Species of bacterium
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Aeromonas veronii is a Gram-negative, rod-shaped bacterium found in fresh water and in association with animals. In humans "A. veronii" can cause diseases ranging from wound infections and diarrhea to sepsis in immunocompromised patients. In leeches, this bacterium is thought to function as a symbiote aiding in the digestion of blood, provision of nutrients, or preventing other bacteria from growing. Humans treated with medicinal leeches after vascular surgery can be at risk for infection from "A. veronii" and are commonly placed on prophylactic antibiotics. Most commonly ciprofloxacin is used but there have been reports of resistant strains leading to infection. 
Protective effect.
A 2005 study showed the potential for using probiotics for controlling "Streptococcus iniae" infection in trout. This study used the gastrointestinal contents of rainbow trout to screen for bacteria that inhibited growth of "S. iniae" and "Lactococcus garvieae". They identified "A. veronii" as a potential candidate for control of "S. iniae" and "L. garvieae" infections in aquaculture. "A. veronii", given live in the feed, protected the trout when challenged with "S. iniae" or "L. garvieae".
References.
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