Superfund Research Program
Molecular Insight into Polyaromatic Toxicant Degradation by Microbial Communities
Project Leader: James M. Tiedje
Grant Number: P42ES004911
Funding Period: 2000 - 2013
Final Progress Reports
Year: 2012 2004
The genome sequence of the outstanding PCB degrader, Burkholderia xenovorans strain LB400, has been completed. Dr. Tiedje’s team has performed genome-wide analysis of biphenyl/benzoate/PCB metabolism by a combined transcriptomics, proteomics, physiology and genetic analysis. Both biphenyl and ortho+para-chlorinated PCBs are inducers of the O2-dependent bph-pathway, and several chaperone proteins and an alkyl hydroperoxide reductase may contribute to detoxification of oxygenated byproducts of this process. The latter is consistent with finding the CoA ligation benzoate pathway to be a major route in biphenyl metabolism along with classic O2-dependent ortho-(β-ketoadipate) pathway. Complex co-regulation of all 3 “benzoate” pathways including ortho-(β-ketoadipate) and two CoA ligation routes has also been implicated by condition-induced shifts among those both in wild-type and single- and double knockout mutants. The transcriptome experiments have outlined the global response of LB400 to PCBs. The researchers have also noted a remarkable tolerance to high concentrations of the Aroclor 1242. The have found no or insignificant toxicity of PCB molecules (500 ppm, in the absence of PCB degradation), based on fatty acid, viability and genome-wide differential expression analyses. The researchers also found LB400 successfully overcome “metabolic” toxicity of PCBs (500 ppm, induced PB degradation), via employing multiple mechanisms as outlined in differential gene expression studies, among those are (exo)polysaccharide biosynthesis and glycogen synthase, C-1 pathway for detoxification of formaldehyde, and alternative shift to the benzoate β-ketoadipate pathway to minimize toxicity of PCB intermediates. The team has also outlined putative transport genes potentially involved in PCB uptake and initiated genetic analysis of their role.
Based on implications from the genome sequence and differential gene expression studies, the team addressed LB400 behavior in regard to environmental conditions important to the microbe’s success, such as N2 fixation and temperature effects on growth and PCB degradation, biofilm formation, and carbon starvation. Formation of biofilm and consumption of sorbed biphenyl (from organically coated sand) correlated with a higher transcription of chemotaxis and bph genes as compared to Bph-batch grown cells. The researchers have shown that LB400 can grow by N2-fixation, the trait presumed beneficial in bioremediation in situ. While the global physiological shift during transition to N2 fixation appeared to some degree to affect tolerance to high concentrations of Aroclor 1242, LB400 still grew in the presence (500 ppm) and degraded PCBs. Contrary to the past laboratory practices, the team found LB400 to also be psychrophilic capable of growth within a range of at least 4°C to 35°C. Importantly for in situ bioremediation goals, LB400 grew on and mineralized PCBs (defined ortho+para-PCB MixM) at low temperatures. They also found the affinity to cold and temperate growth to be unique to Burkholderia LB400 as it was not present in other Burkholderia relatives, underlining the suitability of these organisms for safe environmental biotechnology.
The researchers continued attempts to obtain highly enriched populations of anaerobic PCB dechlorinating organisms. Using group-specific nested PCR based TRFLP, they found presence or abundance of known dehalogenating geners such as Desulfuromonas, Dehalobacter, Dehalococcoides, and Desulfitobacterium in serial enrichments.