Research Goals: The main objective of my research is to examine the ecology of metal resistance genes within microbial communities existing in long-term, metal-contaminated soils.

Overview: Contaminated soils through out the United States may contain various trace metals (e.g. arsenic, cadmium, chromium, cobalt, copper, lead, mercury, nickel, and zinc) in addition to organic pollutants (e.g. polycyclic aromatic hydrocarbons). Both types of contamination often result from industrial activities such as coal gasification, tanneries, wood treatment, metal plating, and petroleum processing. The pollutants can negatively impact human health because of their mutagenic and carcinogenic effects. Many of the organic contaminants are known to undergo microbial degradation, resulting in a reduction of the contaminant concentrations over time in polluted soils. Metals, however, cannot be degraded, resulting in their persistence in the soil environment. At the high concentrations found in contaminated soils, metals act as toxins, altering the indigenous microbial community. Previous research has shown that microbial community composition, activity, and function are altered in metal contaminated soil environments. Trace metals pose a threat to microbial life because the cells can mistakenly incorporate the metal as an analog for an essential element or by up-take of a toxic dose of an essential metal. Once inside the cell, the metals can interfere with cellular processes, alter proteins, inactivate enzymes, and damage DNA.

Metal tolerant microorganisms do exist. They are classified by their survival on media containing metals. Among these bacterial isolates, many of the genes responsible for metal resistance have been identified and characterized. Currently, metal resistance genes are known for both essential and nonessential trace metals: lead, cadmium, zinc, copper, arsenic, mercury, nickel, tellurium, selenium, and silver. The basic function of all known metal resistance is to prevent cellular damage. Our current knowledge about metal resistance microorganisms and the genes conferring resistance is based on the 0.1-1% of culturable bacteria isolated from environmental and clinical settings. The remaining 99% of microorganisms is an untapped resource that can be used to determine the distribution of metal resistance genes within soils environments and how microbial communities adapt to the stress of metal contamination.

This research has two objectives: 1) evaluating the role of plasmids in the in situ transfer of metal resistance genes, and 2) determining how metabolically active bacteria respond to metal contamination and if these bacteria posses metal resistance genes. These two projects will be investigated using soil microcosms as the experimental setting.

Progress to Date: Small pilot studies have been set up to optimize the procedures used to examine plasmid transfer and to assess changes in bacterial community composition.

Application of Knowledge: The main importance of this work is to determine how bacteria communities adapt to the stresses imposed by metal contamination and if the bacterial communities are still able to degrade organic contaminants in the soil environment.

Future Directions:  Identify members of the bacterial community able to tolerate metal-contamination using 16s rDNA analysis.

Techniques Incorporated: Exogenous Plasmid Isolation, Denaturing Gradient Gel Electrophoresis (DGGE), PCR, Minimum Inhibitor Concentration (MIC) testing.

Keywords:
DGGE
Community composition
Metal-resistance
Metal-contamination