Mechanics Of Bacterium's Toxin Being Unraveled

In a presentation today at the 102nd annual meeting of the American Society for Microbiology, Brenda A. Wilson described her basic research and recent findings involving Pasteurella multocida, a bacterium that once left her hospitalized and near death. The bacterium, she said, offers a window to view the mechanics of many toxin-mediated bacterial diseases, including anthrax, which left five people dead from acts of terrorism last year despite extensive treatment with antibiotics.

“A big problem now is antibiotic resistance, but we also need alternative strategies for attacking toxin-mediated disease after exposure to toxins,” she said in an interview in advance of her talk. “Current strategies, such as vaccine therapy or treatment with antitoxins or other inhibitors, are focused on blocking a toxin from binding to cells. My studies consider that exposure has already occurred. Once the toxin is in and hits its target, what do we do? I want to understand what a toxin does after it hits the target.”

Pasteurella multocida is a well-known pathogen in veterinary medicine. Its various strains affect domesticated and agricultural animals, leading usually to serious, and often deadly, respiratory infections. Contact with animals sometimes results in respiratory problems in humans, and skin infections can occur after being bitten by an animal. The bacterium is even part of the Komodo dragon’s deadly bite.

Disease doesn’t always occur, Wilson said, but a synergistic effect with another microorganism, such as Mycoplasma or Bordatella, often has serious consequences.

In 1997, Wilson discovered that the Pasteurella multocida toxin’s target is a protein known as Gq, which regulates a variety of hormonal activities inside cells. “The role that Gq plays in a particular cell will determine what form the cellular damage takes when the toxin acts on it,” she said.

Antiobiotics until recent years have killed many kinds of bacteria, but even as they die some bacteria still can release toxins into the body. In many cases, just one toxin can enter a cell and alter its structure or kill it. Once toxins are released, she said, “you reach a point of no return, where you have a toxin disease no longer treatable with antibiotics even if you have completely removed the bacterium from the body.” In her talk, Wilson announced the construction of a tool “that allows us to visualize the pathway a toxin takes into a cell.” Her unpublished technique utilizes a synthetic green-fluorescent protein attached to the toxin protein. The added green protein, when visualized with a fluoresence microscope, accompanies the toxin during invasion and entry into a cell.

The tool, she said, will allow researchers to test inhibitors or other blocking agents that might be developed to fight toxin-mediated infections.

Wilson also discussed new findings from the May 3 issue of Circulation Research, in which she and her Columbia University collaborators, Susan F. Steinberg and Abdelkarim Sabri, reported that the Pasteurella multocida toxin attacks cardiac cells in two distinct ways. They found that at low concentrations the bacterium causes cardiac hypertrophy, an indicator of heart disease in which cells proliferate and enlarge the organ. At higher toxin levels, heart cells become susceptible to rapid destruction by other damaging agents. The toxin, Wilson said, can now be used as a potent tool to study heart disease processes.

She also provided a brief overview of her progress studying the bacterium’s toxic attack on skin and cells, particularly on the toxin’s ability to completely block fat accumulation. She theorizes that what she is seeing may partially explain the “wasting syndrome” often observed in infected animals.