The Genome Giant of the Soil
Hidden in the soils of Central Germany and across the globe is a microscopic record-holder: Sorangium cellulosum. This bacterium possesses the largest known bacterial genome, a circular chromosome containing 13,033,779 base pairs. Its genetic code is immense, containing 9,367 predicted genes. This dwarfs the genome of the common lab bacterium Escherichia coli, which has about 4.6 million base pairs, and is even larger than that of some simple eukaryotes like baker's yeast.
S. cellulosum is a myxobacterium, a group known for its complex social behaviors. These soil-dwelling, Gram-negative bacteria move by gliding and, under stressful conditions like starvation, will congregate to form multicellular structures called fruiting bodies. The species is a saprophyte, meaning it derives its nutrition by decomposing organic material—specifically cellulose—in environments like soil, tree bark, and animal feces. The particular strain that was fully sequenced, So ce56, was originally isolated from a soil sample in Indonesia, but the species is globally distributed. The sequencing was completed in 2007 by a team at the German Research Centre for Biotechnology (now the Helmholtz Centre for Infection Research) in Braunschweig.
A Microscopic Pharmaceutical Factory
The bacterium's massive genome is a a sophisticated chemical factory. Sorangium cellulosum dedicates a large portion of its genetic code to producing secondary metabolites—complex organic compounds that are not essential for basic growth but provide a competitive advantage. This single genus is responsible for about half of all known secondary metabolites isolated from myxobacteria. The genome of strain So ce56 contains at least 17 distinct gene clusters for making these valuable molecules.
Among the most significant of these products is a class of compounds called epothilones. Epothilones are powerful anticancer agents that work by stabilizing microtubules, structures essential for cell division. This action disrupts the division of cancer cells, leading to their death in a manner similar to the well-known chemotherapy drug Taxol (paclitaxel). Epothilones have shown effectiveness against cancer cells that have developed resistance to taxanes. The bacterium also produces other potent compounds, including the antifungal soraphen A, the antiviral sulfangolids, and chivosazol, which destroys the cellular skeleton of fungi and mammalian cells. The sheer genetic investment in these chemical pathways makes S. cellulosum a subject of intense scientific interest for discovering new drugs.