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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)

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