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,
CI95% = [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 CI95% range spanned 0 (the median increase
was 109 transcripts per 104 16S rRNA, CI95% =
[−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.