title: Survival and replication of Piscirickettsia salmonis in rainbow trout head kidney macrophages.
abstract: Piscirickettsia salmonis is pathogenic for a variety of cultured marine fish species worldwide. The organism has been observed within host macrophages in natural disease outbreaks among coho salmon and European sea bass. In vitro studies, incorporating transmission electron microscopy (TEM) and ferritin loading of lysosomes, have confirmed that P. salmonis is capable of surviving and replicating in rainbow trout macrophages. Certain features of this intracellular survival underline its difference to other intracellular pathogens and suggest that a novel combination of defence mechanisms may be involved. Escape into the macrophage cytoplasm is not used as a means to avoid phago-lysosomal fusion and the organism remains at least partly enclosed within a vacuole membrane. While the piscirickettsial vacuole is often incomplete, survival and replication appear to require occupation of a complete, tightly-apposed, vacuolar membrane which does not fuse with lysosomes. Unlike some mammalian rickettsiae, actin-based motility (ABM) is not used as a means of intercellular spread. It is postulated that the presence of numerous small vesicles within vacuoles, and at gaps in the vacuolar membrane, may result from the blebbing of the piscirickettsial outer membrane seen early in the infection.
authors.affiliation: Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK. una_mccarthy@umit.maine.edu
authors.forename: Una M
authors.initials: UM
authors.lastname: McCarthy
authors.forename: James E
authors.initials: JE
authors.lastname: Bron
authors.forename: Linton
authors.initials: L
authors.lastname: Brown
authors.forename: Fazel
authors.initials: F
authors.lastname: Pourahmad
authors.forename: Ian R
authors.initials: IR
authors.lastname: Bricknell
authors.forename: Kim D
authors.initials: KD
authors.lastname: Thompson
authors.forename: Alexandra
authors.initials: A
authors.lastname: Adams
authors.forename: Anthony E
authors.initials: AE
authors.lastname: Ellis
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D007668:Kidney / Q000166:cytology*; D008264:Macrophages / Q000382:microbiology* / Q000648:ultrastructure; D017686:Oncorhynchus mykiss; D053522:Piscirickettsia / Q000502:physiology*