Aeromonas veronii is a gram-negative, rod-shaped bacterium found in fresh water and in association with animals. It can be a pathogen of humans and a beneficial symbiont of leeches. In humans A. veronii can cause diseases ranging from wound infections and diarrhea to septicemia in immunocompromised patients. In leeches, this bacterium is thought to function in the digestion of blood, provision of nutrients or preventing other bacteria from growing.
REf: . Acessado em 25/jan/2011
Higher order taxa
Bacteria; Proteobacteria; Gammaproteobacteria; Aeromonadales; Aeromonadaceae; Aeromonas
Description and significance
A. veronii is a rod shaped, motile, gram negative, facultative anaerobe. The bacteria are usually not found in groups or pairs but as individual cells. A. veronii is commonly found in soil and various water systems all over the world. It is most often associated with the leech. The blood digested by the bacteria in the leech has been found to contain various antimicrobial properties. It is capable of lowering high concentrations of bacteria through the activatons of the membrane attack complex. This complex creates permeable membranes in a foreign bacteria, essentially inactivating the bacteria. The A. veronii seem to be unsusceptible to this complex, allowing it to proliferate while other bacteria can not. This leads to a very limited number of microbial flora in the digestive tract of the leech, which is extremely uncommon. The population of Aeromonas veronii is greatly effected by the consumption of blood. Tests have shown that dramatic changes occur during this time, the majority of A. veronii bacteria are found not in the epithelial tissue but in the IntraLuminal Fluid (ILF).
The bacteria is made up of 2758 bp of linear DNA. Studies have found that certain genes ( Ast, Alt, and Act) may play a significant role in infection of host organisms. The aeroslysin-hemolysin genes were found to cause diarrhea in some patients who had A. veronii in their digestive system. 
Cell structure and metabolism
The bacterial cell contains a cytoplasm membrane , a thin layer of peptidoglycan and an outer layer composed of lipopolysaccharides (LPS). The catalase gene is important for the degradation of toxic hydrogen peroxide to much more useful molecules, water and oxygen. The expression of the catalase gen is influenced by introduction to extremely low levels of H2O2 during growth and the stationery phases.It has been suggested that H2O2 is used as an antimicrobial by the host cell to damage the DNA, RNA, proteins of invading pathogens. Only those microbes that are able to metabolize hydrogen peroxide would be able to survive in a host cell. 
The A. veronii bacteria can be found in a number of habitats, including humans, mosquitos and leeches. It is primarily found in the digestive tract of the leech where it maintains a symbiotic relationship with its host. The medicinal leech, Hirudo medicinalis is capable of consuming six times its own body weight. The crop is the area of the digestive tract colonized by A. veronii. It is also the area where blood is stored after ingestion, and where water and salt are absorbed from the blood. . Blood is stored in the crop of the digestive tract. Studies have suggested that one of the reasons A. veronii is one of the two predominant microbial flora of the digestive tract is due to the antimicrobial properties of ingested blood. A. veronii provides a number of contributions to the symbiotic relationship it share with the leech. It appears the bacteria helps maintain the flora of the digestive tract, helps in digestion of blood and it also provides necessary nutrients, such as vitamin B complex, not found in abundance in blood.
Medicinal leeches are used after reconstructive or plastic surgery due to their anticoagulating properties and relative inexpense. Studies have shown that without prior antibiotic treatment, up to 20% of patients receiving leech treatment become infected with Aeromonas. Aeromonas species have been shown to have pathogenic properties in a human host. The problem arises if other more pathogenic bacteria are transmitted by leech therapy. Studies looked at whether other bacteria could proliferate or or persist inside the digestive tract for an extended period of time. Virulence is caused by a number of factors such as the pili, flagella and S-layer though non e have been shown to be the sole cause of symptoms during infection.
Currently, studies are being conducted on the medicinal leech, Hirudo medicinalis due to its popularity as an anticoagulant after plastic and reconstructive surgery. These studies focus on the flora of the digestive tract, primarily to determine how effective they are against bacteria that may be pathogenic to humans. The studies look at the whether or not A. veronii is able to contain growth of other bacteria and remain the dominating flora. Current research is also studying the role of A. veronii in a human host. Without antibiotic treatment prior to leech therapy, patients are highly susceptible to infections caused by the bacteria. Though leech therapy is a cost effective treatment with many benefits, it can pose harm to humans. Continual studies of the H. medicinalis and its microial flora are essential to learning more about the complexity of communities withing a host organism. The simple community within the H. medicinalis makes it a perfect model for future studies. Research has been done on the catalase gene of the A. veronii and the role it plays in the symbiotic realtionship between symbiont and host. This is a great model for studying other symbiotic relationships and how their environment may effect growth.
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REF.: http://microbewiki.kenyon.edu/index.php/Aeromonas_veronii . Acessado em 25/jan/2011