734 Modern Food Microbiology
Regarding etiology, Hudson and Brown
50 believe the evidence does not favor histamine per se as the
agent responsible for the syndrome. They suggest a synergistic relationship involving histamine and
other as yet unidentified agents such as other amines or factors that influence histamine absorption.
This view is based on the inability of large oral doses of histamine or histamine-spiked fish to produce
symptoms in volunteers. On the other hand, the suddenness of onset of symptoms is consistent with a
histamine reaction, and the association of the syndrome with scombroid fish containing high numbers
of histidine–decarboxylase-producing bacteria cannot be ignored. Although the precise etiology may
yet be in question, bacteria do play a significant if not indispensable role.
Aeromonas
This genus consists of several species that are often found in gastrointestinal specimens. Among
these are A. caviae, A. eucrenophila, A. schubertii, A. sobria, A. veronii, and A. hydrophila. An
enterotoxin has been identified in A. caviae
76 and A. hydrophila (see below), and the other species
noted are associated with diarrhea. As A. hydrophila has received the most study, the discussion that
follows is based on this species. The aeromonads are basically aquatic forms that are often associated
with diarrhea, but their precise role in the etiology of gastrointestinal syndromes is not clear.
A. hydrophila is an aquatic bacterium found more in salt waters than in fresh waters. It is a significant
pathogen to fish, turtles, frogs, snails, and alligators and is also a human pathogen, especially in
compromised hosts. It is a common member of the bacterial biota of pigs. Diarrhea, endocarditis,
meningitis, soft-tissue infections, and bacteremia are caused by A. hydrophila.
Virulent strains of A. hydrophila produce a 52-kDa single polypeptide that possesses enterotoxic,
cytotoxic, and hemolytic activities. This multifunctional molecule displays immunological crossreactivity with the cholera toxin.
92 According to some investigators,
116 it resembles aerolysin while
others contend that it is aerolysin.
78 Aerolysin is a pore- or channel-forming toxin that kills cells by
forming discrete channels in their plasma membranes.
12 Ion channels are created by the oligomerization
of toxin molecules. Cytotonic activity has been associated with an A. hydrophila toxin, which induced
rounding and steroidogenesis in Y-1 adrenal cells. Also, positive responses in the rabbit ileal loop,
suckling mouse, and CHO assays have been reported for a cytotonic toxin.
27
A large number of studies have been conducted on A. hydrophila isolates from various sources. In
one study, 66 of 96 (69%) isolates produced cytotoxins, whereas 32 (80%) of 40 isolates from diarrheal
disease victims were toxigenic, with only 41% of nondiarrheal isolates being positive for cytotoxin
production. Most enterotoxigenic strains are VP (Voges-Proskauer test) and hemolysin positive and
arabinose negative
13 and produce positive responses in the suckling mouse, Y-1 adrenal cell, and rabbit
ileal loop assays. In a study of 147 isolates from patients with diarrhea, 91% were enterotoxigenic,
whereas only 70% of 94 environmental strains produced enterotoxin as assessed by the suckling
mouse assay.
14 All but four of the clinical isolates produced hemolysis of rabbit red blood cells. Of
116 isolates from the Chesapeake Bay, 71% were toxic by the Y-1 adrenal cell assay, and toxicity
correlated with lysine decarboxylase and VP reactions.
56 In yet another study, 48 of 51 cultures from
humans, animals, water, and sewage, produced positive responses in rabbit ileal loop assays with 10
3
or more cells, and cell-free extracts from all were loop positive.
3
Isolates from meat and meat products have been shown to possess biochemical markers that are
generally associated with toxic strains of other species, with the mouse median lethal dose (LD 50 )
being log 8–9 colony-forming units (cfu) for most strains tested.
85 The latter investigators suggested
the possibility that immunosuppressive states are important factors in food-associated infections by
this organism, a suggestion that could explain the difficulty of establishing this organism as the sole
etiological agent of foodborne gastroenteritis.
