Carsonella ruddii
Tara Banaszek

It was once proposed that the smallest bacteria would need at least 300 genes. Yet, Carsonella ruddii only has 182 genes, just over half of the proposed minimum. The DNA of C. ruddii is only 160 kilobases, with the next smallest genome coming in over 2.5 times its size, at 460 kilobases. How is a 160 KB genome possible? The small pox virus is bigger than this little guy !

C. ruddii lives inside of psyllids known as “jumping plant lice.” These sap-feeding lice have a structure known as a bacteriome. This bacteriome contains bacteriocytes, which contain vesicles which enclose C. ruddii. These gram-negative vesicle walls are made by the bacteriocyte; the limited genome of C. ruddii does not encode for proteins to make a cell wall or reproductive enzymes. This endosymbiont bacteria is believed to have infected one psyllid long ago, and some of its genome may have been transferred to its host’s DNA, allowing this bacteria to shed some of its genome to its current minimalist size.

In addition to its small stature. C. ruddii has a highly unusual genome. It is incredibly A-T rich, with an almost unbelievably low G-C content of 19% (18% in coding region). The chromosomes of DNA are circular and are found in 3 fragments. They lack the Shine-Delgarno ribosomal binding site characteristic of most prokaryotic cells, making them highly unusual. In addition, the open reading frames (ORF) of the genes are overlapping, which is not rare in prokaryotes, but the amount of overlapping is especially pronounced in C. ruddii.

The known metabolic actions of C. ruddii are the pentose phosphate pathway, pyruvate metabolism, oxidative phosphorylation, carbon fixation, methane metabolism, butanoate metabolism, and C5- branched dibasic acid metabolism, based off of the enzymatic content of the bacteria. In addition, it possess the ability to breakdown and/or make most of the amino acids.

The reason that scientists are interested in C. ruddii is because of its near organelle status. Although it is classified as with the smallest genome of any living creature, most scientists are not using it as a basis to determine the minimal amount of genes needed, because it is so dependent on its host. The eukaryotic host organism requires the bacteria to survive, and produces machinery necessary for the bacteria to survive. It is unclear what function the bacteria serves the psyllid, though all the plant lice contain the bacteria and will not survive without it. Some strains of the lice have additional symbionts, but it appears that the only vital symbiont is C. ruddii. It is hypothesized that this bacteria produces some amino acid or other nutrient to the cell. The bacteria are maternally passed down to offspring, which is characteristic of eukaryotic mitochondrion. This relationship between the eukaryote and prokaryote gives reason to believe that C. ruddii is in the process of becoming what we would classify as an organelle. By studying C. ruddii scientists believe they can use this as evidence towards the endiosymbiotic theory of mitochondrion and chloraplasts as well.


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Thao, M.L., Clark, M.A., Baumann, L., Brennan, E.B., Moran, N.A., and Baumann, P. “Secondary Endosymbionts of Psyllids Have Been Acquired Multiple Times.” Current Microbiology Vol. 41 (2000), pp. 300–304. Accessed online: <>.

*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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