Aeromonas Hydrophila

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A Microbial Biorealm page on the genus Aeromonas Hydrophila

Classification

Higher order taxa

Cellular organism; Bacteria; Proteobacteria; Gammaproteobacteria; Aeromonadales; Aeromonadaceae; Aeromonas.

Species

NCBI: Taxonomy

Aeromonas hydrophila

Subspecies: Aeromonas hydrophila subsp. anaerogenes, Aeromonas hydrophila subsp. decolorationis, Aeromonas hydrophila subsp. dhakensis, Aeromonas hydrophila subsp. hydrophila ATCC 7966, and Aeromonas hydrophila subsp. ranaei [19].

Description and significance



Aeromonas hydrophila is the most well known of the six species belonging to the genus Aeromonas. It is rod- shaped, non- spore forming, oxidase- positive, glucose- fermenting, facultative anaerobic, gram- negative bacterium that are inhabitant to aquatic environments [1, 2]. This bacterium can be found in fresh, brackish, estuarine, marine, chlorinated and unchlorinated water supplies worldwide, with highest numbers obtained in the warmer climates [1, 2, 3, 4]. The bacterium has optimal growth at 28°C but can also grow at the limits from 4°C to 37°C [1]. Some strains of Aeromonas hydrophila are capable of causing disease in fish and amphibians as well as in humans who may acquire infections through open wounds or by ingestion of an adequate number of the organisms in water or food. This bacterium can digest materials such as gelatin, hemoglobin, and elastin. Aeromonas hydrophila was isolated from diseased cold- and warm- blooded animals for over 100 years and from humans since the early 1950s [1, 5]. It is also hard to kill because it is a resistant bacterium. Aeromonas hydrophila is resistant to chlorine, refrigeration or cold temperatures [24].



Genome structure

The complete genome of Aeromonas hydrophila strain ATCC 7966T was sequenced [14]. The genome is comprised of a single circular 4,744,448 bp chromosome with 61.5% GC content [20]. Its entire genome consists of 4,122 protein coding genes and 159 RNA genes: 128 tRNA genes amd 30 rRNA genes. It was possible to assign putative functions to 72.3% of the CDSs, while 21.5% possessed similarity to genes of unknown function, and no function could be proposed for 6.2% of the CDSs [21]. The complete genome sequence of Aeromonas hydrophila ATCC 7966T reveals mechanisms contributing to virulence and metabolic condition that allow the organism to grow in a variety of environment and explain how Aeromonas hydrophila is able to survive in polluted or oxygen- poor environments and to colonize and cause illness in humans and other hosts. However, two recognized virulence markers, a type III secretion system and a lateral flagellum, that are reported in other Aeromonas hydrophila strains are not identified in the sequenced isolate, ATCC 7966T. Given the ubiquity and free- living lifestyle of this organism, there is relatively little evidence of fluidity in terms of mobile elements in the genome of this particular strain. Thus, the Aeromonas hydrophila genome sequence provides valuable insights into its ability to flourish in both aquatic and host environments [14].

A number of virulence factors produced by Aeromonas species, however, their association with diarrhea have not been clearly linked [14]. Recently, a cytotoxic enterotoxin (Act), a heat- unstable cytotonic enterotoxin (Alt), and a heat- stable cytotonic enterotoxin (Ast) has been characterized from a diarrheal isolated of Aeromonas hydrophila. The Act is a single- chain polypeptide that is one of the primary genes to make this species pathogenic. Act is aerolysin related and has hemolytic, cytotoxic, and enterotoxic activities. Alt and Ast also have genes that contribute to its toxicity, but are not related to cholera toxin [6].

Cell structure and metabolism

Aeromonas hydrophila are gram- negative straight rods with rounded ends. They usually grow from 0.3 to 1.0 μm in diameter and 1.0 to 3.5 μm in length. Aeromonas hydrophila does not form endospores, and can grow in temperatures as low as 4°C. This bacterium is motile by polar flagella. Aeromonas hydrophila are heterotrophic organisms. They can exist in both aerobic and anaerobic environments, and ferment glucose. Aeromonas hydrophila can also digest gelatin, hemoglobin, and elastin [23, 24].

