Superfund Research Program
The Role of Bacterial Gene Transfer In Cleaning-Up Metal and Organic Contaminated Soils
Some bacteria possess an exceptional ability to break down synthetic chemical contaminants into simpler non-toxic compounds. The microbes that are able to decompose synthetic chemicals have found a way to use the chemicals as a food source. This natural phenomenon is increasingly being used to treat contaminated soil on hazardous waste sites in a technology known as "bioaugmentation," which involves the introduction of non-indigenous microbes into soil to enhance the biodegradation of contaminants.
Although some introduced microbes actually break down contaminants in the soil, others that cannot survive the stresses of the environment have been found to pass on their specialized biodegradative capabilities to the indigenous soil microbes. Bacteria accomplish this feat through transfer of information rich "plasmids." Plasmids are small, mobile pieces of DNA that are independent of the bacterial chromosome and carry spare genetic information, such as metal resistance or the ability to degrade a specific contaminant. Because plasmids can be transferred from one bacterium to another, and even to bacteria of a different species, the potential exists to create efficient biodegrading microbial communities suited to the unique environment and contamination of a particular hazardous waste site.
Scientists at the University of Arizona are investigating the potential of bacterial gene transfer to enhance the remediation of organic contaminants. They are particularly interested in creating microbial communities that can degrade organic contaminants in the presence of metals, which are generally toxic to most microbes and tend to inhibit biodegradative activity. The scientists are carrying out several interrelated investigations. First, they are developing methods for measuring plasmid transfer events. In addition, they are determining how readily plasmid transfer occurs from bacteria with biodegrading abilities to those without. Finally, they are investigating whether bacteria with metal resistance can enhance the remediation of soils co-contaminated with metals and organics.
Initially, the researchers developed a model gene transfer system so that plasmid transfer events could be easily measured. In this model system, the scientists determined that the degradative genes of Alcaligenes eutrophus JMP134, which are contained in a plasmid called pJP4, can be transferred to a recipient microbe, Variovorax paradoxus. Plasmid pJP4 allows the microbes to degrade the organic compound, 2, 4-dichlorophenoxyacetic acid (2,4-D). Transfer occurred readily in culture. But in these first experiments, transfer decreased in sterile soil and only occurred at very low levels in nonsterile soil.
In a follow-up study, the scientists introduced A. eutrophus JMP134 into nonsterile soil microcosms from Madera Canyon, Arizona to which high levels of 2,4-D were added. In this amended soil, A. eutrophus became non-detectable after one week of incubation. But many transconjugants (indigenous microbes that accept the plasmid) containing the pJP4 plasmid were found. Over a 6 week period, transconjugants survived and increased greatly in numbers. Thus, the biodegradation rates of 2,4-D were enhanced in the soil through gene transfer and subsequent growth of large populations of transconjugants.
The next study investigated the potential of metal-resistant microbes to enhance the biodegradation of soil co-contaminated with cadmium and 2,4-D. The researchers studied six cadmium-resistant microbes that had been isolated from both metal-contaminated and uncontaminated soils. These cadmium-resistant isolates were studied in both pure culture and in soil microcosms containing cadmium and 2,4-D. The isolates were able to detoxify varying levels of cadmium, which allowed the cadmium sensitive A. eutrophus JMP134 to degrade 2,4-D. Organic degradation was substantially enhanced with the presence of the metal resistant bacteria.
Gene transfer is likely involved in enhancing 2,4-D degradation in the presence of cadmium; however, the underlying mechanisms have not yet been elucidated. It is possible that degradative genes were transferred from the degrading organism to the metal-resistant isolate, or that the metal resistance genes were transferred to A. eutrophus JMP134. Investigations are underway to determine if transfer of either cadmium-resistance or 2,4-D degradation genes are occurring to facilitate the biodegradation of organic contamination.
The contamination of soils with both metal and organic wastes is a major concern in the United States because this type of co-contamination is found on over 30% of the country's hazardous waste sites and has generally proven difficult to remediate in an inexpensive and effective manner. This research is a significant step towards developing microbial-based bioremediation technologies for co-contamination. Thus far, the findings are very promising in that they suggest microbial communities can be created to treat co-contaminated soils. In addition, these results add important information to the knowledge base of biodegradation strategies and contribute to the basic understanding of bacterial adaptations to co-contaminated environments.
For More Information Contact:
University of Arizona
Department of Chemistry
Tucson, Arizona 85721
To learn more about this research, please refer to the following sources:
- DiGiovanni GD, Neilson JW, Pepper IL, Sinclair NA. 1996. Gene transfer of Alcaligenes eutrophus JMP134 plasmid pJP4 to indigenous soil recipients. Appl Environ Microbiol 62(7):2521-2526. PMID:8779592
- Neilson JW, Josephson KL, Pepper IL, Arnold RG, DiGiovanni GD, Sinclair NA. 1994. Frequency of horizontal gene transfer of a large catabolic plasmid (pJP4) in soil. Appl Environ Microbiol 60:4053-4058. PMID:7993092
- Neilson JW, Josephson KL, Pillai SD, Pepper IL. 1992. Polymerase chain reaction and gene probe detection of the 2,4-dichlorophenoxyacetic acid degradation plasmid pJP4. Appl Environ Microbiol 58:1271-1275. PMID:1599246
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