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Background  The zebrafish embryo is an important  in vivo  model to study the host innate immune response towards microbial infection. In most zebrafish infectious disease models, infection is achieved by micro-injection of bacteria into the embryo. Alternatively,  Edwardsiella tarda , a natural fish pathogen, has been used to treat embryos by static immersion. In this study we used transcriptome profiling and quantitative RT-PCR to analyze the immune response induced by  E. tarda  immersion and injection.    Results  Mortality rates after static immersion of embryos in  E. tarda  suspension varied between 25-75%, while intravenous injection of bacteria resulted in 100% mortality. Quantitative RT-PCR analysis on the level of single embryos showed that expression of the proinflammatory marker genes  il1b  and  mmp9  was induced only in some embryos that were exposed to  E. tarda  in the immersion system, whereas intravenous injection of  E. tarda  led to  il1b  and  mmp9  induction in all embryos. In addition, microarray expression profiles of embryos subjected to immersion or injection showed little overlap.  E. tarda -injected embryos displayed strong induction of inflammatory and defense genes and of regulatory genes of the immune response.  E. tarda -immersed embryos showed transient induction of the cytochrome P450 gene  cyp1a . This gene was also induced after immersion in  Escherichia coli  and  Pseudomonas aeruginosa  suspensions, but, in contrast, was not induced upon intravenous  E. tarda  injection. One of the rare common responses in the immersion and injection systems was induction of  irg1l , a homolog of a murine immunoresponsive gene of unknown function.    Conclusions  Based on the differences in mortality rates between experiments and gene expression profiles of individual embryos we conclude that zebrafish embryos cannot be reproducibly infected by exposure to  E. tarda  in the immersion system. Induction of  il1b  and  mmp9  was consistently observed in embryos that had been systemically infected by intravenous injection, while the early transcriptional induction of  cyp1a  and  irg1l  in the immersion system may reflect an epithelial or other tissue response towards cell membrane or other molecules that are shed or released by bacteria. Our microarray expression data provide a useful reference for future analysis of signal transduction pathways underlying the systemic innate immune response versus those underlying responses to external bacteria and secreted virulence factors and toxins.

Comparison of static immersion and intravenous injection systems for exposure of zebrafish embryos to the natural pathogen Edwardsiella tarda