Monstrous microbes are just big bacteria.

Eight years ago, biologists discovered a strange organism in the guts of surgeonfish. Visible to the naked eye, these single-celled, half-millimeter-long creatures should be classified as protozoans, or so scientists assumed. They were a million times larger than bacteria such as Escherichia coli; yet under the electron microscope, their insides looked so much like those of bacteria that microbiologists didn't really know what to make of them.

So Kendall D. Clements at James Cook University in Townsville, Australia, sent some fish guts from the Great Barrier Reef to the United States and asked Norman R. Pace and Esther R. Angert at Indiana University in Bloomington to have a look.

Pace and Angert isolated genetic material from the ribosomes (protein-building structures) of the unusual organisms and compared it with ribosomal RNA from other microbes. They also examined genetic material from similar bacteria found in surgeonfish in the Red Sea. Their analyses showed that the presumed protozoan, Epulopiscium fishelsoni, should instead consider gram-positive bacteria its closest kin. They report their findings in the March 18 NATURE.

Microbiologists thought bacteria could not grow very big because nutrients would not diffuse throughout giant cells. But these organisms, orders of magnitude bigger than any other known bacterium, prove otherwise. Bacteria probably organize their interiors to get around diffusion limitations, Pace says. Unfortunately, no one has succeeded in growing the giant bacteria in the lab, so they have not been studied further.

In the past, researchers have used size to classify organisms as eukaryotes or prokaryotes. The giant bacteria, however, will force scientists to reexamine the fossil record and to reevaluate their ideas about the evolution of eukaryotes, says Mitchell L. Sogin of the Marine Biological Laboratory in Woods Hole, Mass.