For its formation Belland and Trust 1985 reasoned that the A layer subunits

For its formation belland and trust 1985 reasoned

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For its formation, Belland and Trust (1985) reasoned that the A-layer subunits pass though the periplasm and across the outer membrane for assembly on the cell surface. A requirement for the presence of O-polysaccharide chains, for which the AbcA protein is involved in biosynthesis (Noonan and Trust, 1995) on the LPS was reported as necessary for the assembly of A-layer (Dooley et ai, 1989). These virulent, auto-agglutinating forms produce characteristic deep-blue colonies on CBB agar (Wilson and Home, 1986; Bernoth, 1990). Sakai (1986a, b) postulated that a possible mechanism for auto-agglutination and adhesion could be attributed to the presence of net negative electrical charge in the interiors or on the surfaces of cells. In particular, pathogenic cultures were highly adhesive (Sakai, 1987). It should be emphasised that Udey and Fryer (1978) determined that strains maintained for long periods in laboratory conditions were not auto-agglutinating, and demonstrated reduced virulence. Conversely, it was observed that fresh isolates, obtained from epizootics, were of the aggregating type. From the results of experiments, Udey and Fryer (1978) concluded that the presence of the A-layer was necessary for virulence. However, they contended that more work was needed to establish whether or not the A-layer alone could confer virulence. The discovery of the A-layer generated much interest, resulting in further study of its chemical composition and its specific role in fish pathology. Kay et <2/. (1981) succeeded in purifying the A-layer from virulent isolates, and concluded that it was composed of a surface-localised protein with a molecular weight of 49kDa. Phipps et al. (1983) continued with work on purification and characterisation of the substance, determining that it
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Pathogenicity 299 was hydrophobic in nature, present on the entire cell surface, did not possess any enzymic activity, but instead constituted a macromolecular, refractive protein barrier which was essential for virulence. Meanwhile, an independent parallel investigation of Evenberg et al. (1982) highlighted the relationship between auto-agglutination and the presence of the A-layer. This group examined the cell envelope protein patterns of a variety of isolates obtained from a wide range of geographical locations and different fish species (i.e. carp, minnow, goldfish and salmonids). These fish were suffering from either furunculosis, CE or ulcer disease. A major protein (molecular weight = 54 kDa) was found in all auto-aggregating strains, but little or no trace occurred in isolates which were not auto-agglutinating. When examinations for the presence of the protein were carried out after a change of growth medium, i.e. replacement of horse serum by synthetic sea salt, it was observed that an almost complete loss of the additional cell envelope and the auto-agglutinating ability of the isolate had occurred. Using gel immunoradio assays, it was also determined that the extra cell envelope proteins of all the isolates, irrespective of fish host, type of infection or geographical source, were immunologically related. Evenberg and
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