A green fluorescent protein (GFP) expressing strain of Yersinia ruckeri was created by the transposition of a Tn10-GFP-kan cassette into the genome of Y. ruckeri Strain YRNC10. The derivative, YRNC10-gfp, was highly GFP fluorescent, retained the gfp-km marker in the absence of kanamycin selection, and exhibited in vitro growth kinetics similar to those of the wild type strain. YRNC10-gfp colonized and caused mortality in immersion and intraperitoneally challenged rainbow trout Oncorhynchus

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... s, although it was modestly attenuated compared to the wild type strain. The distribution and location of YRNC10-gfp in infected fish was visualized by epifluorescence microscopy. Abundant extracellular bacteria and a small number of intracellular bacteria were observed in the kidney, spleen and peripheral blood. To determine the percentage of trout cells containing intracellular bacteria, GFP fluorescence was measured by flow cytometry. A small population of GFP positive leukocytes was detected in peripheral blood (1.6%), spleen (1.1%) and anterior kidney (0.4%) tissues. In summary, this is the first report of the construction of a virulent, GFP-tagged Y. ruckeri, which may be a useful model for detecting and imaging the interactions between an aquatic pathogen and the natural salmonid host. KEY WORDS: Yersinia ruckeri · Green fluorescent protein · Flow cytometry · Epifluorescence microscopy Resale or republication not permitted without written consent of the publisher Editorial responsibility: Carey Cunningham,