Dehalococcoides mccartyi Strain GEO12 Has a Natural Tolerance to Chloroform Inhibition

Cocontamination by chloroform and chloroethenes often confounds bioremediation efforts. Here, we describe Dehalococcoides mccartyi strain GEO12 that dechlorinates trichloroethene to ethene in 14 μM (1.6 mg·L^-1) chloroform. The same chloroform concentration effectively inhibited dechlorination in Dehalococcoides strains ANAS2, 11a, and BAV1. Successive transfers of strain GEO12 in increasing concentrations of chloroform led to culture GEO12CF that tolerated 83 μM (10 mg·L^-1) chloroform. The genome of strain GEO12 revealed seven reductive dehalogenase homologous (rdh) genes, including tceA and vcrA. Transcriptional analyses showed that chloroform (45 μM; 5.3 mg·L^-1) in culture GEO12CF enhanced the transcription of tceA to a statistically significant degree (the median increase was 55.4 transcripts per 10⁴ 16S rRNA, CI_95% = [12.9, 125]). The increase of vcrA transcripts in the presence of chloroform (45 μM; 5.3 mg·L^-1) in culture GEO12CF was not statistically significant because the CI_95% range spanned 0 (the median increase was 109 transcripts per 10⁴ 16S rRNA, CI_95% = [-13.6, 246]). Inhibition of dehalogenation by chloroform is often seen in Dehalococcoides, but the mechanism remains unknown. Our results suggest that culture GEO12CF may overcome chloroform inhibition by rdh upregulation. The chloroform-adapted culture GEO12CF provides insights into the metabolic flexibility of Dehalococcoides and could be used to fight chloroethene contamination where chloroform is a cocontaminant.