Higher order taxa
Kingdom (Bacteria); Domain (Bacterial); Phylum (Firmicutes); Class (Bacilli); Order (Bacillales); Family (Bacillales Incertae Sedis); Genus (Exiguobacterium)
Description and significance
E. sibiricum can be found in geological layers frozen for up to 3 million years, at temperatures ranging from -5 to 40 degrees Celcius and is prevalent in the Siberian permafrost. It is significant as a microorganism that can both survive freezing temperatures and endure long periods of time in sub-freezing temperature conditions. Thus, E. sibiricum can be used as a model for observing molecular mechanisms used to enable an organism to adapt to below-freezing temperatures and thrive in low temperatures, the subject of which is still not well-understood. Understanding the mechanisms for enduring very cold temperatures is pertenant as 70% of the earth’s surface has an average temperature of 4 degrees Celcius, including ocean and land. E. sibiricum's ability to exist in such cold temperatures is attributed to physiological and metabolic adaptions, including preserving the flexibility, topology and interactions of macromolecules including DNA, RNA and proteins. Other mechanisms observed include the production of osmoprotectants and the ability to maintain intracellular diffusion rates and enzyme kinetics.
A better understanding of E. sibiricum could be applied to examining and understanding possible exobiological niches on other planets, as they have been seen to survive as low as -12 degrees Celcius.
The genome of E. sibiricum consists of 1 chromosome, 2 plasmids and 3,015 encoding genes. Different genes were seen to be expressed at different temperatures, displaying the genes needed to adapt to temperature extremes tolerable to exist in. Between 4 and 28 degrees Celcius there was little difference in gene expression, while different genes were seen to be expressed when observed at -2.5, 10, 28 and 39 degrees Celcius. 27.7% of genes do not have a known function, though it is speculated that they play a role in the organism’s adaptability to freezing temperatures. E. sibiricum has genomic similarities to Psychrobacter arcticus 273-4, which is a gram negative microorganism that is found in the Siberian permafrost as well.
When grown at different temperatures, it was observed that over 500 proteins were expressed. 39 proteins were uniquely expressed as cold acclimation proteins and 3 as cold shock proteins at 4 degrees Celcius.
Cell structure and Metabolism
E. sibiricum is a free-living, psychrotrophic, nonsporulating, gram positive, facultative aerobe. It is rod shaped, 1 um long and rounded at its edges. Size and shape have been seen to vary with temperature. It is yellow-orange in appearance and motile via peritrichous flagella, though at -2.5 degrees Celcius the flagella is absent. Regarding cellular arrangement, it can be single, linked with another or linked in a long chain. The peptidoglycan in the cell wall was identified as A3alpha L-Lys-Gly and the major fatty acids were identified as iso-C13:0, anteiso-C13:0, iso-C15:0, C16:0 and iso-C17:0.
Studies have shown that E. sibiricum are more partial to sugars and carbohydrate polymers as a source for carbon, evidenced by the presence of genes specified for glucose-involved energy pathways.
E. sibiricum is found typically in geological layers at temperatures ranging between -5 to 40 degrees Celcius, but has been found at as low as -12 degrees Celcius. When extracted for experimental use, E. sibiricum is taken from the Siberian permafrost, but has also been isolated from open oceans. It is a free-living organism, so does not require a host. It is capable of enduring long periods of time, seen up to 3 million years.
E. sibiricum has not been observed to spread disease.
 Rodrigues, Debora F, et al. "Architecture of thermal adaptation in an Exiguobacterium sibiricum strain isolated from 3 million year old permafrost: A genome and transcriptome approach." BMC Genomics. 2008. DOI:10.1186/1471-2164-9-547
 Qiu, Yinghua, Sophia Kathariou, and David M Lubman. "Proteomic analysis of cold adaptation in a Siberian permafrost bacterium – Exiguobacterium sibiricum 255–15 by two-dimensional liquid separation coupled with mass spectrometry." Proteomics 2006, 6, 5221–5233. 2006. DOI:10.1002/pmic.200600071
 "Exiguobacterium sibiricum 255-15, DSM 17290." DOE Joint Genome Institute. 2012. <http://genome.jgi-psf.org/exigu/exigu.home.html>.
 Rodrigues, Debora F, et al. "Characterization of Exiguobacterium isolates from the Siberian permafrost. Description of Exiguobacterium sibiricum sp. nov." NCBI. Aug. 2006. Extremophiles. 2006 Aug;10(4):285-94.
 "Exiguobacterium sibiricum 255-15, DSM 17290." Genomes Online Database. N.p., 23 Sept. 2011. http://genomesonline.org/cgi-bin/GOLD/bin/GOLDCards.cgi?goldstamp=Gc00762
Edited by Andrew Aikens of Dr. Lisa R. Moore, University of Southern Maine, Department of Biological Sciences, http://www.usm.maine.edu/bio