178
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 precipitation was not an important indicator, whereas Mittal et al. ( 1980 ) and Karunasagar
et al. ( 1990 ) reported that settling after boiling was indeed an important measure of
virulence.
Scavenging for Iron
The ability of a potential pathogen to scavenge successfully for iron (in iron-limited
conditions) will infl uence 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 ).
Enterotoxigenicity
Strains have been attributed with enterotoxigenicity, as assessed by the rabbit ilealloop technique, and cytotoxicity (Boulanger et al. 1977 ; Jiwa 1983 ; Paniagua et al.
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 analyses LPS
(considered to constitute an O-antigen) from virulent strains, which autoagglutinated 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 polysaccharide of the LPS carried three antigenic
determinants.
There is evidence that macrophage cytotoxicity leading to apoptosis is induced
by Aer. hydrophila (Banerjee et al. 2012 ). Apoptosis commences with phagocytosis
of viable bacterial cells into the head kidney macrophages; this leads to an alteration
4 Aeromonadaceae Representatives (Motile Aeromonads)
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 precipitation was not an important indicator, whereas Mittal et al. ( 1980 ) and Karunasagar
et al. ( 1990 ) reported that settling after boiling was indeed an important measure of
virulence.
Scavenging for Iron
The ability of a potential pathogen to scavenge successfully for iron (in iron-limited
conditions) will infl uence 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 ).
Enterotoxigenicity
Strains have been attributed with enterotoxigenicity, as assessed by the rabbit ilealloop technique, and cytotoxicity (Boulanger et al. 1977 ; Jiwa 1983 ; Paniagua et al.
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 analyses LPS
(considered to constitute an O-antigen) from virulent strains, which autoagglutinated 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 polysaccharide of the LPS carried three antigenic
determinants.
There is evidence that macrophage cytotoxicity leading to apoptosis is induced
by Aer. hydrophila (Banerjee et al. 2012 ). Apoptosis commences with phagocytosis
of viable bacterial cells into the head kidney macrophages; this leads to an alteration
4 Aeromonadaceae Representatives (Motile Aeromonads)