Naegleria

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Naegleria Template:IPAc-en is a genus consisting of 47 described species of protozoa often found in warm aquatic environments as well as soil habitats worldwide.<ref name=":0">Template:Cite journal</ref> It has three life cycle forms: the amoeboid stage, the cyst stage, and the flagellated stage, and has been routinely studied for its ease in change from amoeboid to flagellated stages.<ref name=":0" /> The Naegleria genera became famous when Naegleria fowleri, the causative agent of the usually fatal human and animal disease primary amoebic meningoencephalitis (PAM), was discovered in 1965.<ref name=":0" /> Most species in the genus, however, are incapable of causing disease.<ref name=":0" />

Etymology

The genus Naegleria is named after the German protozoologist, Kurt Nägler.<ref name=":1">Template:Cite journal</ref>

History

In 1899, Franz Schardinger discovered an amoeba that had the ability to transform into a flagellated stage.<ref name=":2">Template:Cite journal</ref> He named the organism Amoeba gruberi,<ref name=":2" /> which was later changed to the genus Naegleria in 1912 by Alexeieff.<ref name=":0" /> Before 1970, the genus was generally used as a model organism to study the changes from amoeboid to flagellated stages.<ref name=":0" /> However it garnered much more attention when a human pathogenic species (Naegleria fowleri) was discovered in Australia in 1965, and described in 1970.<ref>Template:Cite journal</ref>

Habitat and ecology

Naegleria is found worldwide in typically aerobic warm aquatic environments (freshwater such as lakes and rivers) and soil habitats.<ref name=":0" /> As a typically free living genus, it feeds on bacteria and can be maintained on a diet of gram negative bacteria.<ref name=":0" /> It feeds via phagocytosis.<ref name=":3">Template:Cite journal</ref> The few species that are pathogenic seem to be characteristically thermophilic, preferring warmer temperatures such as nuclear power plant cooling water.<ref name=":4">Template:Cite journal</ref> One species, Naegleria fowleri, can be an opportunistic and usually fatal pathogen of humans if it enters the depths of the nasal cavity.<ref name=":5">Template:Cite journal</ref>

Description of organism

Morphology/anatomy

Naegleria are free-living amoebae,<ref>="CDC2023">Template:Cite web</ref> with some strains being opportunistic pathogens.<ref name=":4" /> Cells range from 10-25 um depending on the life stage it is currently in.<ref name=":1" /> Species are not classified morphologically anymore but historically have been by flagellar shape.<ref name=":1" /> New species are often defined by ribosomal DNA sequences.<ref name=":1" /> The unicellular organism's cytoplasm has distinct separations of an ectoplasm (outer) and endoplasm (inner).<ref name=":1" /> As a mitochondriate, aerobic organism it has many mitochondria in the endoplasm.<ref name=":1" /> The endoplasm also contains ribosomes, food vacuoles, contractile filaments/vacuoles, and protoplasmic filaments.<ref name=":1" /> Notably, Golgi is not visibly identifiable although expression of Golgi-associated machinery has been identified.<ref>Template:Cite journal</ref> It has a nucleus with a prominent nucleolus.<ref name=":1" />

Life cycles

Naegleria has 3 different life cycle stages: amoebae, cyst, and flagellate.

The amoebae stage is the feeding stage and has blunt pseudopodia (lobopodia) that give the cell an overall irregular, yet generally cylindrical shape.<ref name=":1" /> The overall size is usually around 10–20 um at this stage. The pseudopodia are actin based extensions of the body and form at irregular regions of the cell.<ref name=":1" /> Movement occurs in this stage via extending the pseudopodia, and having the cytoplasmic internal contents follow subsequently.<ref name=":1" /> As the feeding stage of the organism, pseudopodia are also used to engulf prey, such as bacteria.<ref name=":1" /> This is also the stage that the organism spends the most time in, and also the reproductive phase.<ref name=":0" /> Reproduction occurs here by binary fission and it can reproduce every 1.6 hours on a bacterial diet.<ref name=":0" /> Reproductive division involves promitosis, or intranuclear mitosis, which does not occur with nuclear envelope breakdown.<ref name=":0" /> Sexual reproduction has not been observed in this genus but the genes for meiosis do exist in the genome.<ref name=":0" />

The cyst stage is a double walled spherical stage.<ref name=":6">Template:Cite journal</ref> The double wall consists of a thick endocyst and a thin endocyst.<ref name=":6" /> The cyst contains usually 2-8 pores (often depending on the species) and is formed when conditions become adverse, such as residing in non optimal temperature.<ref name=":6" /> Cysts are favourable as they are naturally resistant to environmental hardships.<ref name=":6" /> When adverse conditions are restored to normal, the organism can escape the cyst through the pores in its amoeboid form.<ref name=":6" /> Cysts have been observed to be formed in all but one species where the ability to form a cyst is inhibited by a bacterial parasite.<ref name=":6" />

