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 does not produce disease or illness to its host.

Application to Biotechnology

Dehalococcoides ethenogenes is only known bacteria that can fully degrade PCE to ethene. 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". Field studies at industrial sites have documented the full transformation of PCE to ethene in groundwater bioaugmented with strain 195. Data shows high initial concentrations of PCE followed by degradation and time offset spikes and declines of each intermediate compound, until the final data documents high ethene concentration and low presence of all other states.

Current Research

Presence of a single 16S rRNA cannot prove the purity of a culture, as prior believed. Microbes with similar genes may have different dehalogenation characteristics. Study of reductive dehalogenase genes will prove more useful in expanding knowledge of processes and mechanisms than research of less specific ribosomal DNA. Documented use of varied halogenated compounds as electron acceptors attests to the evolution of dehalorespiring microbes. The basic process is likely ancient. However, as anthropogenically introduced compounds pose increasing challenges, dehalorespirating microbes adapt to answer the call.

Balance between concentration and production of hydrogen allow for most efficient reductive dehalogenation to take place. The functions and relationships of reductive dehalogenase encoding genes need to be further defined. In order to gain understanding of the synergistic and competitive interactions of D. ethenogenes, more research is needed to investigate the bioremediation potential of dehalogenting consortia in situ.

References

Edited: Aulenta, F. et al. Enhanced anaerobic bioremediation of chlorinated solvents: environmental factors influencing microbial activity and their relevance under field conditions. 2006. J Chem Technol Biotechnol. 81: 1463-1474.

Cupples, A. Real-time PCR quantification of Dehalococcoides populations: Methods and applications. 2008. Journal of Microbiological Methods. 72: 1-11.

Hiraishi, A. Biodiversity of dehalorespiring bacteria with special emphasis on polychlorinated biphenyl/dioxin dechlorinators. 2008. Microbes Environ. 23: 1-12.

Original: 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 Angelique Tacia and Tracy Svanda of Jay Lennon April 2008 Original by Tim Hou of Rachel Larsen and Kit Pogliano