It is commonly believed that bacteria are microscopic – stealthy and hidden from the naked eye. But the bacterium, Epulopiscium fishelsoni, is a microbial behemoth you can see with the human eye alone. First discovered in the intestines of a brown surgeonfish in the Red Sea in 1985, this giant bacterium was first classified as a protist because of its large size. In 1993, rRNA sequencing revealed that this organism is actually a bacterium.
Epulopiscium varies between 10- to 20-fold in length and has a volume more than 2,000 times that of a typical bacterium. They range from 200 – 700 microns in length, about the size of a grain of table salt. But being big does have its downsides.
As cell size increases, both the surface area and volume increases. The trouble comes because volume increases much faster than surface area. This increased volume poses challenges to giant bacteria that the typical bacterium does not face.
With a small size, bacteria can use simple diffusion to transport molecules and nutrients inside their cell membranes and within the cell. When the cell size increases, diffusion becomes much too slow to be functional. To overcome the diffusion problem, the Epulopiscium cell membrane is wrinkly instead of a flat, smooth surface. The wrinkles lend to more surface area of the membrane where diffusion and transport of molecules into the cell can take place.
Large cells, such as those within eukaryotic organisms, evolved internal structures to more efficiently transport molecules within the cell. Epulopiscium cells, however, do not contain these structures. But they have evolved other ways to deal with the transport problem.
In 2008, researchers found an extremely large amount of DNA within the cell. Epulopisicium cells can contain anywhere between 85 pg and 250 pg of DNA, or about 14 to 42 times as much DNA as a human cell. An E. coli cell contains 0.017 pg of DNA meaning Epulopsicium contains 5,000 to 15,000 times as much DNA as the typical bacterium. This immense amount of DNA is not just one chromosome – each cell contains thousands of copies of its genome. This means that copies of the same gene are scattered about the cell resulting in gene expression that can happen locally, without the need of large-scale diffusion.
Epulopiscium is yet another example of the diverse lifestyles of microbes and reminds us that we find microbes in the most obscure places and in the most astounding forms. Epulopsicium defies the conventional view of microbe and leaves us wondering: how big is too big for a bacterium?