Regarding etiology, Hudson and Brown
50 believe the evidence does not favor histamine per se as the
agent responsible for the syndrome. They suggest a synergistic relationship involving histamine and
other as yet unidentified agents such as other amines or factors that influence histamine absorption.
This view is based on the inability of large oral doses of histamine or histamine-spiked fish to produce
symptoms in volunteers. On the other hand, the suddenness of onset of symptoms is consistent with a
histamine reaction, and the association of the syndrome with scombroid fish containing high numbers
of histidine–decarboxylase-producing bacteria cannot be ignored. Although the precise etiology may
yet be in question, bacteria do play a significant if not indispensable role.
Aeromonas
This genus consists of several species that are often found in gastrointestinal specimens. Among
these are A. caviae, A. eucrenophila, A. schubertii, A. sobria, A. veronii, and A. hydrophila. An
enterotoxin has been identified in A. caviae
76 and A. hydrophila (see below), and the other species
noted are associated with diarrhea. As A. hydrophila has received the most study, the discussion that
follows is based on this species. The aeromonads are basically aquatic forms that are often associated
with diarrhea, but their precise role in the etiology of gastrointestinal syndromes is not clear.
A. hydrophila is an aquatic bacterium found more in salt waters than in fresh waters. It is a significant
pathogen to fish, turtles, frogs, snails, and alligators and is also a human pathogen, especially in
compromised hosts. It is a common member of the bacterial biota of pigs. Diarrhea, endocarditis,
meningitis, soft-tissue infections, and bacteremia are caused by A. hydrophila.
Virulent strains of A. hydrophila produce a 52-kDa single polypeptide that possesses enterotoxic,
cytotoxic, and hemolytic activities. This multifunctional molecule displays immunological crossreactivity with the cholera toxin.
92 According to some investigators,
116 it resembles aerolysin while
others contend that it is aerolysin.
78 Aerolysin is a pore- or channel-forming toxin that kills cells by
forming discrete channels in their plasma membranes.
12 Ion channels are created by the oligomerization
of toxin molecules. Cytotonic activity has been associated with an A. hydrophila toxin, which induced
rounding and steroidogenesis in Y-1 adrenal cells. Also, positive responses in the rabbit ileal loop,
suckling mouse, and CHO assays have been reported for a cytotonic toxin.
27
A large number of studies have been conducted on A. hydrophila isolates from various sources. In
one study, 66 of 96 (69%) isolates produced cytotoxins, whereas 32 (80%) of 40 isolates from diarrheal
disease victims were toxigenic, with only 41% of nondiarrheal isolates being positive for cytotoxin
production. Most enterotoxigenic strains are VP (Voges-Proskauer test) and hemolysin positive and
arabinose negative
13 and produce positive responses in the suckling mouse, Y-1 adrenal cell, and rabbit
ileal loop assays. In a study of 147 isolates from patients with diarrhea, 91% were enterotoxigenic,
whereas only 70% of 94 environmental strains produced enterotoxin as assessed by the suckling
mouse assay.
14 All but four of the clinical isolates produced hemolysis of rabbit red blood cells. Of
116 isolates from the Chesapeake Bay, 71% were toxic by the Y-1 adrenal cell assay, and toxicity
correlated with lysine decarboxylase and VP reactions.
56 In yet another study, 48 of 51 cultures from
humans, animals, water, and sewage, produced positive responses in rabbit ileal loop assays with 10
3
or more cells, and cell-free extracts from all were loop positive.
3
Isolates from meat and meat products have been shown to possess biochemical markers that are
generally associated with toxic strains of other species, with the mouse median lethal dose (LD 50 )
being log 8–9 colony-forming units (cfu) for most strains tested.
85 The latter investigators suggested
the possibility that immunosuppressive states are important factors in food-associated infections by
this organism, a suggestion that could explain the difficulty of establishing this organism as the sole
etiological agent of foodborne gastroenteritis.