388 ◾ Fundamental Food Microbiology
and freshwater environments and are pathogenic to fish. They are also found in the intestinal contents of humans and animals. Human can acquire Aeromonas from wild animals, pets, the environment, water, and from foods. Their growth temperature ranges between 3°C and 42°C with an
optimum between 15°C and 20°C; a few strains can grow at 1°C. They are facultative anaerobes,
but grow better in an aerobic environment. Pasteurization effectively kills the cells. Factors such
as pH (below 4.5), NaCl (above 4%), and low temperature (below 3°C) can reduce their growth.
Food Association
Because of the nature of its normal habitat, Aer. hydrophila is found in many foods, especially in
foods of animal origin. It may be isolated from milk, finfish, seafood, red meat, and poultry; in
some foods, it occurs at a level of 10 5 cells/g or /mL. The strains have been implicated in the spoilage
of foods. Because of the psychrotrophic nature, they can grow in foods at refrigerated temperatures
and, even from a low initial load, can reach a high population with time during storage. Their presence in high numbers in food can be controlled by heat treatment, preventing post-heat contamination, and using one or more of the growth-limiting parameters, such as low pH and low A W .
Disease-Causing Potential
Aeromonads are pathogenic to amphibians, fish, and reptiles. It causes ulcerative stomatitis in
snakes and lizards, “red leg” disease in frogs, septic arthritis in calves, and septicemia in dogs. Some
species of Aeromonas have been suspected to cause gastroenteritis (diarrhea) in humans, especially
when consumed in large numbers and when individuals have impaired health. Diarrhea may last
up to 14 days. Other symptoms associated with Aeromonas infection include dysentery, ulcerative
colitis, haemolytic anemia, and renal dysfunction. Several virulence factors (cytotoxic enterotoxins, hemolysins, adhesins, and siderophores) and enzymes (protease, DNase, RNase, lecithinase,
chitinase, gelatinase) may be responsible for pathogenesis. No confirmed cases of foodborne illness caused by Aer. hydrophila have been reported. Several incidences of gastroenteritis from the
consumption of food and water contaminated with this species were reported. Many strains of
Aer. hydrophila, especially those isolated from foods, were found to produce cytotoxins and hemolysins. It is not definitely known whether these toxins can cause gastroenteritis in humans.
Plesiomonas shigelloides 2
Characteristics
Plesiomonas shigelloides is a facultative anaerobic, motile, nonsporulating, and Gram-negative rod.
The species has many characteristics similar to those of Aeromonas spp. and was previously classified as Aer. shigelloides. The organism is isolated from the intestinal contents of humans and warmand cold-blooded animals. It is found in fresh and brackish water and in fish and oysters harvested
from water. Most strains grow between 8°C and 45°C with optimum growth at 25°C–35°C. The
cells are killed by pasteurization. Low temperatures (below 10°C), low pH (below 4.5), and NaCl
(above 5%) can be used, especially in combinations of two or more, to reduce growth.
Food Association
Ple. shigelloides strains are isolated from foods of aquatic origin, such as fish and shellfish. They
are present in higher frequencies and levels in oysters collected during warmer months and from
and freshwater environments and are pathogenic to fish. They are also found in the intestinal contents of humans and animals. Human can acquire Aeromonas from wild animals, pets, the environment, water, and from foods. Their growth temperature ranges between 3°C and 42°C with an
optimum between 15°C and 20°C; a few strains can grow at 1°C. They are facultative anaerobes,
but grow better in an aerobic environment. Pasteurization effectively kills the cells. Factors such
as pH (below 4.5), NaCl (above 4%), and low temperature (below 3°C) can reduce their growth.
Food Association
Because of the nature of its normal habitat, Aer. hydrophila is found in many foods, especially in
foods of animal origin. It may be isolated from milk, finfish, seafood, red meat, and poultry; in
some foods, it occurs at a level of 10 5 cells/g or /mL. The strains have been implicated in the spoilage
of foods. Because of the psychrotrophic nature, they can grow in foods at refrigerated temperatures
and, even from a low initial load, can reach a high population with time during storage. Their presence in high numbers in food can be controlled by heat treatment, preventing post-heat contamination, and using one or more of the growth-limiting parameters, such as low pH and low A W .
Disease-Causing Potential
Aeromonads are pathogenic to amphibians, fish, and reptiles. It causes ulcerative stomatitis in
snakes and lizards, “red leg” disease in frogs, septic arthritis in calves, and septicemia in dogs. Some
species of Aeromonas have been suspected to cause gastroenteritis (diarrhea) in humans, especially
when consumed in large numbers and when individuals have impaired health. Diarrhea may last
up to 14 days. Other symptoms associated with Aeromonas infection include dysentery, ulcerative
colitis, haemolytic anemia, and renal dysfunction. Several virulence factors (cytotoxic enterotoxins, hemolysins, adhesins, and siderophores) and enzymes (protease, DNase, RNase, lecithinase,
chitinase, gelatinase) may be responsible for pathogenesis. No confirmed cases of foodborne illness caused by Aer. hydrophila have been reported. Several incidences of gastroenteritis from the
consumption of food and water contaminated with this species were reported. Many strains of
Aer. hydrophila, especially those isolated from foods, were found to produce cytotoxins and hemolysins. It is not definitely known whether these toxins can cause gastroenteritis in humans.
Plesiomonas shigelloides 2
Characteristics
Plesiomonas shigelloides is a facultative anaerobic, motile, nonsporulating, and Gram-negative rod.
The species has many characteristics similar to those of Aeromonas spp. and was previously classified as Aer. shigelloides. The organism is isolated from the intestinal contents of humans and warmand cold-blooded animals. It is found in fresh and brackish water and in fish and oysters harvested
from water. Most strains grow between 8°C and 45°C with optimum growth at 25°C–35°C. The
cells are killed by pasteurization. Low temperatures (below 10°C), low pH (below 4.5), and NaCl
(above 5%) can be used, especially in combinations of two or more, to reduce growth.
Food Association
Ple. shigelloides strains are isolated from foods of aquatic origin, such as fish and shellfish. They
are present in higher frequencies and levels in oysters collected during warmer months and from