The use of root-colonizing recombinant bacteria to enhance rhizosphere degradation of polychlorinated biphenyls

Rhizoremediation is an attractive option for the remediation of polychlorinated biphenyls (PCBs) in the shallow surface profile because it can potentially lead to complete in-situ destruction of the PCBs. By utilizing plant/microorganism systems, enhanced PCB rhizoremediation may be possible by constructing recombinant rhizosphere bacteria. The use of root-colonizing bacteria as the recipient for the degradation genes heightens the potential survival, transport and impact of the microorganisms because the plants provide a consistent environment, which is selective for the recipient bacteria. In earlier work, we showed that genetically engineered root-colonizing bacteria thrive in real soils when returned to the host root organism.; In this work, Sinorhizobium meliloti which has a well characterized nodulating relationship with Medicago sativa (alfalfa). S. meliloti 1021 was engineered with the bph genes. The recombinant S. meliloti retained similar growth rates as the wild-type strain. The engineered S. meliloti strains grew on biphenyl and monochlorobiphenyl.; Viable microorganisms were isolated from fescue (Festuca spp.), obtained from a historically PCB-contaminated site, using root exudates, root biomass, and biphenyl. The isolated microorganisms were classified phlyogenetically using methods from Bergey's Manual. Many microorganisms had the natural capability to degrade lower chlorinated PCBs. The bph genes were inserted into selected isolates and presence of the genes was confirmed with PCR. These engineered isolates maintained similar growth rates as the native isolates, however resting cells assays showed no degradation and they had minimal survival in situ. This research showed using engineered rhizosphere microorganisms for PCB remediation is currently not an optimum approach.