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Applied and Environmental Microbiology, August 2000, p. 3297-3304, Vol. 66, No. 8
Laboratory of Microbial Ecology and
Technology,1 Laboratory of
Microbiology,2 and Laboratory of
Phytopathology,3 Ghent University, B-9000 Ghent,
Belgium
Received 30 March 2000/Accepted 2 June 2000
Transfer of the 2,4-dichlorophenoxyacetic acid (2,4-D) degradation
plasmids pEMT1 and pJP4 from an introduced donor strain, Pseudomonas putida UWC3, to the indigenous bacteria of two
different horizons (A horizon, depth of 0 to 30 cm; B horizon, depth of 30 to 60 cm) of a 2,4-D-contaminated soil was investigated as a means
of bioaugmentation. When the soil was amended with nutrients, plasmid
transfer and enhanced degradation of 2,4-D were observed. These
findings were most striking in the B horizon, where the indigenous
bacteria were unable to degrade any of the 2,4-D (100 mg/kg of soil)
during at least 22 days but where inoculation with either of the two
plasmid donors resulted in complete 2,4-D degradation within 14 days.
In contrast, in soils not amended with nutrients, inoculation of donors
in the A horizon and subsequent formation of transconjugants
(105 CFU/g of soil) could not increase the 2,4-D
degradation rate compared to that of the noninoculated soil. However,
donor inoculation in the nonamended B-horizon soil resulted in complete
degradation of 2,4-D within 19 days, while no degradation at all was
observed in noninoculated soil during 89 days. With plasmid pEMT1, this enhanced degradation seemed to be due only to transconjugants (105 CFU/g of soil), since the donor was already
undetectable when degradation started. Denaturing gradient gel
electrophoresis (DGGE) of 16S rRNA genes showed that inoculation of the
donors was followed by a shift in the microbial community structure of
the nonamended B-horizon soils. The new 16S rRNA gene fragments in the
DGGE profile corresponded with the 16S rRNA genes of 2,4-D-degrading
transconjugant colonies isolated on agar plates. This result indicates
that the observed change in the community was due to proliferation of
transconjugants formed in soil. Overall, this work clearly demonstrates
that bioaugmentation can constitute an effective strategy for cleanup
of soils which are poor in nutrients and microbial activity, such as
those of the B horizon.
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Effect of Dissemination of 2,4-Dichlorophenoxyacetic Acid (2,4-D)
Degradation Plasmids on 2,4-D Degradation and on Bacterial Community
Structure in Two Different Soil Horizons
*
Corresponding author. Mailing address: Laboratory of
Microbial Ecology and Technology, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium. Phone: 32 (0)9 264 59 12. Fax: 32 (0)9 264 62 48. E-mail: Eva.Top{at}rug.ac.be.
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