Mycoplasma genitalium

See: Human Pathogen, Virtual Organism, Prokaryote.

References

http://en.wikipedia.org/wiki/Mycoplasma_genitalium
- QUOTE: 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.^[1] There is a difference between smallest parasitic bacteria and smallest free living bacteria. The smallest known free living bacteria is Pelagibacter ubique with 1.3 Mb.^[2]

↑ Aside from viruses—however, it is not agreed upon as to whether or not viruses constitute life.
↑ Stephen J. Giovannoni, H. James Tripp et al. (2005). "Genome Streamlining in a Cosmopolitan Oceanic Bacterium". Science 309 (5738): 1242–1245. doi:10.1126/science.1114057. PMID 16109880.