Yersinia ruckeri is the etiological agent of enteric redmouth disease, a systemic infection which mainly affects salmonids. Although this important freshwater pathogen was discovered in 1966, little is known about its virulence mechanisms. In the present study, the interactions with rainbow trout head kidney macrophages were investigated. In vitro experiments were performed to measure uptake, intracellular survival, respiratory burst response and macrophage viability after exposure to Y. ruckeri. Additionally, the fate of Y. ruckeri in the head kidney after immersion infection was studied in vivo. Results show that Y. ruckeri induced the production of reactive oxygen species and that this response peaked at around 3 h after exposure. Despite these toxic substances, Y. ruckeri is able to survive in vitro inside trout macrophages for at least 24 h. Transmission electron microscopy demonstrated that Y. ruckeri bacteria are sequestered in autophagocytic compartments without fusion with primary lysosomes. Inside these compartments, bacteria were capable of replicating. Immersion infection of juvenile rainbow trout resulted in steadily increasing numbers of bacteria in the head kidney over time. As the infection progressed, Y. ruckeri shifted from a predominantly extracellular phase during the first week after infection to an intracellular phase inside the host macrophages from day 7 onwards. In conclusion, this study clearly demonstrates the capacity of Y. ruckeri to survive in rainbow trout macrophages in vitro as well as in vivo, confirming its facultative intracellular nature.
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