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MICROBIOLOGY LEGEND
CYCLE 28 – ORGANISM 3
Aeromonas hydrophila
Aeromonas hydrophila is a heterotrophic, gram-negative, rod shaped bacterium, mainly
found in areas with a warm climate. This bacterium can also be found in fresh, salt, marine,
estuarine, chlorinated, and un-chlorinated water. Aeromonas hydrophila can survive in
aerobic and anaerobic environments. This bacterium can digest materials such as gelatin,
and hemoglobin. This bacterium is the most well known of the six species of Aeromonas. It
is also highly resistant to multiple medications. Aeromonas hydrophila is resistant to
chlorine, refrigeration and cold temperatures.
Structure
Aeromonas hydrophila are Gram-negative straight rods with rounded ends (bacilli to
coccibacilli shape) usually from 0.3 to 1 µm in width, and 1 to 3 µm in length. Aeromonas
hydrophila does not form endospores, and can grow in temperatures as low as four degrees
Celsius. These bacteria are motile by a polar flagella.
Aeromonas hydrophila - Gram stain
Pathology
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Because of Aeromonas hydrophila’s structure, it is very toxic when it enters the body of
its victim. It travels through the bloodstream to the first available organ and 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 only infect immunocompromised hosts. Though Aeromonas
hydrophila is considered a pathogenic bacterium, scientists have not been able to prove
that it is the actual cause of some of the diseases it is associated with. It is believed that
this bacterium aids in the infection of diseases, but does not cause the diseases
themselves.
Pathogenic mechanism
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. is remain
unknown. The recently proposed type III secretion system (TTSS) mediated pathogenic
mechanism has been proven to play a pivotal role in Aeromonas pathogenesis.
The TTSS is specialized protein secretion machinery that export virulence factors
delivered directly to host cells. These factors subvert normal host cell functions in ways
that are beneficial to 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, translocated through TTSS and delivered into the host cytoplasm which 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.
Occurrence of exposure
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 infected with the bacterium, such as seafood, meats, and even
certain vegetables such as sprouts.
Human diseases
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.
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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 immune systems.
Treatments
Aeromonas hydrophila can be eliminated using one percent sodium hypochlorite solution
and two percent calcium hypochlorite solution. Antibiotic agents such as chloramphenicol,
florenicol, tetracycline, sulfonamide, nitrofuran derivatives, and pyrodinecarboxylic acids
are used to eliminate and control the infection of Aeromonas hydrophila.
References
1. http://en.wikipedia.org/wiki/Aeromonas
2. Hayes, John. "Aeromonas hydrophila." Oregon State University.
3. Arrow Scientific. "Aeromonas hydrophila."
Questions
1. What procedures would you follow to identify Aeromonas hydrophila in your lab?
2. What are the morphological characteristics of Aeromonas hydrophila?
3. Discuss the pathophysiology and treatment of Aeromonas hydrophila.
Thistle QA is a SANAS accredited organisation, No: PTS0001
Accredited to ISO guide 43 and ILAC G13
Certificate available on request or at www.sanas.co.za