Biomedical Engineering Reference
In-Depth Information
Dhc
strain BAV1 was facilitated by the enrichment culture's ability to generate hydrogen, which
is the required electron donor for strain BAV1 to dechlorinate VC to ethene, from acetate.
Acetate fermentation is thermodynamically feasible at low hydrogen partial pressures, and
hydrogen consumption by strain BAV1 maintained conditions conducive for mesophilic acetate
oxidation (He et al.,
2002
).
Although dechlorination with acetate as the only source of reducing equivalents was slow
and required long incubation periods to demonstrate significant VC dechlorination with
Dhc
growth, repeated transfers under these conditions eliminated contaminating organisms and
ultimately enabled the isolation of
Dhc
strain BAV1 by ampicillin treatment and dilution-to-
extinction series in low melting agarose dilution tubes (He et al.,
2003a
;L¨ffler et al.,
2005
).
Unfortunately, the organisms responsible for syntrophic acetate oxidation in this enrichment
culture have not been identified. Strain BAV1 grows in a defined medium that, in addition to
acetate and vitamins, contains L-cysteine, as well as low amounts of sulfide (0.1-0.2 mM) as
reducing agents.
Strain BAV1 grows with all DCE isomers, VC and 1,2-dichloroethane as electron acceptors
and generates ethene and inorganic chloride as products. Strain BAV1 does not grow with PCE
or TCE but is able to dechlorinate these compounds when growth-supporting DCEs or VC are
available, in which case BAV1 converts all chlorinated ethenes to ethene. The cometabolic
dechlorination of PCE and TCE generates growth-supporting
cis
-DCE. This represents a unique
form of cometabolism because strain BAV1 ultimately benefits from the fortuitous dechlori-
nation of PCE and TCE by generating the growth-supporting electron acceptor
cis
-DCE.
Similar enrichment strategies yielded
Dhc
isolate GT and culture VS (M
¨
ller et al.,
2004
;
Sung et al.,
2006b
). The isolation efforts for strain GT yielded a culture that only contained
Dhc
cells and a single 16S ribosomal ribonucleic acid (rRNA) gene sequence was detected. Interest-
ingly, quantitative real-time polymerase chain reaction (qPCR) monitoring of
Dhc
16S rRNA
genes and the reductive dehalogenase genes
tceA
,
bvcA
, and
vcrA
(see below) revealed that this
culture consisted of multiple
Dhc
strains that shared an identical 16S rRNA gene. Consecutive
enrichments with different chlorinated ethenes yielded a pure culture that consisted of a single
Dhc
strain designated strain GT (Sung et al.,
2006b
).
Dhc
strains GT and VS generate ethene as dechlorination end product but, in contrast to
strain BAV1, use TCE as growth-supporting electron acceptor. Both strains GT and VS harbor
the VC RDase VcrA, which is different from the VC-dechlorinating enzyme system BvcA of
strain BAV1. Interestingly, both strain GT and strain VS fail to dechlorinate PCE even in the
presence of a growth-supporting electron acceptor (e.g., TCE) suggesting that both strains
possess a unique TCE RDase that differs from the TCE-dechlorinating enzyme system(s) of
strains FL2 and 195.
2.3.5 Isolation of
Dhc
Strain MB
Dhc
populations have been implicated in PCE and TCE dechlorination to
trans
-DCE rather
than
cis
-DCE (Griffin et al.,
2004
). The recently described isolate MB produced predominantly
trans
-DCE; however, this organism cannot dechlorinate DCEs and VC to ethene (Cheng and
He,
2009
). Isolate MB was obtained from a PCE-to-DCE-dechlorinating microcosm established
with San Francisco Bay sediment. The isolation process used the dilution-to-extinction principle
in a minimal medium amended with acetate, hydrogen and PCE. Sequential transfers in the
presence of ampicillin increased the
trans
-DCE to
cis
-DCE ratio, presumably by inhibiting PCE-
to-
cis
-DCE dechlorinators, which also were present in the enrichment cultures (Cheng and He,
2009
). While the original microcosms dechlorinated PCE to about equal amounts of
trans
-DCE
and
cis
-DCE, isolate MB produces about seven times more
trans
-DCE than
cis
-DCE.
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