Aeromonas hydrophila is a ubiquitous gram-negative aquatic bacterium classified within the phylum proteobacteria. It lives in fresh or brackish water worldwide, and is found in large numbers especially in warm conditions e.g. 25-35oC (77-95oF). This bacterium is hearty and tolerates aerobic and anaerobic environments, a wide temperature range (it can grow at 4oC), chlorination, pollution, and is also resistant to most common antibiotics.
Aeromonas hydrophila are rods with rounded ends, usually between 0.3 – 1 micrometer in width, 1 – 3 micrometers in length and motile via polar flagella. While it makes up part of the normal, harmless bacterial flora in most aquatic ecosystems, this bacterium is opportunistically pathenogenic in aquatic animals, flaring into disease especially in animals that are stressed, disturbed or unhealthy. Aeromonas hydrophila is the most common cause of red leg, a disease contracted by amphibians which causes lesions on their back legs and internal, sometimes fatal hemorrhaging. Fish infected with Aeromonas hydrophila, develop ulcers, tail and fin rot, and other symptoms and can die quickly after contracting symptoms.
While not as pathogenic in humans as it is in fish and amphibians (it was not associated with human disease until 1968), when ingested in large quantities from contaminated water or foods it may cause gastroenteritis, especially in young children and people who have compromised immune systems. There is some disagreement about what the role of A. hydrophila is in causing gastroenteritis, however. Large outbreaks of Aeromonas-caused gastric illness have never been reported and the frequency of Aeromonas-related disease in the United States is unclear. Aeromonas hydrophila can also enter the body when open skin lacerations are exposed to sources of the bacteria, in these case infection can cause skin inflammation, septicemia (dangerous systemic infection), and rarely it has caused necrotizing fasciitis (flesh-eating disease). An unusual 2012 case of necrotizing fasciitis in a healthy 24-year-old brought public attention to A. hydrophila (Lynch 2012), but this bacteria causes far fewer cases of necrotizing fasciitis than does the Strep A bacterium.
Aeromonas hydrophila was one of the first species described from genus Aeromonas; isolated from humans and animals in the 1950s. Interest in the genus has jumped hugely in the last 20 years, and since 1980 20 new Aeromonas names were given standing in the literature (LPSN bacterio.net). At least 13 “genospecies” are recognized; it is difficult to distinguish species beyond their species complexes. Not all of these are associated with disease, far less is known about most other species of Aeromonas. The full genome of A. hydrophila was sequenced in 2006.
(CDC 1990; US EPA 2012; Janda and Abbott 2010; LPSN bacterionet, web; Lynch 2012; US FDA 2013; WHO 2002, 2006)
- CDC May 25, 1990. Aeromonas wound infections associated with outdoor activities -- California. MMWR weekly 39(20);334-335,341. Retrieved November 21 2013 from http://www.cdc.gov/mmwr/preview/mmwrhtml/00001628.htm.
- U.S. Environmental Protection Agency (EPA), 2012. Aeromonas detection: what does it mean? Retrieved November 21, 2013 from http://water.epa.gov/lawsregs/rulesregs/sdwa/ucmr/data_aeromonas.cfm.
- Janda, J.M. and S.L. Abbott, 2010. The Genus Aeromonas: Taxonomy, Pathogenicity, and Infection Clin. Microbiol. Rev. 23:35-73. Available online at http://cmr.asm.org/content/23/1/35.full.pdf+html.
- LPSN bacterio.net. List of Prokaryotic names with standing in nomenclature. Retrieved November 21 2013 from http://www.bacterio.net/a/aeromonas.html.
- Lynch, R. May 15 2012. Flesh-eating bacteria case: could a simple drug have cured it? Los Angeles Times. Retrieved November 21 2013 from http://articles.latimes.com/2012/may/15/nation/la-na-nn-flesh-eating-bacteria-20120515.
- US Food and Drug Administration (FDA), Department of Health and Human Services, 22 March 2012. Aeromonas hydrophila and other spp. Bad Bug Book: Foodborne Pathogenic Microorganisms and Natural Toxins Handbook Volume 2. Retrieved November 21 2013 from
- World Health Organization 2002. Guidelines for drinking water quality, Addendum: Microbiological agents in drinking water, 2nd Edition, Aeromonas. Retrieved November 21 2013 from http://www.who.int/water_sanitation_health/dwq/en/admicrob2.pdf.
