What is the smallest free-living bacterium, with a genome of about 580,000 base pairs?

The bacterium associated with the approximately 580,000-base-pair genome is Mycoplasma genitalium. Its strain G37 genome contains about 580,070 base pairs, making it one of the smallest known genomes among independently reproducing bacteria.

Researchers chose this organism for the Minimal Genome Project because its compact genome contains close to the minimum genetic toolkit needed for cellular life. The complete genome sequence was published in 1995, the second complete bacterial genome sequenced after Haemophilus influenzae. Later work identified roughly 382 genes as essential under the tested laboratory conditions.

Mycoplasma genitalium is a tiny, flask-shaped bacterium that colonizes human urogenital tissues and is sexually transmitted. It lacks a cell wall, which distinguishes it from many familiar bacteria and makes antibiotics that target cell-wall synthesis ineffective. The species is also known by the newer proposed name Mycoplasmoides genitalium, but Mycoplasma genitalium remains the standard quiz and medical name. Mycoplasma pneumoniae is a different species that causes respiratory infections.