What is the Simplest Organism Known?
Which microbe is the simplest organism depends on your definition of a living organism. If viruses, prions, satellites, nanobes, nanobacteria (non-free-living sub-bacterial organisms) are excluded, the simplest free-living organism known is Mycoplasma genitalium, with a genome of only 580,000 base pairs and 482 protein-coding genes. Mycoplasma genitalium is a tiny parasitic bacteria that lives in the digestive and genital tracts of primates. By comparison, Carsonella ruddii, an endosymbiotic bacteria that lives in plant lice, has a genome of only 159,662 base pairs, with just 182 genes, the smallest known. However, Carsonella ruddii cannot live on its own, and like a virus, depends on the host to survive. Previously, a thermophile that lives around underwater hot springs, Nanoarchaeum equitans, was thought to be the simplest organism, with a genome 490,885 base pairs long and a size of 400 nanometers. Mycoplasma genitalium and other “ultramicroscopic” bacteria have diameters in the bal