Aeromonas hydrophila

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Aeromonas hydrophila
Aeromonas hydrophila.jpg
Scientific classification
A. hydrophila
Binomial name
Aeromonas hydrophila
(Chester, 1901)
Stanier, 1943

Bacillus hydrophilus fuscusSanarelli 1871
Bacillus hydrophilusChester 1901
Proteus hydrophilus(Chester 1901) Bergey et al. 1923
Bacterium hydrophilum(Chester 1901) Weldin and Levine 1923
Pseudomonas hydrophila(Chester 1901) Breed et al. 1948


Aeromonas hydrophila colonies on the blood agar. Aeromonas hydrophila colonies 01.jpg
Aeromonas hydrophila colonies on the blood agar.

Aeromonas hydrophila is a heterotrophic, Gram-negative, rod-shaped bacterium mainly found in areas with a warm climate. This bacterium can be found in fresh or brackish water. It can survive in aerobic and anaerobic environments, and can digest materials such as gelatin and hemoglobin. A. hydrophila was isolated from humans and animals in the 1950s. It is the most well known of the species of Aeromonas . It is resistant to most common antibiotics and cold temperatures and is oxidase and indole positive.

Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water however, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface.

Gelatin mixture of peptides and proteins derived from connective tissues of animals

Gelatin or gelatine is a translucent, colorless, flavorless food ingredient, derived from collagen taken from animal body parts. Brittle when dry and gummy when moist, it is also called hydrolyzed collagen, collagen hydrolysate, gelatine hydrolysate, hydrolyzed gelatine, and collagen peptides. It is commonly used as a gelling agent in food, medications, drug and vitamin capsules, photographic films and papers, and cosmetics.

Hemoglobin group of iron-containing proteins in animals (except fish family Channichthyidae) whose primary function is to carry oxygen to the tissues

Hemoglobin (American) or haemoglobin (British), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells (erythrocytes) of almost all vertebrates as well as the tissues of some invertebrates. Haemoglobin in the blood carries oxygen from the lungs or gills to the rest of the body. There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 16 grams of haemoglobin in every 100 ml of blood.


A. hydrophila bacteria are Gram-negative, straight rods with rounded ends (bacilli to coccibacilli shape) usually from 0.3 to 1.0 μm in width, and 1.0 to 3.0 μm in length. They can grow at temperatures as low as 4 °C. These bacteria are motile by a polar flagellum.

Flagellum part of a cell of some organisms

A flagellum is a lash-like appendage that protrudes from the cell body of certain bacteria and eukaryotic cells termed as flagellates. A flagellate can have one or several flagella. The primary function of a flagellum is that of locomotion, but it also often functions as a sensory organelle, being sensitive to chemicals and temperatures outside the cell. The similar structure in the archaea functions in the same way but is structurally different and has been termed the archaellum.


Because of its structure, it is very toxic to many organisms. When it enters the body of its victim, it travels through the bloodstream to the first available organ. It produces aerolysin cytotoxic enterotoxin that can cause tissue damage. A. hydrophila, A. caviae, and A. sobria are all considered to be opportunistic pathogens, meaning they rarely infect healthy individuals. A. hydrophila is widely considered a major fish and amphibian pathogen, [1] and its pathogenicity in humans has been recognized for decades. [2] The genomic insights of aeromonads could be a stepping stone into understanding of them [3]

Aerolysin InterPro Family

In molecular biology, aerolysin is a cytolytic pore-forming toxin exported by Aeromonas hydrophila, a Gram-negative bacterium associated with diarrhoeal diseases and deep wound infections. The mature toxin binds to eukaryotic cells and aggregates to form holes leading to the destruction of the membrane permeability barrier and osmotic lysis. The structure of proaerolysin has been determined to 2.8A resolution and shows the protoxin to adopt a novel fold. Images of an aerolysin oligomer derived from electron microscopy have helped to construct a model of the protein in its heptameric conformation, and to outline a mechanism by which this assembly might insert into lipid bilayers to form ion channels.

Aeromonas caviae is a Gram-negative bacterium of the genus Aeromonas isolated from epizootic guinea pigs.

