Epulopiscium fishelsoni ("Fishelson's guest at a fish's banquet") is a species of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are most well known for their large size, ranging from 200–700 μm in length. Until the discovery of Thiomargarita namibiensis in 1999, these were the largest bacteria known. The bacterium has not been grown in the lab, but scientists have gained a better understanding of it through microscopic analysis.
Epulopiscium spp. were first discovered in 1985, by a group of biologists studying the intestines of a brown surgeonfish in the Red Sea. They were initially classified as protists on the basis of their large size, until rRNA analysis by Angert, et al. in 1993 confirmed that Epulopiscium spp. are bacteria. Their research and studies illustrated the symbiotic relationship between the brown surgeonfish and Epulopiscium which, in Latin, means "guest at the banquet of a fish."
Epulopiscium is one of the largest known bacteria, a million times bigger than Escherichia coli or Bacillus subtilis. It is large enough to be seen with the naked eye at 600 µm. However, because it is so big, it must compensate for its surface to volume ratio, regardless of its nutrient-rich environment. It also has special coping mechanisms and structures within it, which were originally thought to be organelles. These structures form a cortex within Epulopiscium that are made of vesicles, capsules, and tubules that excrete and transport waste throughout the cell. 
The bacteria exhibit many unusual characteristics, mostly due to the adaptations necessary for their large size. Epulopiscium spp. are extremely polyploid, with bacterial chromosomes representing hundreds of thousands of copies of the genome. Since bacteria rely on diffusion rather than cytoskeletal transport as in eukaryotes, this extreme polyploidy allows for the production of gene products at numerous sites where they are needed in the cell.
Epulopiscium's functions rely heavily on the daily activity of the brown surgeonfish. During the day, the bacteria is active, restraining the pH within the surgeonfish's gut. It also reproduces during the day (its unique reproductive system is the suggested reason why Epulopiscium grows to be such a large size). During the darker portions of the day, Epulopiscium finishes its reproduction and becomes inactive and immobile, which causes the pH level in the surgeonfish's gut to rise. The control that the bacteria plays over the pH level in the surgeonfish's gut plays a large role in the fish's digestion because the fish is an algae-feeding herbivore that also feeds on detritus.
Epulopiscium is unique because it is so large in size, yet it does not have any organelles or compartments, which has led scientists to question how it could possibly support itself. Its anatomy helps it overcome the size limitations inherent in cell volume. The cell's outer membrane, which is not smooth, contains many folds that increase the effective surface area. These structures may be 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.
Perhaps the most intriguing aspect of the bacterium 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. Although sporulation is widespread among other bacteria (such as Bacillus subtilis) in the phylum Firmicutes, spore formation is usually brought about by overcrowding, the accumulation of toxins in the environment or starvation, rather than a standard form of reproduction. Also, endospores are dormant, while new Epulopiscium cells are active.
The production of multiple endospores has been observed in other large gut symbionts such as (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 allow transfer of the bacteria from one host to another.
Different strains of Epulopiscium have been isolated from different surgeonfish species around the world, and scientists have been unable to culture Epulopiscium outside of its natural habitat, suggesting that the relationship between the two is mutually beneficial and symbiotic.
Although the exact biochemical nature of the symbiosis remains unclear, the bacteria likely assist the fish in breaking down algal nutrients. Many bacteria of the genus Clostridium are gut symbionts in a variety of other species, including humans, often involved in breaking down complex carbohydrates.
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