Capsular polysaccharides have been found to develop around cells of Aer sal

Capsular polysaccharides have been found to develop

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Capsular polysaccharides have been found to develop around cells of Aer. sal- monicida in the presence of glucose, phosphate, magnesium chloride and/or trace elements. Production of this material was improved in the presence of yeast extract (Bonet et al, 1993). Interestingly, a striking difference in cells grown in vitro or in vivo reflected the presence or absence of capsules. Using intraperitoneal chambers, it was observed that Aer. salmonicida produced capsules with virulence functions (Garduno and Kay, 1995). Adherence to fish cell cultures was slightly higher in cultures o^ Aer. salmonicida grown in conditions to promote capsule formation. Also, invasion offish cells was more pronounced in the capsulated cells (Merino et al., 1996). Another role for the capsule concerns resistance to complement. Thus, it was recorded that, when grown under conditions promoting capsule development, Aer. salmonicida were partially resistant to complement (Merino et al., 1997a,b). Agglutination offish cells Another cell-associated factor, possibly relevant to virulence of Aer. salmonicida, is the ability of the pathogen to agglutinate trout and mammalian erythrocytes (Mellergaard and Larsen, 1981). The haemagglutination capability is purported to be related to the presence of adhesins, which are structures on the bacterial surface that mediate the attachment of the pathogen to the host's cell surface. Thus, the interest in haemagglutination is due primarily to its use to provide semi-quantitative information on the adhesive potential of a bacterial strain, while the sugar inhibition of haemagglutination has allowed adhesive specificity to be demonstrated (Duguid and Old, 1980). There is good agreement in the literature on the haemagglutinating abihty of Aer. salmonicida. For instance, Jiwa (1983) reported mannose-resistant agglutination of bovine, chicken, human group A and guinea-pig erythrocytes by two Aer. salmonicida isolates which were recovered from diseased brown trout. It has also been demonstrated that smooth strains were unable to agglutinate erythro- cytes, whereas rough strains showed a broad spectrum of haemagglutinating activity. Similarly, Parker and Munn (1985) observed that virulent, auto-agglutinating A- layer^ cells agglutinated trout erythrocytes as well as a range of mammalian ery- throcytes. They also noted that the process was not inhibited by specific sugars, and thus concluded that adhesion was a relatively non-specific process, attributable to the hydrophobic properties of the A-layer. Extracellular products Researchers interested in the biology of Aer. salmonicida have been aware for some time that extracellular substances (produced by the organism) presumably exerted a role in virulence. However, ECPs are not always harmful to fish. For example, Madetoja et al. (2003a) reported that ECPs, which lacked caseinase and gelatinase and had low cytotoxic activity in cell culture, from an atypical strain isolated from Arctic charr did not cause mortalities. Nevertheless, the ECPs have been the focus of numerous investigations. Unfortunately, the work has been rendered more difficult
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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