By David Linzee
School of Medicine researchers have identified a strategy that enables the bacterium Salmonella to adapt to concentrations of iron that otherwise would kill it. The finding might help explain why this bacterium is so widespread throughout the world.
In developed countries, Salmonella typhimurium causes food poisoning, which is unpleasant but not often fatal. But in developing countries, it causes typhoid fever, which kills approximately half a million people each year.
Salmonella genes can be readily altered. Therefore, lessons learned from this bacterium can be applied to other disease-causing microbes that are more difficult to study, such as the tuberculosis bacterium.
The researchers, led by Eduardo A. Groisman, Ph.D., professor of molecular microbiology and a Howard Hughes Medical Institute Associate Investigator, reported their latest findings in a recent issue of Cell, a leading basic science journal.
Like all organisms, Salmonella needs iron. In humans, for example, iron allows red blood cells to carry oxygen around the body. But too much iron is deadly, so cells must regulate its intake, storage and secretion stringently.
Iron metabolism in Salmonella has been intensively studied. But Groisman's group is the first to identify a signaling pathway that senses iron outside the cell and then activates genes that prevent too much iron from getting in. Cells that contain this pathway can survive in 250 times more iron than cells that lack it.
The iron-sensing pathway also turns on genes that make Salmonella resistant to the antibiotic polymixin B, which is secreted by a soil bacterium. Although the threat from iron toxicity may seem unrelated to a threat from another microbe, Salmonella is likely to encounter both in the same environment. Groisman suggests that natural selection could have favored the dual response.
"When Salmonella isn't living in a human or animal host, it makes its home in water or soil," Groisman said. "In fact, it can survive in soil for a year. Its ability to persist for so long may help explain why it is so widely transmitted."