the possibility that Aer salmonicida is a facultative intracellular pathogen

The possibility that aer salmonicida is a facultative

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the possibility that Aer. salmonicida is a facultative intracellular pathogen able to survive within phagocytes. Indeed, Munn and Trust (1984) demonstrated that A- layer^ bacteria (i.e. bacteria with an A-layer) were able to multiply within the principal phagocytic organs, e.g. the spleen, following experimental infection. Sub- sequently, it has become established that Aer. salmonicida is capable of repHcation in macrophages, where the pathogen is presumed to be able to resist reactive radicals (Garduno et al, 1997). It has been argued that the surface layer constitutes the first Hne of defence for Aer. salmonicida, with an inducible catalase and manganese super- oxide dismutase as second defensive systems against macrophage-mediated killing via reactive oxygen species. The A-layer has also been impHcated in a role concerning adhesion to fish tissues. By means of in vitro experiments, Parker and Munn (1985) examined the abihty of avirulent (A-layer ~) cells to adhere to cells of baby hamster kidney and rainbow trout gonad in tissue culture. Attachment of A-layer^ Aer. salmonicida to both types of cells was greater than for the A-layer~ derivative. As a result, Parker and Munn (1985) proposed that since attachment to epithelial cells may be the primary step in the pathogenic process, their observations could account for the association of virulence with the presence of an extra outer membrane layer. Another function of the A-layer is a possible interference with the antibacterial peptides, namely magainin, cecropins and defensins (Henry and Secombes, 2000). To summarise, the accumulating body of evidence indicts the A-layer as a principal virulence determinant, even though its precise functions and the mechanism of action obviously require further clarification. However, blithe acceptance of an absolute relationship between virulence and possession of an A-layer must unfortu- nately be cautioned against. This is in view of reports by Johnson et al. (1985) and Ward et al. (1985) on the occurrence of virulent, auto-agglutinating strains that have no detectable A-layer. Conversely, Olivier (1990) recovered non-virulent A-layer^ isolates. Thus, the association between presence of the extracellular layer and viru- lence, but not between auto-agglutination and virulence, appears to be open to question. It is important that the extent of this problem should be determined, particularly because the use of A-protein as an antigenic component of a potential vaccine for control of diseases caused by Aer. salmonicida has been advocated. This is due to the apparent immunological relatedness of the A-protein among isolates from different locations and a variety offish hosts (Evenberg et al, 1982). In view of the existence of virulent, auto-agglutinating Aer. salmonicida strains apparently lacking the A-layer, the effectiveness of such a vaccine would possibly be subject to severe limitations.
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Pathogenicity 301 The type III secretion system A type III secretion system, which utiHses a 140 kbp plasmid and chromosome-
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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