The Genus Dehalococcoides

Abstract

Dehalococcoides, now called Dehalococcoides mccartyi, was first discovered in an enrichment culture from sewage sludge that reductively dechlorinated the groundwater pollutants tetrachloroethene (PCE) and trichloroethene (TCE) to vinyl chloride (VC) and ethene, in contrast to other organohalide-respiring bacteria that dechlorinated PCE and TCE only as far as dichloroethenes (DCEs). The first isolate, strain 195, was a tiny disk-shaped bacterium in the phylum Chloroflexi that had an S-layer protein subunit cell wall lacking peptidoglycan. It was a strict anaerobe using only H2 as the electron donor and organohalides as respiratory electron acceptors. Other D. mccartyi strains are similar and use a variety of halogenated aliphatic and aromatic compounds as electron acceptors. The genomes of D. mccartyi are highly streamlined, varying from 1.34 to 1.5 MB, yet contain 10–36 different copies of rdhAB operons predicted to encode reductive dehalogenases (RDases), most with adjacent genes predicted to encode transcriptional regulators, indicating that organochloride respiration is a highly evolved and regulated process in D. mccartyi. The presence of D. mccartyi at chloroethene-contaminated groundwater sites appears necessary for dechlorination of PCE and TCE past DCEs, and molecular tests for D. mccartyi and its associated rdhAB genes have become part of contaminated site characterization. Moreover, D. mccartyi-containing cultures have been commercially developed for bioaugmentation of those sites to abet dechlorination to ethene, especially cultures that contain D. mccartyi strains that can efficiently convert VC to nontoxic ethene in a respiratory process, like strains BAV1 and VS. This tiny unusual bacterium is now considered to be an important player in the restoration of chloroethene-contaminated sites.