Pathogenic mechanism

The pathogenicity of Aeromonas species was believed to be mediated by a number of extracellular proteins such as aerolysin, lipase, chitinase, amylase, gelatinase, hemolysins, and enterotoxins. However, the pathogenic mechanisms are unknown. The recently proposed type-III secretion system (TTSS) has been linked to Aeromonas pathogenesis. TTSS is a specialized protein secretion machinery that exports virulence factors directly to host cells. These factors subvert normal host cell functions to the benefit of invading bacteria. In contrast to the general secretory pathway, the TTSS is triggered when a pathogen comes in contact with host cells. ADP-ribosylation toxin is one of the effector molecules secreted by several pathogenic bacteria and translocated through the TTSS and delivered into the host cytoplasm, which leads to interruption of the NF-κB pathway, cytoskeletal damage, and apoptosis. This toxin has been characterized in A. hydrophila (human diarrhoeal isolate), A. salmonicida (fish pathogen), and A. jandaei GV17, a pathogenic strain which can cause disease both in humans and fish.

Occurrence of exposure

A. hydrophila infections occur most often during sexual changes, stressors, changes in temperature, in contaminated environments, and when an organism is already infected with a virus or another bacterium. It can also be ingested through food products contaminated with the bacterium, such as seafood, meats, and even certain vegetables such as sprouts. It can also be transmitted by leeches. [4]

A stressor is a chemical or biological agent, environmental condition, external stimulus or an event that causes stress to an organism. Psychologically speaking, a stressor can be events or environments that an individual would consider demanding, challenging, and or threaten the individual's safety.

Fish and amphibians

A. hydrophila is associated with diseases mainly found in freshwater fish and amphibians, because these organisms live in aquatic environments. It is linked to a disease found in frogs called red leg, which causes internal, sometimes fatal hemorrhaging. When infected with A. hydrophila, fish develop ulcers, tail rot, fin rot, and hemorrhagic septicemia. Hemorrhagic septicaemia causes lesions that lead to scale shedding, hemorrhages in the gills and anal area, ulcers, exophthalmia, and abdominal swelling.

Ulcer (dermatology) type of cutaneous condition

An ulcer is a sore on the skin or a mucous membrane, accompanied by the disintegration of tissue. Ulcers can result in complete loss of the epidermis and often portions of the dermis and even subcutaneous fat. Ulcers are most common on the skin of the lower extremities and in the gastrointestinal tract. An ulcer that appears on the skin is often visible as an inflamed tissue with an area of reddened skin. A skin ulcer is often visible in the event of exposure to heat or cold, irritation, or a problem with blood circulation. They can also be caused due to a lack of mobility, which causes prolonged pressure on the tissues. This stress in the blood circulation is transformed to a skin ulcer, commonly known as bedsores or decubitus ulcers. Ulcers often become infected, and pus forms.

Fin rot is a symptom of disease or the actual disease in fish. This is a disease which is most often observed in aquaria and aquaculture, but can also occur in natural populations.

Haemorrhagic septicaemia is one of the most economically important pasteurelloses. Haemorrhagic septicaemia in cattle and buffaloes was previously known to be associated with one of two serotypes of P. multocida: Asian B:2 and African E:2 according to the Carter-Heddleston system, or 6:B and 6:E using the Namioka-Carter system.

Human diseases

A. hydrophila is not as pathogenic to humans as it is to fish and amphibians. One of the diseases it can cause in humans, gastroenteritis, occurs mostly in young children and people who have compromised immune systems or growth problems. This bacterium is linked to two types of gastroenteritis. The first type is a disease similar to cholera, which causes rice-water diarrhea. The other type is dysenteric gastroenteritis, which causes loose stools filled with blood and mucus. Dysenteric gastroenteritis is the most severe out of the two types, and can last for several weeks. A. hydrophila is also associated with cellulitis. It also causes diseases such as myonecrosis and eczema in people with compromised or suppressed (by medication) immune systems. [5] In very rare cases, A. hydrophila can cause necrotizing fasciitis. [6]

Gastroenteritis Inflammation of the stomach and small intestine

Gastroenteritis, also known as infectious diarrhea, is inflammation of the gastrointestinal tract—the stomach and small intestine. Symptoms may include diarrhea, vomiting and abdominal pain. Fever, lack of energy and dehydration may also occur. This typically lasts less than two weeks. It is not related to influenza, though it has been called the "stomach flu".

