Aeromonas Bacteria

Members of Aeromonas genus are Gram-negative rod-shaped bacteria which grow with or without oxygen. They do not form spores. The genus Aeromonas used to be included in Vibrionaceae family, but recently has been classified in a new family, the Aeromonadaceae which includes at least 14 species. These can be divided into two groups based on the temperature range at which strains are able to grow: psychrotrophic and mesophilic (those not able to grow at 10° C).

Aeromonas are widely distributed in nature and commonly found in rivers, streams, canals, and tap water. They can survive on moist surfaces and can be recovered from moistened paper towels. Aeromonas species release a number of toxins that cause various infections in humans and animals. It is highly probable that people are generally unaffected by enteric Aeromonads which form a part of gut microbiota of healthy individuals. However, it cannot be ruled out that age, other underlying conditions, infection dose, and sufficient virulence factors, such as toxins, may aid in the ability to cause disease.⁸

Aeromonas hydrophila ("water-loving, gas-producing") bacteria which was first isolated from frogs, produces alpha- and beta- hemolysins which cause skin cell necrosis. The bacterium also widely occurs in foods, both fresh and processed, sludge and sewage. Aeromonas hydrpphila can survive for a long time in water, particularly at low temperatures, and can be found in chlorinated water, including chlorinated drinking water that is free from E. coli. It is thought that their presence in chlorinated water is the result of post-treatment contamination or inefficiencies in chlorination process. Motile Aeromonas have been associated with gastrointestinal disease.⁴

Sources of Contamination

Water is the most common source of Aeromonads in foods, with fish and seafood most often contaminated, although they are commonly found in household environments, such as drains and sinks, and soil. Edible land snails, egg salad, and shrimp cocktails have also been reported contaminated with these bacteria. Motile Aeromonas are often found on refrigerated animal products such as chicken, beef, pork, lamb, cheese (especially high pH cheese produced from raw milk), ice-cream, whipped cream, pasteurized and raw milk, and vegetables. Some Aeromonads have been isolated from frozen food after storage for about two years. Aeromonas species are unlikely to grow when the salt levels are more than 3-3.5% and pH values are below 6.0.^3,4

Human Diseases Associated With Aeromonas

Aeromonas species produce enterotoxins that may cause diarrheal disease in susceptible individuals.⁷ Gastroenteritis associated with Aeromonas species is most frequently reported in young children, although it can occur in individuals of any age. The most common signs are watery diarrhea and mild fever, sometimes accompanied by vomiting. The less common form is disentery-like illness with bloody diarrhea and mucus in the stools. The disease usually lasts 1-7 days, occasionally several months.⁴ In persons with weakend immune defenses Aeromonas may produce septicemia characterized by high fever and shock. In persons with liver cirrhosis and malignancies, Aeromonas septicemia can be rapidly fatal.² Meningitis caused by Aeromonas has been reported in children.¹

Wound infections with Aeromonas species have been associated with contaminated soil and water-related activities, such as swimming, diving, boating, and fishing. Septicemia can follow from such infections.⁵ Infections via deep puncture wounds may cause skin ulcers, cellulitis, and deep muscle necrosis.⁶

Uncommon infections with Aeromonas include urinary tract infections, ear infections, liver and gall bladder infections, endocarditis, hemolytic uremic syndrome (HUS), peritonitis, respiratory tract disease, and rarely eye infections (conjunctivitis and corneal ulcers).^2,8

Animal Diseases Caused By Aeromonas

Aeromonas salmonicida, Aeromonas allosaccharophila, Aeromonas sobria, Aeromonas bestiarum, Aeromonas caviae, and Aeromonas veronii are animal pathogens. Bacterial septicemia (furunculosis) caused by Aeromonas salmonicida is an important disease because it causes large economic losses of salmon and trout at commercial fish farms. Signs of infection include skin ulcers, abnormal protrusion of the eyes (exophthalmos), bleeding from nares and vent, and distended lower abdomen. Aeromonas hydrophila is often associated with the protozoan Epistylis in causing widespread epidemic skin lesions known as red-sore disease. Atlantic salmon are most susceptible; rainbow trout are most resistant. Goldfish, common carp, American and Japanese eels are most often affected, but pike and catfish are also susceptible. Aeromonas salmonicida infection has also become a serious problem in marine fish. This bacterium has also been recovered from sea lice and plankton.⁶

Most Aeromonas species are susceptible to antibiotic drugs; some produce β-lactamase, an enzyme that inactivates the antibiotic ampicillin.

References:
1. Oski's pediatrics: principles & practice By Julia A. McMillan, Ralph D. Feigin, Catherine DeAngelis, M. Douglas Jones
2. Koneman's color atlas and textbook of diagnostic microbiology By Washington C. Winn, Elmer W. Koneman
3. Compendium of methods for the microbiological examination of foods By Frances Pouch Downes, Keith Ito
4. The food safety hazard guidebook By Richard Lawley, Laurie Curtis, Judy Davis
5. Guidelines for Drinking-water Quality: Recommendations By World Health Organization
6. Fish Disease: Diagnosis and Treatment By Edward J. Noga
7. The Prokaryotes: A Handbook on the Biology of Bacteria: Proteobacteria: Gamma Subclass Martin Dworkin, Stanley Falkow
8. Microbiology of Waterborne Diseases by Martha Embrey, Paul Hunter