Epulopiscium fishelsoni
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Epulopiscium fishelsoni is a giant rod-shaped bacterium, about 80 μm in diameter and between 200 and 700 μm in length, making it one of the largest bacteria. It was first discovered in 1985 by the Israeli scientist Lev Fishelson from TAU (Tel Aviv University), apparently inside the intestines of a brown surgeonfish. The name means "guest at a fish's banquet". It is placed among the Clostridia. Its daily cycle is integrated with that of its surgeonfish host.
Life Cycle
The daily life cycle of Epulopiscium fishelsoni correlates well with the daily activities of the surgeonfish. In the mornings, E. fishelsoni cells (isolated from a surgeonfish gut, not a culture) were found to contain compact, spherical nucleoids at the apices of the cells which elongated during the day. Throughout the day, the average length of the cells increased with the nucleoids making up a large percentage of the parent cell volume. In the late afternoons and evenings, these nucleoids reached a maximum of approximately 50 - 75% of the length of the parent cells. During the night, over 70% of the E. fishelsoni cells found in the gut contained two nucleoids; the rest of the cells were smaller and lacked incipient daughter cells. These smaller cells, which were almost always found only in early morning samples, were assumed to be the released daughter cells; the parent cells are destroyed in the process of releasing daughter cells. Because of this daily process it is almost impossible to grow them in a culture.
During the day when the nucleoids are elongating and the bacteria are reaching their full size, the surgeonfish would be at its most active feeding frequently and filling its gut with algal food materials. In addition, the metabolism of the E. fishelsoni at this time suppress the pH of the gut fluids. During the night when the fish would be inactive in reef shelters, the modal cell size declines and the bacteria does not suppress the pH (Bresler et. al 1998).
It is not known if the abnormally large size of Epulopiscium fishelsoni has anything to do with its unique reproduction in which one to seven daughter cells form inside of the parent cell. These cells grow along the body of the parent cell until they fill the cell and finally burst the cell wall. Metabacterium polyspora is phylogenetically related to E. fishelsoni and is thought by some to point towards the evolution of the special reproductive system of Epulopiscium. M. polyspora live in the intestines of rodents, grow to an unusual size (not as large as Epulopiscium, but unusual nonetheless), and reproduce by forming two or more refractile endospores per cell.
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