Bacillus hydrophilus fuscus Sanarelli 1871
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.
Aeromonas hydrophila infections occur most 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.
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.
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 Aermonas hydrophila. These treatments include disinfectants and Acriflavine.
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