- World Health Organization 2006. Guidelines for drinking water quality. Second edition, Volume 2. Health Criteria and other supporting information. ISBN 92 4 154480 5. PDF retrieved November 21 2013 from http://www.who.int/water_sanitation_health/dwq/2edvol2p1.pdf
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. Aeromonas hydrophila was isolated from humans and animals in the 1950s. It is the most well known of the six species of Aeromonas. It is resistant to most common antibiotics and cold temperatures.
Aeromonas hydrophila are Gram-negative straight rods with rounded ends (bacilli to coccibacilli shape) usually from 0.3 to 1 micrometer in width, and 1 to 3 micrometers in length. Aeromonas hydrophila can grow in temperatures as low as four degrees Celsius. These bacteria are motile by a polar flagellum.
Because of Aeromonas hydrophila’s 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 (ACT), a toxin that can cause tissue damage. Aeromonas hydrophila, Aeromonas caviae, and Aeromonas sobria are all considered to be opportunistic pathogens, meaning they rarely infect healthy individuals. Aeromonas hydrophila is widely considered a major fish and amphibian pathogen, and its pathogenicity in humans has been recognized for decades.
It was believed that the pathogenicity of Aeromonas spp. is mediated by a number of extracellular proteins such as aerolysin, lipase, chitinase, amylase, gelatinase, hemolysins and enterotoxins. However the pathogenic mechanisms of Aeromonas spp. are unknown. The recently proposed type III secretion system (TTSS) has been linked to Aeromonas pathogenesis. The 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, type III secretion system 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 TTSS and delivered into the host cytoplasm leads to interruption of NF-κB pathway, cytoskeletal damage and apoptosis. This toxin has been characterized in Aeromonas hydrophila (human diarrhoeal isolate), Aeromonas salmonicida (fish pathogen) and Aeromonas jandaei GV17, a pathogenic strain which can cause disease both in human and fish.
Occurrence of exposure[clarification needed]
Aeromonas hydrophila infections occur most often during environmental changes, stressors, change in the 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 that have already been contaminated with the bacterium, such as seafood, meats, and even certain vegetables such as sprouts. It can also be transmitted by leeches.
Fish and amphibians
Aeromonas 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 Aeromonas 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.
Aeromonas hydrophila is not as pathogenic to humans as it is to fish and amphibians. One of the diseases it can cause in humans is gastroenteritis. This disease can affect anyone, but it occurs most 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 of disease 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 multiple weeks. Aeromonas hydrophila is also associated with cellulitis, an infection that causes inflammation in the skin tissue. It also causes diseases such as myonecrosis and eczema in people with compromised or suppressed (by medication) immune systems.
Though Aeromonas hydrophila can cause serious diseases, there have never been serious outbreaks. There was an outbreak inside the intestinal tract of lizards in Puerto Rico. There were 116 different strains found in the lizards. On May 1, 1988 there was a small Aeromonas hydrophila outbreak in California. There were 225 isolates and 219 patients admitted in the hospital because of the bacterium. Confidential Morbidity Report cards were used to report the cases of the bacterium 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.
Brage et al, 1990 recommends fluoroquinolone administration as a prophylactic treatment during medicinal leech application.
Antibiotic agents such as chloramphenicol, florenicol, tetracycline, sulfonamide, nitrofuran derivatives, and pyrodinecarboxylic acids are used to eliminate and control the infection of Aeromonas hydrophila.
It is ill-advised to transfer fish from hatchery to hatchery without any sanitation. Hatchery workers should clean the fish, and check for bacterial infection between each operation. To avoid contamination oxygen levels in fish should be maintained, and fish should always be handled gently, to avoid injury. Prophylactic treatments can also be used when trying to prevent Aeromonas hydrophila. These treatments include disinfectants and Acriflavine.
- Prevalence and distribution of Aeromonas hydrophila in the United States
- Clinical and microbiological features of Aeromonas hydrophila-associated diarrhea
- Evolving concepts regarding the genus Aeromonas: an expanding panorama of species, disease presentations, and unanswered questions
- Necrotizing fasciitis caused by Aeromonas hydrophila
- 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
- Fulton, MacDonald. "The Bacterium Aeromonas hydrophila from Lizards of the genus Anolis in Puerto Rico". Louisiana State University Medical Center, New Orleans.
- Hayes, John. "Aeromonas hydrophila." Oregon State University.
- Arrow Scientific. "Aeromonas hydrophila."
- "Aeromonas hydrophila." Bad Bug Book Foodborne Pathogenic Microorganisms and Natural Toxins Handbook. US Food and Drug Administration.
- "Aeromonas hydrophila and Related Bacteria." International Specialty Supply.
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