role of horizontal gene transfer in the evolution of microbial me resistance
gene transfer (HGT) is the process through which microbes, organisms that
reproduce asexually, exchange genes. This mechanism facilitate rapid adaptation
to changing and pollution impacted environments. Studies have revealed
that HGT is not a marginal phenomenon, as it has been thought of for many
years, but a common process which revolutionizes our view of the “tree
of life” (see figure below), where frequent gene transfer during
the evolution of life has modified the tree to resemble more a “net
of life” by the horizontal movement of genes from one “branch”
to the other.
illustrates our view of HGT; it affects the evolution of life
yet it does not erase or alter the integrity of the tree of life
(If you would like to know more about HGT and the tree of life
click here for a recent journal
article about this topic).
three major mechanisms of gene transfer between microorganisms.
These are illustrated in the adjoining figure. Our current research
is focused on transfer of metal resistance genes on bacterial
plasmids among bacteria in subsurface soils and how this process
enables life in metal and radionuclide contaminated aquifers.
cartoon depicts the three major mechanisms of gene transfer as
they occur in the environments where these processes are known
to affect the fitness of the microbial community to the environment.
Our lab is actively studying conjugation in soils and its affects
the ability of the community to withstand metal stress
study is important because the deep subsurface has been contaminated with
metals and radionuclide due to the production of nuclear weapons and energy.
This raises a public health concern with the mobilization of toxic metals
and radionuclides to groundwater. The most feasible approach to controlling
this problem is the stimulation of microbes whose activities reduce mobility
of these contaminants in the soil. These microbes however are exposed
to high concentrations of toxic metals and therefore their metal resistance
is critical to the immobilization of metals and radionuclides. HGT of
metal resistance genes in subsurface microbial communities is therefore
a mechanism that enhances resistance to metals and therefore bioremediation.
Initial studies conducted in our lab by Dr. Jonna
Coombs showed that some but no all genes for resistance to the toxic
metal lead in subsurface bacteria have evolved by HGT. Jonna used a combination
of PCR amplifications, DNA sequencing, and bioinformatic analyses in her
work. Click here if you would like to read more about Dr. Coombs's studies
To explore the possible involvement of conjugation in the documented HGT
among subsurface bacteria a graduate student, Aspa
Chatziefthimiou, is using an oligonucleotide microarray hybridization
to show that metal resistance genes are located on plasmids that were
extracted from the subsurface bacteria as shown in the figure below. Plasmids
labeled with a fluorescence dye are hybridized with the array to produce
signals with those probes for which homologous genes are present on the
plasmid. The DNA of such plasmids are then sequenced and analyzed for
the presence of elements known to mediate gene transfer.
figure illustrates our protocol for DNA microarray hybridization
that identifies metal resistance genes on plasmids. First, plasmid
DNA is separated by gel electrophoresis (left). Second, the isolated
plasmid DNA is labeled with fluorescent dye and hybridizated to
the probes arrayed on a glass slide (center); the colored circle
indicates a probe with a significant signal. Third, the signals
are quantitated and plotted (right). Strain Bacillus sp. Y7 is one
of the subsurface strains that Aspa has been studying. It has at
least six different plasmids (left) and one among them, designated
Y7-5 carries lead resistance gene as indicated by the circled hybridization
signal (center) and the pink bar (right).
studies are being conducted in collaboration with Dr. Patty
Sobecky from Georgia Tech, Dr.
Søren Sørensen from the University of Copenhagen , and
Dr. Niels Kroer from the National
Environmental Research Institute, Denmark with support from the NABIR
program of the Dept. of Energy.