Plasmids are important factors to consider when studying the ecology of bacteria, because they are essential for transferring genetic information among different bacterial species. Before we get ahead of ourselves we need to go over two important concepts: the bacteria within which plasmids reside, and the molecule that bacterial chromosomes and plasmids are made of, DNA.

DNA, a brief historical outline

Plasmid History


Plasmids are extra-chromosomal DNA molecules that can replicate in a cell separately from the chromosome. Frequently, plasmids have accessory genes that encode functions beneficial to the host cell. They are important for understanding how genes can be moved about between different bacteria and what that means for us.

Originally identified in bacteria, plasmids have also been found in yeasts and other fungi. Most bacterial plasmids are circular, although some are linear, and they require at least an origin of replication to be maintained in the cell.

Many plasmids can transfer between bacterial cells by a process called conjugation, and some are even able to transfer to yeast or plants.

The genes on a bacterial plasmid can be categorized in two major groups, ‘backbone’ genes and ‘accessory’ genes.

  1. Backbone genes are those that contribute to the replication and maintenance of the plasmid or help it transfer between host cells; these genes are often conserved among members of the same family of plasmids and used to categorize plasmids into groups.
  2. Accessory genes encode functions that are frequently beneficial to the host cell. These functions benefit the host cell in many different ways, e.g. degrading environmental pollutants and using them as a carbon or nitrogen source, or providing resistance to an antibiotic or a heavy metal. These accessory genes, as the name suggests, are not necessary for the stable replication or transfer of the plasmid and are not always essential for the survival of their host, although they can provide functions that are critical for survival of the cell in specific environments, or at least give the cell an advantage over others that do not have these genes. An example of this would be genes that encode for resistance to antibiotics, which are essential for survival of the host cell in the presence of antibiotics.

Among the backbone genes the transfer genes facilitate the transfer of a plasmid copy to other bacterial cells, including very different species, by a process called conjugation. These genes often take up half of the backbone; the other half is composed of genes that allow the plasmid to replicate and be stably maintained in the host.

The transfer of plasmids from one cell to another by conjugation is now well recognized as a major mechanism by which bacteria can easily and rapidly acquire a variety of phenotypic traits and thus rapidly adapt to changing environments; plasmids can be spread among bacterial cells, such that a phenotypic trait on a plasmid can be distributed among a wide range of bacterial species. Unfortunately, many pathogens have acquired one or several genes for resistance to antibiotics through this system of gene exchange. Furthermore, virulence genes can also be transferred on plasmids and distributed throughout the bacterial world.

While these activities of plasmids can be threatening to our health, they also have contributed to humans through the dissemination of genes that degrade environmental pollutants, such as herbicides or pesticides.

Dr. Top's Research

Professor Eva Top ’s research is focused on the evolution and ecology of plasmids that transfer to and replicate in a broad range of bacterial hosts. These plasmids play an important role in the rapid adaptation of their hosts to changing environments. A good example is the current epidemic of antibiotic resistance in human pathogens, which is in part due to the spread of drug resistance plasmids.

Dr. Top is currently focusing her efforts on three projects, which are explained in more detail on her website.

  1. The first project aims at understanding if and how the host range of drug resistance plasmids can expand, contract or shift over time.
  2. The second project is focused on revealing the diversity and evolutionary history of the extant pool of broad-host-range plasmids through sequence analysis.
  3. The third project is developing mathematical models to better understand and predict population dynamics of plasmids in spatially structured populations such as biofilms.

Understanding the principles that guide transfer and persistence of plasmids will help us predict and influence how genetic properties are moved among bacterial species in different environments. This is important because we will learn how to accelerate the spread of plasmids that carry ‘good’ genes, such as those that code for degradation of toxic pollutants, and we may be able to slow down the spread of unwanted genes, like those that cause resistance to drugs in pathogens.

For more information please visit Dr. Top's Lab site at:, or visit one of the links above to open a new window to a section of Dr. Top's Lab site.

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The Virtual Genome Project is funded by the National Science Foundation Microbial Genome Sequencing Program, Grant number: EF-0627988.
For more information contact: Dr. Eva Top, Professor of Biology, Department of Biological Sciences, University of Idaho, Moscow, I.D. 83844-3051 U.S.A.
email: etop [at]