Bad Bug Book:
Foodborne Pathogenic Microorganisms and Natural Toxins Handbook
A new version of the Bad Bug Book was released in 2012, below is a previous version.
Aeromonas hydrophila, Aeromonas caviae, Aeromonas sobria & (Aeromonas veronii?)
Aeromonas hydrophila is a species of bacterium that is present in all freshwater environments and in brackish water. Some strains of A. hydrophila are capable of causing illness in fish and amphibians as well as in humans who may acquire infections through open wounds or by ingestion of a sufficient number of the organisms in food or water.
Not as much is known about the other Aeromonas spp., but they too are aquatic microorganisms and have been implicated in human disease.
A. hydrophila may cause gastroenteritis in healthy individuals or septicemia in individuals with impaired immune systems or various malignancies.
A. caviae and A. sobria also may cause enteritis in anyone or septicemia in immunocompromised persons or those with malignancies.
At the present time, there is controversy as to whether A. hydrophila is a cause of human gastroenteritis. Although the organism possesses several attributes which could make it pathogenic for humans, volunteer human feeding studies, even with enormous numbers of cells (i.e. 10^11), have failed to elicit human illness. Its presence in the stools of individuals with diarrhea, in the absence of other known enteric pathogens, suggests that it has some role in disease.
Likewise, A. caviae and A. sobria are considered by many as "putative pathogens," associated with diarrheal disease, but as of yet they are unproven causative agents.
Two distinct types of gastroenteritis have been associated with A. hydrophila: a cholera-like illness with a watery (rice and water) diarrhea and a dysenteric illness characterized by loose stools containing blood and mucus. The infectious dose of this organism is unknown, but SCUBA divers who have ingested small amounts of water have become ill, and A. hydrophila has isolated from their stools.
A general infection in which the organisms spread throughout the body has been observed in individuals with underlying illness (septicemia).
A. hydrophila can be cultured from stools or from blood by plating the organisms on an agar medium containing sheep blood and the antibiotic ampicillin. Ampicillin prevents the growth of most competing microorganisms. The species identification is confirmed by a series of biochemical tests. The ability of the organism to produce the enterotoxins believed to cause the gastrointestinal symptoms can be confirmed by tissue culture assays.
A. hydrophila has frequently been found in fish and shellfish. It has also been found in market samples of red meats (beef, pork, lamb) and poultry. Since little is known about the virulence mechanisms of A. hydrophila, it is presumed that not all strains are pathogenic, given the ubiquity of the organism.
The relative frequency of A. hydrophila disease in the U.S. is unknown since efforts to ascertain its true incidence have only recently been attempted. Most cases have been sporadic rather than associated with large outbreaks, but increased reports have been noted from several clinical centers.
On rare occasions the dysentery-like syndrome is severe and may last for several weeks.
A. hydrophila may spread throughout the body and cause a general infection in persons with impaired immune systems. Those at risk are individuals suffering from leukemia, carcinoma, and cirrhosis and those treated with immunosuppressive drugs or who are undergoing cancer chemotherapy.
All people are believed to be susceptible to gastroenteritis, although it is most frequently observed in very young children. People with impaired immune systems or underlying malignancy are susceptible to the more severe infections.
A. hydrophila can be recovered from most foods by direct plating onto a solid medium containing starch as the sole carbohydrate source and ampicillin to retard the growth of most competing microorganisms.
For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC.
Literature references can be found at the links below.
Loci index for genome Aeromonas hydrophila
Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.