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Micb 301: Wikipedia Page Byron Brook, 42133099

Dehalococcoides mccartyi (D. mccartyi): Higher order taxa: Bacteria (Domain); Chloroflexi (Phylum); Dehalococcoidetes (Class); Dehalococcoidales (Order); Dehalococcoidacea (Family); Dehalococcoides (Genus); Dehalococcoides mccartyi (Species) (10).

Description: D. mccartyi are specialist “strict organohalide respiring bacteria,” and are restricted to its reductive dehalogenation activity with its use of chlorinated and brominated compounds as their terminal electron acceptor and are dependent on hydrogen as an electron donor (10). This species of bacteria does not utilize fermentation or any other known energy metabolism mechanisms. D. mccartyi are anaerobes (2).

Cell Structure: This non-motile disc-shaped bacteria range in size from 0.3 to 1 µm in diameter with a thickness between 0.1 and 0.2 µm (10). The two flat sides of the disc are distinguishing indentations. The cells have roughly 30 times lower cell volume than E. coli (10). Research to further investigate cell wall structure and other structural features has been limited by the current restriction on obtaining large biomass yields of cells (10). Dehalococcoides species cellular image.

Figure 1: Dehalococcoides Image from: Bacmap Genome Atlas, (4). Of note in the above image is the characteristic indentation on one side of the disc, as well as the filamentous extension. Dehalococcoides can bind to solid features in a separatory column (3) and the filamentous extension may be responsible for the observed binding (10). The cell wall has also been observed to have an odd structure that resembles the S-layer of Archaea (11). There was no visible peptidoglycan layer (10).

Significance: Vinyl chloride, a product of D. mccartyi’s partial dehalogenation of tetrachloroethene (PCE) and of trichloroethene (TCE) has been listed as a carcinogen (6). The dehalogenation process is described in figure 2 below. Vinyl chloride has also been noted as a toxic compound (8). Image of dehalcoccoides pathway - make it from http://umbbd.ethz.ch/tce2/tce2_map.html  ??

Figure 2: Dehalogenation of tetrachloroethene to ethene. PCE (tetrachloroethene), TCE (trichloroethene), DCE (dichloroethene), vinyl chloride. Reductive enzymes were represented by the numbers: 1 (tetrachloroethene reductive dehalogenase), 2 (trichloroethene reductive dehalogenase), 3 (dichloroethene reductive dehalogenase), and 4 (vinyl chloride reductive dehalogenase). Image adapted from a flow chart described by L. Ellis, and S. Anderson (5) with the corresponding sources for images: PCE chemical image from 12, TCE chemical image from 13, Cis-dichloroethene chemical image from 14, Trans-dichloroethene chemical image from 15, Vinyl chloride chemical image from 16, Ethene chemical image from 17. Oxygen levels over 4 mg/mL can permanently inhibit the vinyl chloride degradation step catalyzed by enzyme 4 in the above figure (2). In some strains of Dehalococcoides the oxygen inhibition was reversible, though this was a subset (2) The trichloroethene degradation to vinyl chloride was not inhibited by oxygen (2) which could lead to vinyl chloride accumulation and may cause concerning health and environmental side effects, as described earlier. Various Dehalococcoides strains are described in Table 1 with further information for reference.

Table 1: Details different isolated strains with various abilities to degrade organohalides (10). VC is vinyl chloride, and DCE is di-chloroethene.

An interesting biotechnology application could arise if D. maccartyi growth was optimized and if it could be cultured with oxygen inhibition to produce vinyl chloride. The vinyl chloride generated, with further development, could possibly be used to produce the third highest produced plastic, polyvinyl chloride (19). Some believe that Dehalococcoides are the most important microbial processors for generating ethene (9). Harnessing ethene production from a dry-cleaning waste product such as PCE may be beneficial in industrial settings to reduce costs paying for PCE disposal and also be useful in generating an ethene product in the process. Ethene is commonly known for its use in producing plastics through reacting it to produce polyethylene.

Growth Conditions: Dehalococcoides mccartyi growth can occur in a controlled medium supplemented with haloorganic compounds including chlorine and bromine to act as the terminal electron acceptor (10). Acetate was provided as a carbon source, hydrogen as an electron source, and vitamin B12 was necessary for growth (10). It was possible to increase the doubling time from 3 days to 0.8 days through the addition of enrichment cultures previously designed for other Dehalococcoides, though the specific components involved have not yet been defined (10). Optimum growth conditions occurred from pH 6.5-8.0 and between 25-35oC, with permitted growth as low as 15oC (10).

Genome Content: Through gene sequencing it has been found that D. mccartyi lack the traditional genes for sulfate, nitrate, and fumarate reduction (10). This lends evidence to this bacteria being a strict organohalide respiring organism. Other genes identified to be missing were genes responsible for motility, peptidoglycan synthesis, and sporulation (10). These features are part of the identifying mechanisms used for the bacteria. It has been proposed that D. mccartyi evolved its dechlorinating ability through a horizontal gene transfer event (18).

