The largest bacterium

THE large, morphologically peculiar microorganism Epulopiscium fishelsoni^1,2 inhabits the intestinal tract of Acanthurus nigrofuscus, a brown surgeon fish (family Acanthuridae) from the Red Sea. Similar microorganisms have been found in surgeonfish species from the Great Barrier Reef³. As these microorganisms have only been seen in surgeonfish and no free-living forms have been found, they are considered to be specific symbionts of surgeonfish, although the nature of the symbiosis is unclear^1-4. Initial reports considered them to be eukaryotic protists, based primarily on their size^1-4, with individuals being larger than 600 μm by 80 μm. But their cellular morphology in the electron microscope is more like that of bacterial than eukaryotic cells^1,2,54. To resolve the nature of these symbionts, we have isolated the genes encoding the small subunit ribosomal RNA from two morphotypes and used them in a phylogenetic analysis. In situ hybridization with oligonucleotide probes based on the cloned rRNA sequences confirmed the source of the rRNA genes. Our results identify the symbionts as members of the low-(G + C) Gram-positive group of bacteria. They are therefore the largest bacteria to be described so far.