Imagine a prokaryote that you can see with your naked eye! That is the case with Epulopiscium fishelsoni, the longest prokaryote known to humans. This amazing bacterium can reach a size of 600 mm in length by 80 mm in width, large enough to see without a microscope.

Found in the intestines of surgeonfish that live in and around coral reefs of Australia, Hawaii, and other Pacific Rim islands, scientists first thought this microorganism was a protist of sorts. DNA analysis later showed that E. fishelsoni belonged in the gram-positive cell lineage.

While this bacterium cannot yet be grown in the laboratory, scientists have been able to better understand how it lives and grows through detailed microscopic analysis. One of the big questions has been how a cell this big can maintain itself without organelles or compartments. The size of a bacterium was thought to be limited to the ratio of its surface area to its volume. It turns out that E. fishelsoni does not have a smooth outer membrane; it is wrinkled with many pockets and folds. This allows the large bacterium to maintain a balance between surface area and volume.

E. fishelsoni also has a unique and fascinating way of reproducing. Instead of reproducing via binary fission, it generates its offspring within itself. Two daughter cells are formed at one end of the mother cell. Once the daughter cell membranes have completely formed, the two cells are released through a slit in the mother cell.

Figure 1: Photomicrograph of E. fishelsoni. The photo was taken in Nomarski mode and is false-colored by manipulating the condenser. The individual cell shown is approximately 600 mm in length (0.6 mm).

Legend written by:
Kristen Catlin-LeBaron
American Society for Microbiology
Washington, D.C. 20036
klebaron@asmusa.org