Dehalococcoides ethenogenes

A Microbial Biorealm page on the genus Dehalococcoides ethenogenes

Classification

Higher order taxa

Domain: Bacteria; Phylum: Chloroflexi; Class: Dehalococcoidetes; Order: Dehalococcoides

Species

Dehalococcoides ethenogenes (strain 195)

Description and significance

Dehalococcoides ethenogenes is Gram-positive, which generally means it has a very thick cell wall and a single membrane layer. From a three-dimensional prospective, it appears to have an irregular, spherical shape. The species has a circular shape, although not perfectly circular (irregular coccus). The species moves spontaneously and independently. It is mesoplilic, likeing moderate environments around 25 and 40°C, with an optimal temperature of 35°C. D. ethenogenes is anaerobic and cannot use oxygen, nitrate, or sulfate as electron acceptors.

This specific strand of genome was sequenced, and it was discovered to help decontaminate toxic chemicals from many industries. Specifically, this species reduces chlorinated hydrocarbons in contaminated environments to harmless daughter compounds (ethene) . Chlorinated hydrocarbons are significantly toxic to humans and contaminate groundwater where the chemical is not handled properly.

Genome structure

Dehalococcoides ethenogenes has 1,469,720 or 1.5 Mb nucleotide base pairs in its genome. Only one gene encoding reductive dehalogenase has been isolated and characterized. Strain 195 is the only known bacterium, to date, which completely dechlorinates tetrachloroethene (PCE) and trichloroethene, to ethene. Strains show >98% nucleotide and >85% amino acid similarity; however, different strains utilize different ranges of haloorganic compounds as electron acceptors.

Dehalococcoides ethenogenes has plasmids called pUA969, pUA970, pUA971. These plasmids work to bind D. ethenogenes LexA gene to Bacillus subtilis. This is significant because it makes the first Gram-negative bacterium sharing LexA binding site like that of B. subtilis.

Cell structure and metabolism

D. ethenogenes mediates reductive dechlorination reaction via hydrogenolysis or dichloroelimination. In hydrogenolysis, chlorine is replaced by hydrogen, with a net input of one proton and two electrons. In dechloroelimination, chlorine substituents are replaced via the formation of a double bond between the two associated carbon atoms. The dechloroelimination reaction has a net input of two electrons. The biological process mostly undergoes hydrogenolysis.

Importantly, D. ethenogenes conserves energy when hydrogen serves as electron donor, halogenated compounds are electron acceptors, and the enzyme reductive dehalogenase catalyzes the reaction. Each intermediate reaction, from PCE to TCE to cis-DCE to VC is energy-yielding for D. ethenogenes; however, the VC to ethene intermediate is cometabolic and does not provide the beneift of energy to the microbe. While the process from VC to ethene is not beneficial to the microbe, it is critical to remediating the contaminated environment.

The redox potentials for each intermediate reaction range from 260 to 570 mV. D. ethenogenes can dehalorespirate using chloroethenes, chlorophenols, and polychlorinated biphenyls/dioxins as terminal electron acceptors.

Ecology

Bioremediation strategies may be critically enhanced if the dehalorespiration process and mechanisms can be thoroughly understood and induced in contaminated groundwater environments. The anaerobic process may prove more effective in removing halogens atoms than aerobic reductive dehalogenation.

Growing pure cultures of strain 195 is difficult, as D. ethenogenes prefer life in consortia with other microbes. It is challenging to maintain the microbe as an axenic culture. Growth is slow and consequently, biomass yields are small. Amplifying samples using real-time PCR methods has been essential to research efforts.

When using bioaugmentation to dose contaminated groundwater with D. ethenogenes, biostimulation with carbon sources should be applied carefully to ensure that carbon concentrations favor growth of strain 195. High carbon concentrations may favor growth of competing strains that cannot reduce PCE completely to ethene.

Pathology

This organism can be known as pathogen-free. It does not produce disease or illness to its host.

Application to Biotechnology

Dehalococcoides ethenogenes is only known bacteria that can breakdown PCE, a dangerous and toxic carcinogen in chemical working environments. This organism converts PCE and hydrogen to chlorine and ethene, which render the toxin completely harmless. The bacteria "come in stainless steel vessels that contain roughly 2000 billion Dehalococcoides bacteria ready for injection into groundwater" (495). This system of removing contamination from groundwater was named "pump-and-treat". Basically, they would inject the bacteria into groundwater to convert PCE into ethylene. Research indicates that the level of ethylene rises; therefore, the injection was an success.

Current Research

1) The solvents tetrachloroethene (PCE) and trichloroethene (TCE) are among the most pollutants at contaminated groundwater sites. Under aerobic conditions, PCE is considered difficult to reduce, while TCE can be broken down to non-toxic products. This poses a problem because PCE is a harmful chemical to the environment, but cannot be controlled when oxygen is present. Therefore, there are flaws that D. ethenogenes cannot fix completely. However under anaerobic conditions, PCE and TCE have been able to reduce to ethylene, which deems them less harmless.

2) Bioremediation Consulting Inc. (BCI), of Watertown, Massachusetts identified at least 4 natural microbes containing Dehalococcoides ethenogenes that are known as TCA-tolerant D. ethenogenes. Also, found growing there was Dehalobacter. TCA is converted to chloroethane by Dehalobacter. There is also dechlorination TCE and DCE to ethene and TCA dechlorinated to chloroethane. This research indicated the difference between using natural microbes or bacteria like Dehalobacter to treat the contaminated areas.

3) Today, in the United States, the bacteria have been used to clean up chlorinated solvents in ten states at 17 sites including Kelly Air Force Base in Texas and Caldwell Trucking Superfund Site in New Jersey. For SiREM, business has been good; 13 of the sites were done in the past 18 months.

References

Seshadri R et al., "Genome sequence of the PCE-dechlorinating bacterium Dehalococcoides ethenogenes.", Science, 2005 Jan 7;307(5706):105-8

Hendrickson, E.R. et al. Molecular analysis of Dehalococcoides 16S Ribosomal DNA from chloroethene-contaminated sites throughout North America and Europe. Applied and Environmental Microbiology 68, 485-495 (February 2002).

Major, D. W. et al. Field demonstration of successful bioaugmentation to achieve dechlorination of tetrachloroethene to ethene. Environmental Science and Technology 36, 5106-5116 (November 2002).

Maymo-Gatell, X. et al. Isolation of a bacterium that reductively dechlorinates tetrachloroethene to ethene. Science 276, 1568-1571 (June 6, 1997).

Duhamel et al. 2002, Water Research, Vol 36, p 4193

Maymo-Gatell, X., 1997, Science, Vol 276

B. Sun et al., 2002, Science, Vol 298 p. 1023

Edited by Tim Hou of Rachel Larsen and Kit Pogliano