In the American government’s biodefense efforts, the potential for terrorists to cause a deadly anthrax outbreak remains a significant concern, six years after the letter attacks that shook the nation shortly after 9/11.
Now, researchers at the University of Michigan Medical School have developed the first complete picture of how anthrax-causing bacteria survive and grow inside unwitting immune cells — their supposed attackers — during the crucial first moments of anthrax infection. They have also identified gene candidates to pursue as possible anthrax drug targets. They say the methods they used to detect the microbe’s activities should become important new tools for other researchers.
Ultimately, the goal in this and other related research is to discover more effective, more easily tolerated treatments than those available now if an anthrax attack occurs, says U-M scientist Nicholas H. Bergman, Ph.D., the lead author of the study, which appears in the July edition of Infection and Immunity. Drugs given to people within a day of exposure, before symptoms develop, can prevent illness and death.
In mouse studies using DNA microarray technology, the U-M scientists were able to track which genes and enzymes play key roles in the bacterium that causes anthrax, while it sneaks inside the immune system’s first-responder cells in the lungs, called macrophages, and begins to multiply. The work is a significant advance because it will make it much easier to identify precise new targets for better anthrax drugs and vaccines, says Bergman, a research assistant professor of Bioinformatics at the U-M Medical School.
In strategies to quell the anthrax microbe, timing is everything. During most of its life cycle, the organism has formidable defenses. These make it a challenge for scientists to find a prime moment when future drugs, without the digestive-tract side effects and other drawbacks of those used now, can effectively stop the bug in its tracks.
Bacillus anthracis can quickly transform from a dormant spore (the white powder sent to U.S. lawmakers and others in the mail in 2001) into an active, quickly-multiplying organism once it gets inside the warm lungs of a host. Bacillus anthracis can cause infection elsewhere in the body, but is most serious and potentially deadly when its spores are inhaled.
Bergman’s team focused on the mystifying step in anthrax infection when the bacteria pass unrecognized inside macrophages, the primary immune cells able to kill most bacteria.
“Somehow the bacterium avoids being killed and actually hijacks these phagocytes (microbe-killing cells),” Bergman says. New drugs, he says, should target the bug during the brief “window of vulnerability” when the bacteria transform from dormant spores into active, growing organisms. That chance exists for a few hours when the invaders are inside immune cells in the lung and then pass from the lungs to the lymph nodes.
Once the bacteria reach the bloodstream, they become unrecognizable to immune cells there. At this stage, they cause death from septicemia in essentially all people infected. Even with modern ICU support, the mortality rate for infections that progress to this stage is greater than 50 percent.
To understand what happens in the anthrax microbe as it activates inside the defender macrophages, Bergman’s team used DNA microarrays, a technology emerging in the last decade, to examine mouse macrophage cells infected with the attenuated version of the microbe. It is modified so that it cannot infect laboratory workers but remains infectious in mice and other animals. The form is also used in animal anthrax vaccines.
The scientists were able to profile all the significant genetic activities in the microbe at several points in time as it invaded the macrophage, germinated, killed its host, and then escaped to spread further. They identified several pathways and functions that helped the microbe survive and thrive, which could be targets for future drugs.
Among a large number of genes shown to be highly active, the scientists picked one to study further, a previously uncharacterized gene in the MarR family that possibly regulates transcription. When they infected mouse cells with a Bacillus anthracis strain altered to lack the gene, they found the bacteria were significantly less able to cause disease. The next step will be to screen compounds that could potentially block the action of this gene and other genes identified in the study.
A new generation of anthrax drugs is needed because antibiotics given now to people exposed to anthrax spores, though they work, cause serious gastrointestinal effects during the 60 days people need to take them. Many people do not complete the full course of treatment. An improved drug would knock out the anthrax microbe but leave the normal good bacteria in the gut alone. As a first step, U-M scientists plan to screen compounds to find ones able to block specific processes they have identified in the anthrax microbe.
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