Aeromonas hydrophila can exist either as as a free-living aquatic habitant or inside of its host as a parasite to a variety of vertebrates including humans, fish, frogs, and crustaceans. Similarly to Vibrio parahaemolyticus and Legionella pneumophila, this organism is found worldwide throughout aquatic environments including in bottled water, swimming pools, chlorinated water, well water, and heavily polluted water. A substantial amount these bacteria were found in theHurricane Katrina Flooding floodwater following Hurricane Katrina as well as in Thailand following the 2004 tsunami.

Aeromonas hydrophila has been known to grow either aerobically or anaerobicly in a wide range of temperatures, conductivities, pHs, and turbidities, however, its optimal temperature range is from 25 to 37°C. While it is still able to survive, it does not thrive under very extreme conditions such as salinity, temperature, and pollution. Because oceans have lower salinity concentrations in deeper water, it is found more prevalently 200m below sea level. As a free-living aquatic organism, it has adapted mobility by means of a polar flagella and it feeds as a primary consumer in this particular niche as you can read about in the nutrition section. Also, because of its nutritional needs, it often forms a biofilm on surfaces where nutrients collect. However, since sudden changes in temperature and water level can negatively affect the immune system of marine species, the bacteria can more easily attack and grow in the immunosuppressed fish when the environment changes rapidly. Because of this, it is more prevalent in a host during seasonal changes. For more on the occurrence and distribution of Aeromonas hydrophila, read about its impact on public health.
Not only can this species infect a variety of organisms, but it also has the potential to infect a variety of systems within the individual host. The pathogen will inhabit different parts of its host depending on where it has lodged itself. For example, if a human were to consume food or water contaminated by A. hydrophila, the bacterium would reside and deteriorate the gastrointestinal tract. If it entered the host via a wound that has come in contact with contaminated water, the bacteria will inhabit the soft tissue surrounding the wound. It has also been reported to infect the eyes, respiratory tract, urinary tract, and the blood. Furthermore, it is important to note that this species is not a normal inhabitant of an organism's body.

To understand the nutrition and reproductive strategies that this species utilizes in these particular habitats, go to the next page all about food and sex.

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