It's no mystery that inhaling anthrax spores usually means death, but scientists have long been mystified by how the bacteria sneak around the human immune system to do their dirty work.
Dr. Roland Strong of the Hutchinson Center's Basic Sciences Division, in collaboration with chemists at the University of California, Berkeley, and microbiologists at the University of Mississippi Medical Center, discovered a trick that anthrax bacteria use to evade the body's defenses, but which may turn out to be an anthrax Achilles' heel. They found that anthrax bacteria have a two-pronged molecular approach for grabbing iron from their human hosts. While one route is routinely shut down by the immune system, the other molecular thief proceeds unchecked.
The researchers uncovered the stealthy ploy while studying how these deadly bacteria rob iron — an essential ingredient for growth — from their human hosts to develop and multiply. Humans make a protein called siderocalin to defend against bacteria by interrupting bacteria's efforts to scavenge for iron. Anthrax bacteria produce two small molecules, bacillibactin and petrobactin, to snatch iron away from the human body's iron-transporter molecules. These scavengers, or "siderophores," are essential to anthrax's ability to grow rapidly, especially after the spores are inhaled, though why the bacteria need two siderophores to do the job has been puzzling.
The scientists found siderocalin, the human-immune protein, grabs bacillibactin and effectively sidelines it. But anthrax's second iron scavenger, petrobactin, is not stopped by siderocalin, so the bacteria gets the iron it needs to complete its lethal invasion of the body. The findings from their National Institutes of Health-funded study appeared in the Dec. 5, 2006, print edition of the Proceedings of the National Academy of Sciences.
"So far, we haven't been able to find a naturally occurring protein that binds to petrobactin, but that could be a treatment strategy," said Strong, whose lab focuses on determining the three-dimensional structure of proteins related to the immune system. "The research is going in an exciting direction."
Interestingly, anthrax's less successful siderophore, bacillibactin, is very similar to siderophores produced by several disease-causing bacteria that live in the gut, such as Salmonella enterica and strains of E. coli. In Strong's previous work with these two bacteria, he found they also produce a second siderophore with a molecular structure similar to petrobactin.
The researchers believe that production of a second, stealth siderophore may be a common response by bacteria to the human body's production of siderocalin. The discovery of a similar approach in anthrax suggests that producing more than one siderophore is a general tactic of bad as well as benign bacteria, according to Strong.
The researchers used samples of anthrax siderophores from Dr. B. Rowe Byers' anthrax-research laboratory at the University of Mississippi Medical Center. Because bacteria secrete siderophores, these molecules can be separated from the bacteria and studied without danger of infection.
Strong and his lab members are working with Dr. Kenneth Raymond's team at UC Berkeley to explore how their discovery could be used to diagnose or treat anthrax by launching a counterattack against petrobactin. They hope to create a sensor to detect petrobactin, which is not known to occur in any other bacteria besides anthrax. Additionally, because the iron-capture stage is critical to growth, it is a possible drug target.
Anthrax infection is difficult to diagnose because its initial symptoms are nondescript. As a potential bioweapon, it is nearly always fatal when inhaled. Its long-lived spores grow rapidly in the lungs, leading quickly to breathing problems and shock. The bacteria succeed by forming capsules that invade lung cells, then capturing iron in order to reproduce, and finally, manufacturing a toxin that kills the cells and releases thousands of new spores into the bloodstream. Although a vaccine exists, there is currently no effective treatment.
Anthrax has long been intertwined with human history. It is believed to have been one of the Egyptian plagues at the time of Moses, and cases were clearly recorded by the ancient Romans. Anthrax bacteria are considered the first "germs" proved to cause human disease. Studying anthrax is a useful model for understanding early events in the infectious process and the molecular basis of inflammation.
Benefits of collaboration
Strong said his collaboration with Raymond, who has researched iron-capturing siderophores for 35 years, is a great example of how joint efforts can breathe new life into scientific questions. "When we came up with this study, we immediately decided we needed to collaborate with siderophore chemists," Strong said. "Ken Raymond was about to let his 30-year-old grant die because he thought the whole story was over with siderophores. But we've come along and done some great science with him, and we've gotten some great publications. It's just a totally cool project."
Strong said he plans further collaboration with Raymond's group and scientists at Baylor University to see if it would be possible to redesign siderocalin, the human-immune protein, so that it binds to more siderophores. "Baylor has a computational method to try to do exactly that, but it's very, very difficult," Strong said. "It's a 'Hail Mary' tactic, but we want to try it."
He also intends to determine the structure of petrobactin. "We know chemically there is a carbon here and a carbon there, but we don't yet know how it looks when it grabs onto iron and holds it," Strong said. "We'll need to know the structure to do the redesign of siderocalin. Everyone's expertise is required to get this done. It should be fun."