MICROBIOLOGY LEGEND CYCLE 44 ORGANISM 5 Aeromonas Hydrophila

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MICROBIOLOGY LEGEND

CYCLE 44 ORGANISM 5

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.

Figure 1: Aeromonas hydrophila - Gram stain

Signs and symptoms They can cause diseases in both immunocompetent and immunecompromised individuals. Aeromonas has been linked to causing gastroenteritits, wound infections, bacteremia, and other miscellaneous infections like endocarditis, meningitis, and osteomyelolitis. Various toxins and factors have been identified, but their specific role in virulence is uncertain.

Gastroenteritis caused Aeromonas hydrophila 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 immune systems.

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48 Monte Carlo Crescent Kyalami Business Park, Kyalami Johannesburg, 1684 South Africa www.thistle.co.za Tel: +27 (011) 463 3260 Fax to Email: + 27 (0) 86-557-2232 e-mail : [email protected] Pathology 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.

Laboratory Diagnosis Aeromonas spp. grow well on common laboratory media including blood agar, nutrient, Hecteon enteric agar, MacConkey's, and Luria Bertani. On blood agar, Aeromonas forms circular colonies that are 1-3 mm in diameter that start off grayish in color due to beta-hemolysis and after three days become dark green. DNase culture is positive. Maximal growth is seen when the temperature is between 37 and 44 °C. Colony morphology on: Blood agar - large, round, raised, opaque; most colonies are beta-hemolyitc except A. caviae MacConkey agar - typically non-lactose fermenting; however, some lactose fermenting Aeromonas spp. have been observed

Aeromonas species are facultative anaerobes that are oxidase and catalase positive. This test must be performed on media without a fermentable sugar (ie MacConkey agar), because the fermentation process results in acidification of the medium and a false-negative result may result. Biochemical testing can help to distinguish Vibrio spp. from Aeromonas spp. as some commercially available kits have trouble separating these two genera.

Figure 2: Aeromonas hydrophilia on blood agar Figure 3: Aeromonas hydrophilia on MacConkey agar

Pathogenic mechanism It was believed that the pathogenicity of Aeromonas hydrophilia is mediated by a number of extracellular proteins such as Aerolysin, lipase, chitinase, amylase, gelatinase, hemolysins and enterotoxins. 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.

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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.

Treatments There are few studies that have focused on the susceptibility of Aeromonas species to antimicrobial agents. Fluoroquinolones have been shown to be active against isolates of Aeromonas hydrophila with MICs of less than 1 µg/ml for 90% of the isolates tested. Susceptibility of Aeromonas species to cefotaxime, nalidixic acid, and trimethoprim/sulfamethoxazole have been reported as well. Patients with travelers diarrhea showed varying resistance to chloramphenicol, tetracycline, and cotrimoxazole. Aeromonads also show complete resistance to ampicillin by chromosomally-mediated inducible beta-lactamases.

References 1. http://en.wikipedia.org/wiki/Aeromonas 2. Hayes, John. "Aeromonas hydrophila." Oregon State University. 3. https://path.upmc.edu/cases/case792/dx.html

Questions 1. What procedures would you follow to identify Aeromonas hydrophila in your lab? 2. Discuss the pathogenic mechanism/Pathophysiology of Aeromonas hydrophila? 3. Discuss the Morphological characteristics and treatment of Aeromonas hydrophila.

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