In 2006, Carsonella ruddii was reported as the smallest of the small: this microbe contains the smallest genome identified at the time, clocking in at 159,662 base pairs encoding 182 genes. The Escherichia coli genome on the other hand contains over 4 million base pairs encoding about 4,000 genes.
Unlike E. coli, Carsonella ruddii is not a free-living microorganism. It dwells within bacteriocytes in the abdomen of sap-feeding insects known as psyllids. Since these insects are restricted to plant sap, their diet is high in sugar but lacking in most essential amino acids and vitamins. Thus, C. ruddii was considered an endosymbiont because it also contains genes for the biosynthesis of essential amino acids. (more…)