Epulopiscium fishelsoni
Encyclopedia
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 bacterium known.
, inside the intestine
s of a brown surgeonfish. It was initially classified as a protist
on the basis of its large size, until rRNA analysis by Angert, et al. in 1993 confirmed that it was a bacterium. Epulopiscium can reach up to four times the length of the average Paramecium
.
, with bacterial chromosomes representing as much as 100,000 - 200,000 copies of the genome
throughout the cell at any given time. 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 that helps it 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 tubule
s, 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.
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. 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.
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 nucleoid
s 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
.
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacterium
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
that has a symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
relationship with the surgeonfish. It is most well known for its large size, ranging from 200-700 μm in length, and about 80 μm
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
in diameter. Until the discovery of Thiomargarita namibiensis
Thiomargarita namibiensis
Thiomargarita namibiensis is a gram-negative coccoid Proteobacterium, found in the ocean sediments of the continental shelf of Namibia. It is the largest bacterium ever discovered, in general, wide, but sometimes up to...
in 1999, it was the largest bacterium known.
Discovery
Epulopiscium was first discovered in 1985 by the Israeli scientist Lev Fishelson from Tel Aviv UniversityTel Aviv University
Tel Aviv University is a public university located in Ramat Aviv, Tel Aviv, Israel. With nearly 30,000 students, TAU is Israel's largest university.-History:...
, inside the intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s of a brown surgeonfish. It was initially classified as a protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
on the basis of its large size, until rRNA analysis by Angert, et al. in 1993 confirmed that it was a bacterium. Epulopiscium can reach up to four times the length of the average Paramecium
Paramecium
Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion at speeds of approximately 12 body...
.
Physiology
The bacteria exhibit many unusual characteristics, mostly due to the adaptations necessary for their large size. Epulopiscium is extremely polyploidPolyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
, with bacterial chromosomes representing as much as 100,000 - 200,000 copies of the genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
throughout the cell at any given time. Since bacteria rely on diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
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 that helps it overcome the size limitations inherent in cell volume. Its cell wall contains many folds in order to increase surface area
Surface area to volume ratio
The surface-area-to-volume ratio also called the surface-to-volume ratio and variously denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. The surface-area-to-volume ratio is measured in units of inverse distance. A cube with sides of...
, and it possesses an unusual "cortex" containing tubule
Tubule
A tubule is a very small tube or fistular structure.A system of surface-connected membranes in muscle that enables a nerve impulse to travel to the interior of the muscle fibre.In anatomy, a tubule is a minute tube lined with glandular epithelium....
s, vesicles
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
, 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
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
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
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
by protists.
Reproduction
Perhaps the most intriguing aspect of the bacteria is its unusual, almost viviparousVivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
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. Although sporulation is common among other bacteria (such as Bacillus subtilis
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
), 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
Dormant
Dormant means lacking activity. It can refer to:*Dormancy in an organism's life cycle*Dormant volcano, a volcano that is inactive but may become active in the future...
, 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.
Symbiosis
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
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, the bacteria's compact, spherical nucleoid
Nucleoid
The nucleoid is an irregularly-shaped region within the cell of a prokaryote that contains all or most of the genetic material. In contrast to the nucleus of a eukaryotic cell, it is not surrounded by a nuclear membrane. The genome of prokaryotic organisms generally is a circular, double-stranded...
s 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
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
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
in a variety of other species, including humans, usually involved in breaking down complex carbohydrates
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
.
External links
- Epulopiscium spp. and related surgeonfish symbionts, Department of Microbiology, Cornell University. (Accessed August 2008.)