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Bacterial Breakthrough
Science paper defines a key to bacterial sex and some virulent misconduct
Using an innovative lab test on a bacterium that causes cancer in plants, microbiologists at The University of Texas Medical School at Houston have cracked a longstanding secret of bacterial sex, opening new pharmaceutical possibilities for inhibiting the microbes’ troublesome injection of DNA into each other.
At the UT Medical School at Houston Peter Christie,
Ph.D., left, and Eric Cascales, Ph.D., innoculate
kalanchoe plants in their lab with an agrobacterium.
Their research may help find ways to prevent such
problems as bacterial resistance to antibiotics.
Photo by Scott Merville
Writing in the May 21 issue of the prestigious journal Science, Associate Professor of Microbiology Peter Christie, Ph.D., and post-doctoral fellow Eric Cascales, Ph.D., detail the architecture of a channel through the bacteria’s membrane used in this process, which is known as bacterial conjugation. Bacteria spread such genetic traits as resistance to antibiotics and other virulence factors by injecting DNA through the channel.
“Scientists have studied bacterial conjugation mechanisms for over 50 years and have a general understanding of how these machines function,” Christie said, “but until our study, there was no knowledge of the secretion channel, the route through which DNA is delivered across the bacterial cell’s surface.
“With this knowledge, we are now in a position to collaborate with drug companies in hopes of developing drugs that might inhibit the action of the conjugation machines,” said Christie, who also holds a faculty appointment in the UT Graduate School of Biomedical Sciences at Houston. “Such drugs would be designed to interfere with the actions of the channel components and would prevent or at least reduce the rate at which antibiotic resistance genes, for example, are spread to other bacteria.”
Bacterial resistance to antibiotics is a growing problem, as overuse of the medications has yielded infectious strains that are extremely difficult to kill with drugs.
An accompanying perspectives article in Science by scientists from the American Red Cross Holland Laboratory also notes that some bacteria, such as those that cause Legionnaire’s disease, ulcers and whooping cough, inject proteins into human cells through pathways similar to the one detailed by Christie and Cascales. The article concludes that the Christie’s and Cascales’ findings take the medical
and scientific communities one step closer to hindering the virulence mechanisms of these diseasecausing bacteria.
Bacteria, the planet’s oldest life form, reproduce asexually – they double their chromosomes and then divide into two new cells. They obtain new DNA three ways: through bacterial sex; by taking up DNA molecules from their surroundings (transformation); or infection from viruses (transduction). All of these processes involve passage of DNA across the bacterial cell surface. These mechanisms contribute to the evolution of infectious pathogens by passing along genetic traits that strengthen the bacteria.
Christie and Cascales developed a sensitive laboratory test – or assay – called transfer DNA immunoprecipitation (TrIP) to track the progress of DNA as it exits a cell. Christie credits Cascales with creating the test by modifying an assay used to analyze DNA in yeast.
They applied the test to Agrobacterium tumefaciens, a bacterium known to attack plants through a specific conjugation system called the VirB/D4 type IV secretion system (T4SS). Agrobacterium injects DNA and proteins into plants, causing cell proliferation and finally tumors culminating in crown gall disease, which can devastate crops.
The scientists used the assay to track the path of DNA starting with its recruitment in the cell’s interior by one protein and then its sequential transfer along five other proteins in the system and out of the cell. “We propose that those six proteins are the subunits that make up the secretion channel,” Christie said. Additional structural analysis is under way to more thoroughly characterize this channel.
TrIP, the assay developed by Christie and Cascales, can be used to analyze conjugation systems employed by other bacteria. At least 30 other bacteria (most of them pathogenic) use the VirB/D4 conjugation system or its close relatives to spread DNA or protein virulence factors. Research collaborations are under way studying bacteria that cause Legionnaire’s disease and ulcers.
Bacteria do not limit their sexual activity to their own kind. “Sexual transmission of DNA is not limited to the bacterial kingdom – bacteria can deliver DNA not only to bacterial cells, but also to plant, fungal and mammalian cells,” Christie explained. “Bacterial sex therefore likely has contributed to the evolution of all genomes, from bacteria to man.”
Scientists have long used a modified version of the agrobacterium to genetically engineer plants by replacing the genes that cause disease with beneficial genes to be injected into plant cells.
More broadly, TrIP should prove useful for illuminating how large molecules are transported across membranes, a process that is fundamental to the survival of all types of cells, not only bacteria.
— Scott Merville, Public Affairs