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Curbing C. difficile's toxin production

As if being admitted to the hospital weren't bad enough, patients, once admitted, are at higher risk of becoming infected with a "superbug" bacterium, Clostridium difficile (C. difficile). The toxins produced by C. difficile kill human intestinal cells by causing them to burst open, allowing the bacteria to use them as fuel. This results in severe diarrhea and, in rare cases, death. Abraham "Linc" Sonenshein, PhD, and colleagues from the Department of Molecular Biology and Microbiology at the Tufts University School of Medicine (TUSM) and the Sackler School of Graduate Biomedical Sciences at Tufts, have discovered how a protein called CodY regulates the genes that control production of the dangerous toxins. Understanding the relationship between CodY and C. difficile is an important step toward the development of a drug that may prevent hospital patients from falling ill.

"The C. difficile bacteria only produce toxins when they are in need of food," explains Sonenshein, professor of microbiology at TUSM and corresponding author on the paper to be published in Molecular Microbiology. "We found that the CodY protein, in essence, monitors the hunger level of C. difficile, preventing toxin production when the bacteria have enough to eat." Sonenshein, along with first author Sean Dineen, PhD, and other Tufts colleagues developed a series of experiments to investigate the importance of CodY and how this protein communicates to bacteria that it is time to search for new sources of food.

The researchers first developed a mutant strain of C. difficile bacteria that does not make the protein CodY, and compared the amount of toxin produced by the mutant strain of bacteria to the amount of toxin produced by normal bacteria. The mutant strain produced much higher levels of toxin. The presence of CodY seems to tell the bacterial cells that they are well-fed and there is no reason to make toxin that kills intestinal cells for fuel. Lack of CodY activity, however, indicates to the bacteria that they are lacking key nutrients and that it is time to make the toxins they need to get food from the host cells.

To determine how CodY tells the bacteria not to make toxins, Sonenshein and colleagues removed DNA from the bacteria, and observed the interaction of the DNA and the CodY protein in vitro. They found that CodY targeted the region of the bacterial chromosome that includes the toxin genes. "When CodY senses that the cell has enough nutrients, it binds to this gene region, and prevents the bacterium from making toxin," says Sonenshein. "Conversely, when food is scarce, the CodY protein does not bind to these genes, allowing C. difficile to make the toxins needed to attack intestinal cells."

Knowing what turns on, and more importantly, what turns off the toxin-producing genes in C. difficile opens the door for treatment and prevention options. "It is possible that, based on our findings, a new drug could be developed that would trick CodY into thinking there is enough fuel for the bacteria, causing CodY to remain bound to the toxin gene region and thus suppressing toxin production."

Tufts University, Health Sciences. September 2007.

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