Ecology: D. mccartyi species have been isolated from contaminated environments of “digester sludge, sediments, and aquifers” (10). It was noted that one strain was isolated from a river with no reported halogen contamination (10). This shows that the organism may not require a man-made ecological niche. It is more likely that the bacteria was previously evolved to exploit the occurrence of halogenated compounds in the environment and this population increased with the occurrence of increased human use of PCE (3). A possible organism able to produce various organochlorinated compounds for use as a terminal electron acceptor are a variety of common fungi, though the direct interaction with Dehalococcoides has yet to be determined (7). Fungi, lichen, and bacteria are able to produce chlorinated compounds for use as an antibacterial substance (1) and any of these may be a source of PCE that permitted Dehalococcoides growth before humans changed the environment. This supports the idea that a naturally formed environment may have already been present for D. maccartyi to exploit, and only recently, due to human activity, had widespread tetrachlorine contaminated habitats increased. 

References: 1. Abrahamsson K., Ekdahl, A. Collen, J. and Pedersen, M. “Marine Algae-A Source of Trichloroethylene and Perchloroethylene.” American Society of Limnology and Oceanography, 1995, DOI: not provided

2. Amos B., Ritalahti, K., Cruz-Garcia, C., Padilla-Crespo, E., and Löffler, F. “Oxygen Effect on Dehalococcoides Viability and Biomarker Quantification.” Environ. Sci. Technol., 2008, DOI: 10.1021/es703227g.

3. Amos B., Suchomel, E., Pennell, K., and Löffler, F. “Spatial and Temporal Distributions of Geobacter lovleyi andDehalococcoides spp. during Bioenhanced PCE-NAPL Dissolution.” Environ. Sci. Technol., 2009, DOI: 10.1021/es8027692.

4. “Dehalococcoides sp. GT.” Bacmap Genome Atlas, N.d., DOI: not provided.

5. Ellis, L., and S. Anderson. “Tetrachloroethene Pathway Map (Anaerobic).” UMBBD, Nov 2011, DOI: not provided

6. Jones E., Voytek, M., and Lorah, M. “Effect of Fe(III) on 1,1,2,2-Tetrachloroethane Degradation and Vinyl Chloride Accumulation in Wetland Sediments of the Aberdeen Proving Ground.” Bioremediation Journal, May 2010, DOI:10.1080/10889860490453159

7. Jong E., Field, J., Spinnler, H., Wijnberg, J., and De Bont, J. “Significant Biogenesis of Chlorinated Aromatics by Fungi in Natural Environments.” Applied and Environmental Microbiology, Jan. 1994, DOI: not provided.

8. Kassomenos P., Karayannis, A., Panagopoulos, I., Karakitsios, S., Petrakis, M. “Modelling the dispersion of a toxic substance at a workplace.” Environmental Modelling & Software, Jan 2008, DOI: 10.1016/j.envsoft.2007.05.003

9. Lendvay J., Löffler, F., Dollhopf, M., Aiello, R., Daniels, G., Fathepure, B., Gebhard, M., Heine, R., Helton, R., Shi, J., Krajmalnik-Brown, R., Major, C., Barcelona, M., Petrovskis, E., Hickey, R., Tiedje, J., and Adriaens, P. “Bioreactive Barriers:  A Comparison of Bioaugmentation and Biostimulation for Chlorinated Solvent Remediation.” Environ. Sci. Technol., 2003, DOI: 10.1021/es025985u.

10. Löffler F., Yan, J., Ritalahti, K., Adrian, L., Edwards, E., Konstantinidis, K., Müller, J., Fullerton, H., Zinder, S., and Spormann, A. “Dehalococcoides mccartyi gen. nov., sp. nov., obligate organohalide-respiring anaerobic bacteria, relevant to halogen cycling and bioremediation, belong to a novel bacterial class, Dehalococcoidetes classis nov., within the phylum Chloroflexi.” IJSEM, 2012, DOI:10.1099/ijs.0.034926-0.

11. Maymo-Gatell X., Chien, Y., Gossett, J., and Zinder, S. “Isolation of a Bacterium That Reductively Dechlorinates Tetrachloroethene to Ethene.” SCIENCE, 1977, DOI: not provided. ISSN: 0036-8075

12. Pubchem. “Tetrachloroethylene - Compound Summary (CID 31373).” Pubchem compound. N.d. DOI: not provided.

13. Pubchem. “Trichloroethene - Substance Summary (CID 6575).” Pubchem compound. N.d. DOI: not provided.

14. Pubchem. “cis-Dichloroethylene - Substance Summary (CID 643833).” Pubchem compound. N.d. DOI: not provided.

15. Pubchem. “trans-1,2-Dichloroethene - Substance Summary (CID 638186).” Pubchem compound. N.d. DOI: not provided.

16. Pubchem. “VINYL CHLORIDE - Substance Summary (CID 6338).” Pubchem compound. N.d. DOI: not provided.

17. Pubchem. “Ethene - Substance Summary (CID 6325).” Pubchem compound. N.d. DOI: not provided.

18. Regeard C., Maillard, J., Dufraigne, C., Deschavanne, P., Holliger, C. “Indications for Acquisition of Reductive Dehalogenase Genes through Horizontal Gene Transfer by Dehalococcoides ethenogenes Strain 195.” Applied & Environmental Microbiology, 2005, DOI: 10.1128/AEM.71.6.2955-2961.2005.

19. Veris Consulting Inc. “U.S. PRODUCTION, SALES & CAPTIVE USE.” American Chemistry Council, Inc; April 2012, DOI: not provided.