Schulz et al., 1999
Epulopiscium fishelsoni ("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.
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.)
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 1,000 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 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.
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. 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.
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.