A microbial behemoth
In the warm waters of the Red Sea, inside the intestines of a brown surgeonfish (Acanthurus nigrofuscus), lives an exceptional microbe. Epulopiscium fishelsoni is a giant among bacteria, reaching lengths of up to 600 micrometers (0.6 mm). This makes it large enough to be seen with the naked eye, appearing about the size of a grain of table salt or a printed hyphen. Its volume can be over two thousand times greater than that of a typical bacterium like E. coli.
Discovered in 1985 by a team including Israeli ichthyologist Lev Fishelson, its immense size initially led scientists to classify it as a protist, a type of single-celled eukaryote. It was only in 1993 that ribosomal RNA analysis confirmed its identity as a Gram-positive bacterium. The name Epulopiscium translates to "guest at a banquet of a fish," a nod to its symbiotic home. Tens of thousands of these bacteria can inhabit a single surgeonfish, where they may help their host digest algae.
Being so large presents challenges for a bacterium, which typically relies on simple diffusion to move nutrients across its cell membrane. Epulopiscium overcomes this with a convoluted and wrinkled cell membrane, which dramatically increases its surface area relative to its volume. It is also extremely polyploid, containing tens of thousands, or even hundreds of thousands, of copies of its genome scattered throughout the cell. This lets for localized gene expression without the need for molecules to travel across the vast cell.
Reproduction from the inside out
The most unusual feature of Epulopiscium is its method of reproduction. It does not divide by binary fission, the standard method for bacteria. Instead, it reproduces through a unique process resembling live birth. Up to twelve daughter cells begin to grow inside the mother cell, a reproductive strategy similar to sporulation but with a different outcome.
This reproductive cycle appears to be synchronized with the daily activities of its surgeonfish host. During the day, as the fish feeds, the genetic material inside the bacterium elongates. By late afternoon, the sporulation-like process begins, and daughter cells form and grow. The offspring develop fully inside the parent, eventually consuming it from within. When the mother cell's envelope lyses, the new, active daughter cells are released.
This mode of reproduction is almost unheard of in the bacterial world. While some bacteria form endospores as a survival mechanism in harsh conditions, Epulopiscium uses this internal development as its primary means of propagating. This strategy may protect the developing cells from the harsh environment of the fish's gut and facilitate transfer between hosts. Because of this complex life cycle, scientists have not yet succeeded in culturing Epulopiscium in a laboratory setting.