Precipitation of Aeromonas hydrophila The importance of precipitation after

Precipitation of aeromonas hydrophila the importance

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Precipitation of Aeromonas hydrophila The importance of precipitation after boiling is a debatable issue in screening of Aer. hydrophila isolates for virulence. Santos et al. (1988) considered that precipita- tion was not an important indicator, whereas Mittal et al. (1980) and Karunasagar et al. (1990) reported that settHng after boiling was indeed an important measure of virulence.
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294 Bacterial Fish Pathogens Scavenging for iron The ability of a potential pathogen to scavenge successfully for iron (in iron-limited conditions) will influence the outcome of the infection process. The haemolysins of Aer. hydrophila are iron-regulated, and access to iron in the haemolytic destruction of the host cells may be necessary (Massad et al, 1991). The acquisition of iron from iron-transferrin in serum is dependent on the siderophore amonabactin. Many aeromonads use haem as a sole source of iron for growth. Some have evolved both siderophore-dependent (iron-transferrin) and -independent mechanisms (haem compounds) for the acquisition of iron from host tissues (Massad et al, 1991). Enter otoxigenicity Strains have been attributed with enterotoxigenicity, as assessed by the rabbit ileal loop technique, and cytotoxicity (Boulanger et ai, 1977; Jiwa, 1983; Paniagua et ai, 1990), and correlated with lysine decarboxylase production (Santos et al, 1987). Enterotoxigenic strains have been shown to produce two types of enterotoxins, which appear to be antigenically related, although the mode of action differs (Boulanger et al, 1977). This was an interesting observation because de Meuron and Peduzzi (1979) isolated two types of antigen, of which the K-antigen (this was thermolabile at 100°C) was considered to represent a pathogenicity factor. Possibly, this corresponded to the enterotoxin or cytotoxin as described by Boulanger et al. (1977). However, the O (somatic) antigen, which was heat-stable, may have greater relevance, insofar as most virulent isolates share a common O-antigen (Mittal et al, 1980). In an excellent study, Dooley et al. (1985) used SDS-PAGE to analyse LPS (considered to constitute an O- antigen) from virulent strains, which auto-agglutinated in static broth culture. The LPS contained O-polysaccharide chains of homogeneous chain length. Two strains produced a surface protein array, which was traversed by O-polysaccharide chains and thus exposed to the cell surface. Antigenic analysis revealed that the polysacchar- ide of the LPS carried three antigenic determinants. Clearly, the evidence indicates the involvement of both endo- and exotoxins in the pathogenesis of Aer. hydrophila infections. It still remains for further work to elucidate the precise mechanism of action. Evidence from molecular analyses By comparing virulent and avirulent cultures, suppression subtractive hybridisation (SSH) was used to identify genetic differences, with the results highlighting 69 genomic regions absent from the latter (Zhang et al, 2000). Genes considered to represent known virulence attributes included haemolysin, histone-like protein, oli- goprotease A, OMP and multi-drug resistance protein. Other genes encoded synthesis
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  • Bacteria, representative, gram-negative bacteria

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