cell density reaches its threshold. The ironQS system was further confirmed  in vivo  to present an  in vivo -triggered and cell density-dependent expression pattern in larvae and adult zebrafish. A recombinant  E. tarda  vector vaccine candidate WED(ironQS-G) was established by introducing  gapA34,  which encodes the protective antigen glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from the fish pathogen  Aeromonas hydrophila  LSA34 into ironQS system, and the immune protection afforded by this vaccine was assessed in turbot ( Scophtalmus maximus ). Most of the vaccinated fish survived under the challenge with  A. hydrophila  LSA34 (RPS = 67.0%) or  E. tarda  EIB202 (RPS = 72.3%).    Conclusions  Quorum sensing system has been extensively used in various gene structures in synthetic biology as a well-functioning and population-dependent gene circuit. In this work, the  in vivo  expression system, ironQS, maintained the high expression efficiency of the quorum sensing circuit and achieved excellent expression regulation of the Fur box. The ironQS system has great potential in applications requiring  in vivo  protein expression, such as vector vaccines. Considering its high compatibility, ironQS system could function as a universal expression platform for a variety of bacterial hosts.

A quorum sensing-based in vivo expression system and its application in multivalent bacterial vaccine