Aeromonas hydrophila has comprehensive biosynthetic abilities. The TCA cycle is complete and complemented by TCA cycle intermediates which can be replenished from acetate for use in various biosynthetic reactions. The Entner- Doudoroff and glycolytic pathways are intact, while the pentose phosphate pathway appears to be missing the oxidative branch. Complete multi- step pathways for synthesizing all amino acids are predicted, as are biosynthetic pathways for numerous cofactors such as biotin, glutathione, ubiquinone and menaquinone, pantothenate, thiamine, riboflavin, heme, molybdopterin, iron- sulfur clusters, coenzyme A, and tetrahydrofolate [14]. Aeromonas hydrophila has the oxygen- sensitive IscSUA- HscBA- Fdx system for the biosynthesis of iron- sulfur clusters rather than the oxygen- resistant SUF system, typically associated with aerobic and facultatively anaerobic organisms [14]. Under anaerobic growth conditions of Aeromonas hydrophila, a NiFe uptake hydrogenase may act as electron carrier to appropriate acceptors, and immediately downstream is a putative nickel transporter, followed by various hydrogenase maturations. In addition to, nitrate is converted to nitrite and then to ammonia in the general assimilatory pathway. Nitrate may also act as an electron acceptor for anaerobic respiration [14].

Sulfate assimilation is accomplished by reduction of sulfate to sulfide with the participation of a sulfate adenylyltransferase, adenylylsulfate kinase, and phosphoadenosine phosphosulfate reductase. The sulfite product from the assimilation pathway is reduced by sulfite reductase to sulfide for incorporation into an amino acid, peptide, protein, etc. Additionally, Aeromonas hydrophila has a dissimilatory anaerobic sulfite reductase involved in dissimilation of oxidized anions for energy transduction. This enzyme catalyzes hydrogen sulfide production from sulfite, which is regulated by electron acceptors from hydrogen or an organic substrate, and serves as an important energy- conserving step. It also confers the ability to synthesize cysteine anaerobically. Other electron acceptor sources utilized by Aeromonas hydrophila include tetrathionate, fumarate, and trimethyl- N- oxide [14].

Ecology

Aeromonas hydrophila are ubiquitous bacteria which found in a variety of aquatic environments worldwide, including bottled water, chlorinated water, well water, and heavily polluted waters. The organism is posted in the Contaminant Candidate List by the Environmental Protection Agency, and U.S. water supplies are routinely examined for it. Even Aeromonas hydrophila confer the metabolic versatility to persist in its aquatic habitats or that facilitate ecological interactions with other prokaryotic and eukaryotic organisms [14]. Aeromonas hydrophila infections elevate occurrence with environmental changes, in contaminated environments, change in the temperature, and when an organism is already infected with a virus or another bacterium [23].

Pathology

Aeromonas hydrophila causes a variety of diseases in both fish and human populations. The ubiquitous nature of the bacterium in aquatic environments provides significant opportunity for animals, mainly fish and amphibians to contact and ingest organisms [14]. Aeromonas hydrophila is very toxic to many organisms because of its structure. When it enters the body of fish, amphibians, or human, it travels via the bloodstream to the first available organ. It produces aerolysin cytotoxic enterotoxin (Act) which is one of the major virulence factors. Its toxin is produced and secreted by the cell from a type II secretion system. The toxin binds to high- affinity receptors and undergoes oligomerization to form a heptameric pore- forming complex which allows passage of small molecules in the plasma membrane, resulting in permeabilization of the cell, cell death, and eventually tissue destruction. Aeromonas hydrophila is also known as an opportunistic pathogenic bacterium, meaning they only infect hosts with weakened immune responses [6, 7]. Other virulence functions include a surface layer which inhibits complement- mediated killing, type IV pili for attachment, and a set of extracellular proteases which can cause tissue damage [14]. 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 do not cause the diseases themselves [14].

The Disease in Fish and amphibians

Aeromonas hydrophila cause illness mainly in fish and amphibians because this bacterium lives in aquatic environments. It is related to a disease found in frogs called red leg that causes internal or fatal hemorrhage. When infected with Aeromonas hydrophila, fish develop ulcers, fin rot, tail rot, and hemorrhagic septicaemia [12, 13]. Specially, hemorrhagic septicaemia causes lesions that lead to scale shedding, hemorrhages in the gills and anal area, ulcers, exophthalmia, and abdominal swelling. [1, 23]

The Disease in Humans

Aeromonas hydrophila is also pathogenic to humans. It causes gastroenteritis which is one the disease in humans. This disease can affect anyone, but it most occurs in young children and people who have compromised immune systems or growth problems [12, 13]. This bacterium is linked to two types of gastroenteritis. The first type is a disease similar to cholera which causes rice- water diarrhea. Mild symptoms include fever and chills, but patients who become overwhelming bacterial infection with Aeromonas hydrophila often exhibit abdominal pain, nausea, vomiting, and diarrhea [1, 23]. The other type of disease is dysenteric gastroenteritis that 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 [1, 14, 23].

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

Current Research

Enter summaries of the most recent research here--at least three required

References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by student of Rachel Larsen