Mycoplasma genitalium is a small parasitic bacterium that lives on the ciliated epithelial cells of the primate genital and respiratory tracts. M. genitalium is the smallest known genome that can constitute a cell, and the second-smallest bacterium after the endosymbiont Carsonella ruddii. Until the discovery of Nanoarchaeum in 2002, M. genitalium was also considered to be the organism with the smallest genome. There is a difference between smallest parasitic bacteria and smallest free living bacteria. The smallest known free living bacterium is Pelagibacter ubique with 1.3 Mb.
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