The S-Layer of Pathogenic Strains of Aeromonas hydrophila

Aeromonas hydrophila, a Gram-negative, motile rod, is a pathogen of a wide variety of animals, including man. In man the organism is most commonly implicated as a gastrointestinal pathogen, but has also been isolated from a variety of other clinical conditions (Trust and Chipman 1979). The species is especially important as a pathogen of fish, typically producing a fulminant haemorrhagic septicaemia (Trust 1986). In many instances disease most likely results from opportunistic infection by strains of A. hydrophila from the fishes’ normal flora. In other cases, however, the strains of A. hydrophila producing fish disease clearly behave as primary pathogens (Mittal et al. 1980, DeFigueiredo and Plumb 1977). For example, a group of strains with high virulence for salmonids has been reported by Mittal et al. (1980). These strains exhibit certain cell-surface-associated phenotypic characteristics unique among A. hydrophila isolates. They auto-aggregate in static broth culture, are resistant to the bactericidal activity of normal serum, and form a single thermostable serogroup based on the O-polysaccharide antigens of their lipopolysaccharides (LPS) (Mittal et al. 1980, Dooley et al. 1985). Several members of the serogroup, while exhibiting virulence for fish, were not isolated from fish, but were isolated from sources such as aborted piglet liver, bovine brain, and human diarrhoeal faeces.