Carsonella ruddii

Candidatus Carsonella ruddii is an endosymbiotic gamma proteobacterium. It has the smallest known genome of any bacterium.
The species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids. The endosymbionts occurs in a specialised structure known as the bacteriome.
In 2006 the genome of one strain of "Ca." C. ruddi was sequenced at RIKEN in Japan and the University of Arizona. The genome is a circular chromosome of 159,662 base pairs and has a high coding density (97%) with many overlapping genes and reduced gene length. The number of predicted genes was 182, also the lowest on record (NCBI-Genome). Numerous genes considered essential for life seem to be missing, suggesting that the species may have achieved organelle-like status.
In comparison, "Mycoplasma genitalium", which has the smallest genome of any free-living organism, has a genome of 521 genes.