Although Duff 1937 was the first worker to report the ability of Aer

Although duff 1937 was the first worker to report the

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the presence of an extra antigen in the rough strains. Although Duff (1937) was the first worker to report the ability of Aer. salmonicida to dissociate into several distinct colony types with differences in pathogenicity, a phenomenon which is now widely accepted, it is curious that he ascribed pathogenicity to the smooth colony type. This is in contrast to the view currently held that the rough colony type is, in fact, virulent. Interestingly, the Furunculosis Committee had also reported a variation in colony morphology among isolates (it may be assumed that these corresponded to the rough and smooth variants), but contended that this phenomenon was not accompanied by a difference in virulence. It is regrettable that this initial confusion over dissociation occurred, preventing an earlier realisation of its significance. In fact, the relevance of dissociation of Aer. salmonicida colonies and the relationship to virulence was not made apparent until the work of Udey (1977), almost 40 years later. Early studies provided tentative evidence for a variety of possible pathogenic mechanisms, but
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Pathogenicity 297 there is no doubt that progress in the understanding of Aer. salmonicida pathogenesis and virulence has been accelerated by rapid advances in the knowledge of cell biology and the development of sophisticated biochemical techniques. It is the application of such techniques that continues to yield considerable new information about the manner in which Aer. salmonicida may affect its disease processes in fish. Pathogenicity the value of intraperitoneal chambers An intriguing and significant development concerned the description of intraperito- neal chambers, which could be implanted into fish (Garduno et ai, 1993a,b). These chambers could be filled with pathogens (or for that matter a range of other objects), implanted into fish, and measurements made with time. Garduno and colleagues placed Aer. salmonicida into a chamber, and studied its fate in the peritoneal cavity of rainbow trout. In one set of investigations, these workers oberved that when the pathogen was contained in the chamber killing occurred rapidly as a result of host-derived lytic activity (in the peritoneal fluid). In contrast, free cells had a better chance of survival (Garduno et al., 1993a). Moreover, within the peritoneal chamber, Aer. salmonicida produced novel antigens, as determined by western blots (Thornton et al., 1993). In another pubHcation using the peritoneal chamber, evidence was presented that the capsular layer around Aer. salmonicida permitted the pathogen to resist host-mediated bacteriolysis, phagocytosis and oxidative kilHng (Garduno et al, 1993b). Pathogenicity cell-associated versus extracellular components A variety of pathogenicity mechanisms and virulence factors have been proposed for diseases caused by Aer. salmonicida, namely possession of an extracellular (A) layer (= the surface or "S" layer), a type III secretion system (e.g. Dacanay et al., 2006) and the production of ECP, although there is confusion and even contradiction about the relative merits of the various components in pathogenicity (see Ellis
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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