Microbes Retain Toxicity Tolerance After They Escape Toxic Elements

Ground water microbes living outside a contaminated area contain mobile genetic elements that provide them resistance to heavy metals.

Visualization shows circular plasmids (mobile genetic elements) from two ground water samples taken in Oak Ridge, Tennessee (sample F, blue; sample G; green). Using a newly devised method, researchers discovered more than 600 plasmids. Many of these genetic elements confer resistance to toxic metals.

The Science

Bacteria and other microbes contain mobile genetic elements called plasmids that are circular DNA molecules. Plasmids often encode traits that confer some advantage to the harboring microbe such as antibiotic resistance. A team of researchers has developed a method to purify and study plasmids from ground water microbial communities. Using this method, they discovered several hundred different plasmids in samples from a Department of Energy site in Tennessee. The most common trait encoded in those plasmids was resistance to toxic metals such as mercury. The team also found that the plasmids were diverse but not as much as the microbial community they came from.

The Impact

Studying plasmids from microbial communities that live in ground water is difficult because of the low concentration of cells in those habitats. Researchers knew that heavy metal resistance genes were present in ground water microbes but their new method allowed them to prove that these mobile elements harbor metal resistance genes. The method will also facilitate the discovery of new plasmids in other water environments.


Plasmids are mobile genetic elements that often contain genes that confer important functions to the microbial host. They are composed of circular DNA molecules and are easy to purify. However, plasmid purification methods do not work well if the concentration of cells is low, as is the case in samples of microbial communities from ground water environments. To solve this problem, a team of researchers developed a method to purify and analyze plasmids from such environments. With this method, the team uncovered over 600 different plasmids that showed an enrichment in metal tolerance genes in addition to antibiotic- and virus-resistance genes. Given that the team did not detect heavy metals in the microbial habitat, they hypothesize that plasmids may represent a mechanism to maintain latent resistance within a microbial community. The discovery of new plasmid genes allowed by this research will provide new possibilities for engineering novel and useful microbial strains.


Aindrila Mukhopadhyay
Lawrence Berkeley National Laboratory

Program Managers
Pablo Rabinowicz
Department of Energy, Office of Science

Todd Anderson
Department of Energy, Office of Science


This work was supported by the Department of Energy, Office of Science, Office of Biological and Environmental Research.


A. Kothari, Y.W. Wu, J.M. Chandonia, M. Charrier, L. Rajeev, A.M. Rocha, D.C. Joyner, T.C. Hazen, S.W. Singer, and A. Mukhopadhyay, “Large circular plasmids from groundwater plasmidomes span multiple incompatibility groups and are enriched in multimetal resistance genes.” mBio 10, e02899-18 (2019). [DOI: 10.1128/mBio.02899-18]

Highlight Categories

Program: BER , BSSD

Performer: University , DOE Laboratory