Epulopiscium fischelsoni
Arielle Sager

Epulopiscium fishelsoni was originally thought to belong to the Protoctista kingdom but upon sequencing of the 16S ribosome subunit gene it was found to have low G+C Gram-positive characteristics and was reclassified as a bacteria. Epulopiscium differs greatly from the excepted norms of bacteria because of its very large size, organelle like structures and a unique mode of replication. The most striking difference is that Epulopiscium can grow up to 600nm in length, which is large enough to see with the naked eye or roughly a million times bigger than E. coli. Epulopiscium is not completely atypical in function since it resides in the gut of several species of surgeonfish as a symbiotic partner.

Epulopiscium was first isolated in 1985 from the gut of a Brown Surgeonfish (Acanthurus nigrofuscus) from the red sea. Since that time several similar species have been identified in surgeonfish populations through out the South Pacific from Hawaii to Japan and extending all the way down to the Great Barrier Reef and the waters around Australia. Several very closely related species have been found in the Pacific surgeonfish (A. nigrofuscus and A. lineatus) that share the large cell size and other traits of Epulopiscium and has been subsequently classified as A morphotype. Until fairly recently Epulopiscium was thought to be the largest known bacteria but Thiomargarita namibiensis has replaced it as the record holder that weighs in with a diameter up to 750nm.

At this time two genetically distinct populations of E. fishelsoni have been identified. These two populations and the Epulopiscium spp. as a whole make up a group within the Firmicutes. E. fishelsoni have a natural circadian rhythm that corresponds with the feeding and daily activities of the surgeonfish. During day light hours E. fishelsoni is active, mobile, and inhibits the pH within the surgeonfish’s gut. Surgeonfish feed on primarily algae or other plant materials which further inhibits the pH within its gut. E. fishelsoni also reproduces during day light hours because of the unique reproductive system it has been suggested that this is why E. fishelsoni grows to such a large size. During the dark portion of the circadian rhythm E. fishelsoni finish reproduction and become inactive, immobile, which does not inhibit the pH of the gut, causing it to rise. This cycle corresponds to the natural high, feeding, and low, rest, activity levels through out the day that surgeonfish exhibit.

Fig. 3
Maturing daughter cells within the E. fishelsoni parent cell. A) The daughter cell is ~ 390 by 45 micrometers in size and lacks caps - decondensed DNA is dispersed evenly below the cell wall. B) The daughter cell is ~ 350 by 45 micrometers and has two caps. C) The daughter cell is ~ 350 by 45 micrometers and has a single cap. D) The daughter cell is ~ 360 by 45 micrometers and has two caps and almost completely separated DNA. E) The daughter cell is ~360 by 45 micrometers with two caps and completely separated DNA. From Bresler et al.

E. fishelsoni usually reproduce by forming daughter cells within the original (mother) cell. they can form up to 7 daughter cells which grow until they reach such a size that the mother cell ruptures. This growth is thought to be associated with the circadian rhythm and thus makes it very hard, if not impossible, to grow these organisms in vitro. It has been theorized that E. fishelsoni shares a common ancestor with contemporary endospores forming bacteria because of the many similarities that the reproductive processes share.  Dr. Esther Angert of Cornell University and Dr. Kendall Clements of Auckland University suggested this theory. One of the major differences between endospores forming bacteria, like Bacillus subtilis, and E. fishelsoni is that the daughter cells in E. fishelsoni only incorporate a small portion of the mother cell’s DNA through the partitioning process that the DNA undergoes during the reproduction.

Fig. 4 Epulopiscium Reproductive cycle

Because of the large size of E. fishelsoni cells it must have special coping mechanisms and structural components. Even though E. fishelsoni live in a very nutrient rich environment they must compensate for the surface to volume ratio. These specialized structures were originally thought to be organelles but have now been found to be closer to the structures used in other bacteria for support. These non-organelles form a cortex with in E. fishelsoni cells that appears to be made up of vesicles, capsules and tubules. It has been suggested that these structures are used for waste excretion and transportation within the cell to overcome the diffusion difficulties imposed by such a large volume. Also these structures help to reduce the amount of cytoplasm actually being used for diffusion purposes so that it alters the surface area to volume ratio.

Much is still not known about E. fishelsoni but study is ongoing. Dr Angert’s lab at Cornell is one of the fore runners. Because of the trouble maintaining samples in the lab getting new sample populations can be a lengthy process and can even require special sampling trips to locations in the South Pacific. Since E. fishelsoni have so many unique attributes the possibilities for the greater understanding of bacteria cellular function is large.
















*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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