Cholera Bacterial infection of the small intestine

Cholera is an infection of the small intestine by some strains of the bacterium Vibrio cholerae. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and muscle cramps may also occur. Diarrhea can be so severe that it leads within hours to severe dehydration and electrolyte imbalance. This may result in sunken eyes, cold skin, decreased skin elasticity, and wrinkling of the hands and feet. Dehydration can cause the skin to turn bluish. Symptoms start two hours to five days after exposure.

Human feces Solid or semisolid remains of the food that could not be digested or absorbed in the small intestine of humans

Human feces are the solid or semisolid remains of the food that could not be digested or absorbed in the small intestine of humans, but has been rotted down by bacteria in the large intestine. It also contains bacteria and a relatively small amount of metabolic waste products such as bacterially altered bilirubin, and the dead epithelial cells from the lining of the gut. It is discharged through the anus during a process called defecation. Human feces have similarities to feces of other animals and vary significantly in appearance, according to the state of the diet, digestive system and general health. Normally human feces are semisolid, with a mucus coating. Small pieces of harder, less moist feces can sometimes be seen impacted in the distal end. This is a normal occurrence when a prior bowel movement is incomplete, and feces are returned from the rectum to the large intestine, where water is absorbed.


Though A. hydrophila can cause serious disease, large scale outbreaks have not been reported. Outbreaks among vertebrates have occurred. One such incident occurred in Puerto Rico inside the intestinal tracts of lizards. [7] Some 116 different strains were found in the lizards. On May 1, 1988, a small outbreak happened in California. The 225 isolates in 219 patients caused their hospital admissions. Confidential morbidity report cards were used to report the cases to the local health departments. Investigations were conducted, and reports were sent to the California Department of Health Services for diagnosis and methods in treatment.


A. hydrophila can be eliminated using a 1% sodium hypochlorite solution or 2% calcium hypochlorite solution.

Brage et al., 1990 recommends fluoroquinolone administration as a prophylactic treatment during medicinal leech application. [8]

Antibiotics such as chloramphenicol, florfenicol, tetracycline, sulfonamide, nitrofuran derivatives, and Pyridinecarboxylic acids are used to eliminate and control the infection of A. hydrophila.

Terramycin is placed in fish food during hatchery operations as another chemotherapeutic agent in preventing A. hydrophila.

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  1. Prevalence and distribution of Aeromonas hydrophila in the United States
  2. Clinical and microbiological features of Aeromonas hydrophila-associated diarrhea
  3. Tan, Wen-Si; Yin, Wai-Fong; Chan, Kok-Gan (2 Jan 2015). "Insights into the Quorum-Sensing Activity in Aeromonas hydrophila Strain M013 as Revealed by Whole-Genome Sequencing". Genome Announcements. 3 (1): 1–2. doi:10.1128/genomeA.01372-14. PMC   4293626 .
  4. Snower, DP; Ruef, C; Kuritza, AP; Edberg, SC (1989). "Aeromonas hydrophila infection associated with the use of medicinal leeches". J. Clin. Microbiol. 27: 1421–2. PMC   267578 . PMID   2666448.
  5. Evolving concepts regarding the genus Aeromonas: an expanding panorama of species, disease presentations, and unanswered questions
  6. Necrotizing fasciitis caused by Aeromonas hydrophila
  7. Fulton, MacDonald. "The Bacterium Aeromonas hydrophila from Lizards of the genus Anolis in Puerto Rico". Archived October 31, 2004, at the Wayback Machine Louisiana State University Medical Center, New Orleans.
  8. Braga A, Lineaweaver WC, Whitney TM, Follansbee S, Buncke HJ. Sensitivities of aerononas hydrophila cultured from medicinal leeches to oral antibiotics. J Reconstr Microsurg. 1990; 6(2):135-137

Further reading