Dehalococcoides mccartyi strain GEO12 has a natural tolerance to chloroform inhibition.

Co-contamination 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 transcription of tceA to a statistically significant degree (median increase 55.4 transcripts per 104 16S rRNA, CI95% =[12.9, 125]). Increase of vcrA transcripts in the presence of chloroform (45 µM; 5.3 mg∙L-1) in culture GEO12CF was not statistically significant since the CI95% range spanned 0 (median increase 109 transcripts per 104 16S rRNA, CI95% =[-13.6, 246]). Inhibition of dehalogenation by chloroform was 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 with chloroform as a co-contaminant.