A Microbial Biorealm page on the genus Dehalobacter restrictus
Bacteria; Firmicutes; Clostridia; Clostridiales; Peptococcaceae; Dehalobacter; Dehalobacter restrictus (2)
Description and significance
Dehalobacter restrictus is an anaerobic microbe that is found in soils. It has a flagellated rod shape that stains Gram Negative(5). It uses chlorinated molecules as terminal electron acceptors in metabolism so it has a lot of potential to be used for bioremediation of halogen pollution (5).
D. restrictus strain DSM 9455 has a completely sequenced genome that is 2900kB in length(1). This species has a circular chromosome and does not contain any plasmids. 42% of the genome was found to contain C and G which is consistent with its phylum Firmicutes because it is known for having a low G and C content (1,5).
Cell structure, metabolism & life cycle
D.restrictus is a rod shaped, non-endospore forming, bacteria that stains gram negative, however it is in the family Clostridia which is made up of gram positive bacteria because of genomic analysis (5). Unlike other microbes that stain gram negative, D. restrictus does not have an outer membrane. Instead it has a thin layer of peptidoglycan on its cell wall similar to a gram negative bacteria and a large S-layer made of proteins which is characteristic of a gram positive bacteria. These characteristics result in the gram stain not being retained and giving the negative result (5).
This bacterium has a very unique metabolic system. It undergoes anaerobic respiration using hydrogen gas as the electron donor and chlorinated molecules, especially chloroethenes, as electron acceptors. The only byproduct of this respiration is organic hydrocarbons such as ethene that are much more environmentally friendly than the original receptor (5).
Ecology (including pathogenesis)
D. restrictus is found in anerobic environments in soils. It does not partake in symbiosis with other organisms, however it does compete with other lithotrophs for hydrogen gas and other essential nutrients in the soil (4). It has not been classified as a pathogen because it does not live on or affect on host organisms.
The most interesting feature of D. restrictus is its potential for bioremediation of halogenated pollutants. It has already been shown that it successfully reduces tetrachloroethene (PCE), one of the most abundant halogenated soil pollutants (4). It does this through metabolism in which it utilizes the membrane enzymes hydrogenase and PCE reductase for the electron transport chain. Menaquinone is the biological electron transporter within the electron transport chain that allows for this mechanism to occur(4).
The only problem with using D. restrictus as a bioremediation tool is that it must compete with other lithotrophs in the soil for hydrogen gas. At low concentrations of hydrogen gas, D. restrictus is able to outcompete methanogens, sulfur reducing bacteria, and acetogenic bacteria, but still must compete with nitrate and iron reducing organisms(3). At higher concentrations it must compete with all of these so the threshold at which it is most successful is between hydrogen concentrations of 0.05 and 0.08nM (3). Hydrogen concentration is also thought to determine the extent to which molecules are dehalogenated because in places where there is high competition there is frequently accumulation of molecules that have been only partially dehalogenated (3).
1. "Dehalobacter Restrictus DSM 9455 GOLD CARD." Genomes Online Database. Web. 24 Oct. 2011. <http://genomesonline.org/cgibin/GOLD/bin/GOLDCards.cgi?goldstamp=Gi05571>.
2. "Dehalobacter Restrictus DSM9455." NCBI Taxonomy Browser. NCBI. Web. 24 Oct. 2011. <http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info>.
3. Luijten,M., Roelofsen, W., Langenhoff, A., Schraa, G. and Stams, A. “Hydrogen threshold concentrations in pure cultures of halorespiring bacteria and at a site polluted with chlorinated ethenes.” Environmental Microbiology. 2004. 6(6), p. 646–650
4. Schumacher, W., Vazquez, F. and Holliger, C. “A corrinoid protein involved in reductive dechlorination of tetrachloroethene by Dehalobacter restrictus.” Journal of Inorganic Biochemistry. 1995. 59(2-3), p. 278
5. Slonczewski, J. and Foster, J. Microbiology An Evolving Science. 2nd edition. 2011. W.W. Norton and Company Inc. New York,NY. p. 689-690.