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infection or geographical source, were immunologically related. Evenberg and Lugtenberg (1982) pursued this topic, and described the protein as water-insoluble with an amino acid composition similar to those of the additional surface layers of other bacteria, e.g. the adhesive K88 fimbriae of enteropathogenic strains ofEsch. coli.It is particularly relevant that the findings of Evenberget al.(1982), concerning the auto-agglutinating ability of "atypical" strains from cases of CE and ulcer disease, and the presence of the A-layer, were in excellent agreement with the work of Trustet al.(1980c) and Hamiltonet al.(1981). These earlier studies deduced the presence of an outer layer protein, which was estimated to have a molecular weight of 50 kDa. Evidence was provided by Ishiguroet al.(1981) that loss of the A-layer and loss of auto-agglutinating properties resulted in decreased virulence. After examining the effects of temperatures on the growth ofAer. salmonicida,it was shown that in cells cultured at 30°C (the generally accepted upper limit for the organism) virulence was restricted to <10% of the population. The avirulent, attenuated cells that resulted from use of the higher growth temperature did not auto-agglutinate and, for that matter, did not possess the A-layer. It is interesting to note that higher maximum-growth temperatures were recorded for the attenuated strains, in compar-ison with their virulent counterparts. Perhaps, this is explained by their selection at high temperatures. Because of this observation, Ishiguroet al.(1981) hypothesised that the A-layer is important in determining the physical properties of the cell envelope, and that these properties undergo a change when the A-layer is lost, permitting growth at higher than normal temperatures. If the A-layer is a prerequisite for virulence, it may be assumed that its presence confers advantages on the bacterial cell in its role as a pathogen. Indeed, several prime functions for the A-layer have been proposed. Thus, evidence exists that the extracellular layer protectsAer. sal-monicidacells from the action of protease (Kay and Trust, 1991) and bacteriophage, by shielding its phage receptors (Ishiguroet al.,1981). In addition, the layer may protect the cell from serum complement, insofar as Munn and Trust (1984) demon-strated that virulent strains (with the A-layer) were resistant to complement bacteriocidal activity in the presence (and indeed absence) of specific antibody in
300Bacterial Fish Pathogens rainbow trout serum. Other investigations have revealed that hydrophobicity is conferred upon the bacterial surface by the A-layer (Trustet ai,1983; Van Alstine et al,1986). These workers reported that the hydrophobic A-layer providedAer. salmonicidacells with an affinity for fatty acid esters of polyethylene glycol and an enhanced ability to associate with rainbow trout and mouse phagocytic monocytes (macrophages), in the absence of opsonising antibody. Although Trustet al.(1983) conceded that the advantages to the pathogen of the increased association with macrophages remained to be determined, they suggested as a tentative explanation
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