Using an animal model, scientists have discovered a way to inactivate the lethal toxin produced by the anthrax bacterium.

The findings provide a potential route to therapy of anthrax that could complement antibiotics.

Anthrax infection -- lately in the news because of its potential for use as a biological weapon -- can be contracted by humans through the skin or by inhalation.

Antibiotics can treat the infection, but they must be given quickly. And while they may kill the anthrax bacterium, they cannot inactivate the lethal toxin it produces.

By the time symptoms of anthrax contracted by inhalation have appeared, it is generally too late to rescue an individual with antibiotics, because he or she will succumb to the toxin the bacteria has already produced.

Anthrax releases three nontoxic proteins that assemble themselves into the toxin. Investigators have identified a protein, known as a polyvalent inhibitor, that blocks this assembly.

The researchers have tested the polyvalent inhibitor of toxin action by mixing it with a potentially lethal dose of the lethal toxin and injecting the mixture into rats.

Normally, the lethal dose of toxin will kill the animal within 90 minutes. In the presence of the polyvalent inhibitor, the animals survived and showed no symptoms.

This finding supports the notion that this inhibitor could be used to block toxin action in infected humans and rescue them. According to the researcher, inhalation anthrax is the most deadly form of the disease. It is contracted by inhaling anthrax spores -- durable, hard-shelled "seeds" containing the bacterium.

The spores begin to grow in the lungs and enter the bloodstream, where they produce the anthrax toxin. The toxin acts by killing certain immune system cells that normally guard against bacterial invaders. In so doing it also causes changes in metabolism that lead to death.

Nature Biotechnology October 2001;19:958-961