The flagellate stage consists of two flagella which are induced by de novo assembly of a primarily microtubule cytoskeleton from a former actin based cytoskeleton (from the amoeboid form).<ref name=":4" /> The microtubule skeleton is prominent along with the development of basal bodies.<ref name=":1" /> The entire flagellar structure consists of 200 proteins.<ref name=":1" /> Division of the organism does not occur in this life stage, although two species have been found to divide as an exception.<ref name=":1" /> There is no cytostome (feeding groove) present suggesting that feeding occurs primarily in the amoeboid stage via phagocytosis.<ref name=":1" /> There is a single nucleus which is near the flagellar root.<ref name=":1" /> The flagellated stage is typically encountered when the genus needs to move to a more desirable location, which is often encountered when conditions are not optimal.<ref name=":1" /> Therefore, this flagellated stage is transient and the organism usually reverts to the amoeboid form within an hour, with transformation taking about 100 minutes.<ref name=":1" /> The reversion to the amoeboid form can be induced by changes in ionic concentration of the water it resides in (such as placing it in distilled water);<ref name=":1" /> during which transformation the cell disassembles its microtubules.<ref name="Gelfand-Bershadsky-1991">Template:Cite journal</ref> Notably, five species have never been observed in this flagellate life stage.<ref name=":1" />

Genetics

The genome of Naegleria gruberi has been sequenced and consists of a 41 Mb nuclear genome with 15,727 protein-coding genes.<ref name=":7">Template:Cite journal</ref> It has a 33% GC content, and 57.8% of the genome is coding with about 36% consisting of introns.<ref name=":7" /> This suggests a mean of about 0.7 introns per gene.<ref name=":7" /> There are at least 12 chromosomes present.<ref name=":7" /> About 1% of the genes have homology to bacterial genes suggesting that lateral gene transfer may have occurred at some point.<ref name=":7" /> The genome also notably contains the required genes for Golgi but it is visibly lacking.<ref name=":7" /> Although only seen to be asexual, meiotic genes are also present.<ref name=":7" />

Compared to other protists, Naegleria also has a larger set of mitochondrial genes with about a 50 kb mitochondrial genome.<ref name=":3" /> The mitochondrial genome clearly encodes for aerobic respiration which is seen through its ability to perform oxidative phosphorylation and use oxygen as a terminal electron acceptor.<ref name=":3" /> Remarkably the organism's genome also encodes for an elaborate anaerobic metabolism such as substrate-level phosphorylation and an ability to use fumarate as the terminal electron acceptor.<ref name=":3" /> This anaerobic system is hypothesized to be used in slightly anoxic muddy environments during the cyst life stage.<ref name=":3" />

The genus Naegleria's ribosomal DNA (rDNA) consists of an extrachromosomal plasmid of which about 4000 exist in each cell.<ref name=":0" /> Comparison of 5.8S rDNA is the current way of molecularly classifying new species.<ref name=":0" /> Species can also be distinguished by their internal transcribed spacers type 2 (ITS2) sequences.<ref name=":1" />

Practical importance

One species of Naegleria is known to be a potential pathogen to humansTemplate:SndNaegleria fowleri.<ref name=":8">Template:Cite journal</ref> It is typically free living, but is a thermophilic parasite if it encounters the right host.<ref name=":8" /> Besides being found in freshwater, it can also be found in warm water of industrial plants, as well as poorly chlorinated swimming pools.<ref name=":8" /> It enters through the nose of the host (who is typically found to be in contact through warm water such as thermal nuclear plant cooling water), and attaches to the olfactory epithelium where it goes to the brain by locomotion (pseudopodia).<ref name=":8" /> There it destroys neurons and causes primary amoebic meningoencephalitis (PAM), a very rare, yet fatal disease.<ref name=":8" /> PAM shows symptoms very similar to bacterial meningitis.<ref name=":5" /> N. fowleri is one of four known free living amoebae found in association with human disease.<ref name=":5" /> The end result is almost always death, even in healthy people.<ref name=":5" /> N. fowleri possess secreted proteases, phospholipases, and pore-forming peptides which are characteristics of a pathogenic process.<ref name=":5" />

Two other species, Naegleria austerealiensis and Naegleria italica have been shown to produce disease in experimental animals.<ref>Template:Cite journal</ref> They have been observed to cause central nervous system (CNS) infections in animals such as mice, rats, squirrels, guinea pigs, sheep, as well as the gills of fish.<ref name=":0" />

Another practical importance of the genus is that it is extensively studied for its transformation from the amoeboid phase into the flagellated stage, which can be difficult to induce in other genera.<ref name=":1" /> The transformation from flagellate to amoeboid stage can be induced by changes in ionic concentration, such as placing the organism in distilled water making it a great model organism for doing so.<ref name=":1" />

List of species (or lower taxonomic units)

48 species of Naegleria have been described.<ref name=":1" /> These include: Template:Div col

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References

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Further reading

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