Epulopiscium fishelsoni

Taxobox
name = "Epulopiscium fischelsoni"
regnum = Bacteria
phylum = Firmicutes
classis = Clostridia
ordo = Clostridiales
genus = "Epulopiscium"
species = "E. fishelsoni"
binomial = "Epulopiscium fishelsoni"
binomial_authority = Schulz "et al.", 1999

"Epulopiscium fishelsoni" ("Fishelson's guest at a fish's banquet") is a Gram-positive bacterium that has a symbiotic relationship with the surgeonfish. It is most well-known for its large size, ranging from 200-700 μm in length, and about 80 μm in diameter. Until the discovery of "Thiomargarita namibiensis" in 1999, it was the largest bacteria known.cite journal |author=Angert ER, Clements KD, Pace NR |title=The largest bacterium |journal=Nature |volume=362 |issue=6417 |pages=239–241 |pmid=8459849 |doi=10.1038/362239a0 |year=1993] cite journal |author=Angert ER, Brooks AE, Pace NR |title=Phylogenetic analysis of Metabacterium polyspora: Clues to the evolutionary origin of "Epulopiscium" spp., the largest bacteria |journal=Journal of Bacteriology |volume=178 |issue=5 |pages=1451–1456 |year=1996 |pmid=8631724]

Discovery

"Epulopiscium" was first discovered in 1985 by the Israeli scientist Lev Fishelson from Tel Aviv University, inside the intestines of a brown surgeonfish. It was initially classified as a protist on the basis of its large size, until rRNA analysis by Pace, "et al" in 1993 confirmed that it was a member of the bacteria. "Epulopiscium" can reach up to three times the length of the average paramecium.Fact|date=May 2008

Physiology

The bacteria exhibit many unusual characteristics, mostly due to the adaptations necessary for their large size. "Epulopiscium" is extremely polyploid, with bacterial chromosomes representing as much as 100,000 - 200,000 copies of the genome throughout the cell at any given time.cite journal |author=Mendell JE, Clements KD, Choat JH, Angert ER |title=Extreme polyploidy in a large bacterium |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=18 |pages=6730–4 |year=2008 |month=May |pmid=18445653 |doi=10.1073/pnas.0707522105 |url=http://www.pnas.org/cgi/pmidlookup?view=long&pmid=18445653] cite web |title=Giant bacterium carries thousands of genomes |author=Ledford H |url=http://www.nature.com/news/2008/080508/full/news.2008.806.html |publisher=Nature |date=2008-05-08 |accessdate=2008-05-11 (doi|10.1038/news.2008.806)] Since bacteria rely on diffusion rather than cytoskeletal transport as in eukaryotes, this over-expression may be necessary for proteins to disperse throughout the cell. This polyploidy is also associated with a very high efflux rate, due to the over-expression of genes for export pumps.

"Epulopiscium" has a unique anatomy which is designed to overcome the size limitations inherent in cell volume. Its cell wall contains many folds in order to increase surface area, and it possesses an unusual "cortex" containing tubules, vesicles, and other structures which are usually found in eukaryotes. It may be the case that these structures are involved in intracellular transport, which would provide a unique example of convergent evolution on the cellular level.

While these adaptions allow the bacteria to break the theoretical upper limit for size, the underlying evolutionary reasons for the bacteria to grow to this size in the first place remain speculative. One possible reason could be the ability to avoid predation by protists.

Reproduction

Perhaps the most intriguing aspect of the bacteria is its unusual, almost viviparous reproductive cycle. Unlike most bacteria, which undergo binary fission, "Epulopiscium" reproduces exclusively through an unusual form of sporulation in which anywhere from one to twelve daughter cells are grown inside of the parent cell, until the cell eventually lyses and the new bacteria burst through the cell wall.cite journal |author=Angert ER, Clements KD |title=Initiation of intracellular offspring in Epulopiscium |journal=Molecular Microbiology |volume=51 |issue=3 |pages=827–835 |year=2004 |pmid=14731282 |doi=10.1046/j.1365-2958.2003.03869.x] Although sporulation is common among other bacteria (such as Bacillus subtilis), it is a desperation measure brought about by overcrowding or starvation, rather than a standard form of reproduction. Also, the daughter cells in standard sporulation are usually dormant, while new "Epulopiscium" cells are active.

This form of reproduction has been observed in other large gut symbionts (Metabacterium polyspora), which are phylogenetically related to "Epulopiscium". Since sporulation affords bacteria much more protection from the outside environment than binary fission, it is thought that the evolution of this unusual life cycle may be in order to allow transfer of the bacteria from one host to another, and also provide protection during reproduction from the harsh environment of the digestive system.

ymbiosis

Different strains of "Epulopiscium" have been isolated in most surgeonfish species around the world, and scientists have been unable to culture "Epulopiscium" outside of its natural habitat, meaning that the relationship between the two is probably mutually beneficial and symbiotic.

The daily life cycle of "Epulopiscium" exhibits a correlation with the daily activities of the surgeonfish. During the day, when the surgeonfish feed on algae, the bacteria's compact, spherical nucleoids migrate to the poles of the cell and begin to elongate. As the day goes on, the average length of the cells increase, until the nucleoids make up a large percentage of the parent cell volume, and the sporulation process begins in the late afternoons and evenings, when these nucleoids reach a maximum of approximately 50 - 75% of the length of the parent cells. The pH of the surgeonfish's gut also shows a correlation with the daily life cycle of the bacteria, showing that they suppress it during the day.

Although the exact biochemical nature of the symbiosis remains unclear, it is safe to assume that the bacteria assist the fish in breaking down algal nutrients. Many bacteria of the genus "Clostridia" are gut symbionts in a variety of other species, including humans, usually involved in breaking down complex carbohydrates.

References

Further reading

*Flint JF, Drzymalski D, Montgomery WL, Southam G, Angert ER (2005). "Nocturnal production of endospores in natural populations of "Epulopiscium"-like surgeonfish symbionts". "Journal of Bacteriology" 187:7460-7470.
*Robinow C, Angert ER (1998). "Nucleoids and coated vesicles of Epulopiscium spp". "Archives of Microbiology" 170:227-235.

External links

* [http://author.cals.cornell.edu/cals/micro/research/labs/angert-lab/angert-epulopicium.cfm "Epulopiscium" spp. and related surgeonfish symbionts] , Department of Microbiology, Cornell University. (Accessed August 2008.)
* [http://www.physorg.com/news129394905.html Researchers study bacterium big enough to see] — Physorg.com (7 May 2008. Accessed May 2008.)