Stewart C. Johnson

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Aeromonas salmonicida subsp. salmonicida is a Gram-negative bacterium that is the causative agent of furunculosis, a bacterial septicaemia of salmonid fish. While other species of Aeromonas are opportunistic pathogens or are found in commensal or symbiotic relationships with animal hosts, A. salmonicida subsp. salmonicida causes disease in healthy fish. The(More)
Physiological, immunological and biochemical parameters of blood and mucus, as well as skin histology, were compared in 3 salmonid species (rainbow trout Oncorhynchus mykiss, Atlantic salmon Salmo salar and coho salmon O. kisutch) following experimental infection with sea lice Lepeophtheirus salmonis. The 3 salmonid species were cohabited in order to(More)
The purpose of this study is to provide a point of reference regarding the neurotoxic effects resulting from exposure to environmental contaminants. Benzo(a)pyrene is a member of the polycyclic aromatic hydrocarbon (PAH) family and it is a by-product of combustion processes. Thus, persons living near factories or hazardous waste sites face the danger of(More)
The recently described type III secretion system (TTSS) of Aeromonas salmonicida subsp. salmonicida has been linked to virulence in salmonids. In this study, three TTSS effector genes, aexT, aopH or aopO, were inactivated by deletion, as was ascC, the gene encoding the outer-membrane pore of the secretion apparatus. Effects on virulence were assayed by live(More)
The consequences of high (735 copepodids fish-1) and low (243 copepodids fish-1) level exposures of size-matched juvenile pink and chum salmon to Lepeophtheirus salmonis copepodids were examined. At both levels of exposure the prevalence and abundance of L. salmonis was significantly higher on chum salmon. In addition, the weight of exposed chum salmon(More)
The changes in the activities of mucus hydrolytic enzymes and plasma cortisol levels were examined following infection of Atlantic salmon Salmo salar with the salmon louse Lepeophtheirus salmonis and these changes were compared with those resulting from elevated plasma cortisol. Salmon were infected at high (Trial 1; 178 +/- 67) and low (Trial 2; 20 +/- 13)(More)
To investigate the immunological responses of turbot to nodavirus infection or pIC stimulation, we constructed cDNA libraries from liver, kidney and gill tissues of nodavirus-infected fish and examined the differential gene expression within turbot kidney in response to nodavirus infection or pIC stimulation using a turbot cDNA microarray. Turbot were(More)
'Sea lice' is a common name for a large number of species of marine ectoparasitic copepods, many of which are widespread and important disease-causing agents that infect both cultured and wild fish. Of these copepods, the salmon louse Lepeophtheirus salmonis is the most extensively studied because of its economic impact on the salmonid aquaculture industry(More)
Physiological changes, elicited in animal immune tissues by exposure to pathogens, may be studied using functional genomics approaches. We created and characterized reciprocal suppression subtractive hybridization (SSH) cDNA libraries to identify differentially expressed genes in spleen and head kidney tissues of Atlantic cod (Gadus morhua) challenged with(More)
Deletion mutants of Aeromonas salmonicida subsp. salmonicida were used to determine the effect of the type three secretion system (TTSS) on Atlantic salmon anterior head kidney leucocytes (AHKL). One strain had a deletion in the outer membrane pore gene, ascC; and the other in three effector genes: aopO, aopH and aexT (we call this strain Deltaaop3). Host(More)