|11 September 2001>News Stories>Patient Assasin. Anthrax Germs Turn the Body Against Itself
Patient Assasin. Anthrax Germs Turn the Body Against Itself
Nicholas Wade . NYTimes . 23 October
The anthrax spore lurks in the soil for months or decades, a patient assassin waiting for its sleep to be disturbed. One day, perhaps when a grazing cow yanks up a plant by its roots, the tiny spores shake loose and are eaten, or are sniffed into the cow's lungs, or dust its skin with a deadly bloom. Then begins a precise but terrible execution.
The surfaces of animals' bodies, whether lungs, intestines or skin, are ceaselessly patrolled by scavenging cells that engulf foreign matter and alert the immune system. These sentries are the spores' target. The spores let themselves be ingested by the cells, but as the scavenger cell is taking its prisoner to the nearest lymph node for inspection, the spore stirs from its long torpor. It germinates into an anthrax bacterium, and the bacterium divides and replicates.
The bacteria release poisons that weaken the cell's containing wall. Soon a handful of anthrax bacteria burst out and slip into the blood. The scavenger cell has now betrayed the whole animal it is supposed to defend.
The handful of bacteria in the bloodstream are programmed to execute two tasks: first to kill the animal, then to consume it so utterly that no more bacteria can be made.
The method of execution is highly ingenious. The bacteria's toxin is not particularly poisonous in itself. But the toxins home in on a class of cells known as macrophages, whose natural role is to inflame the tissues. A little inflammation helps the immune system respond to local infestations. But too much inflammation can cause septic shock, a reaction in which blood vessels leak, blood pressure drops and organs fail.
By forcing the macrophages into a mass release of their inflammatory hormones, the bacteria turn the body's immune defense system against itself so violently that death soon follows.
When the animal or person is dead, the anthrax feast begins. The bacteria grow explosively until all food is gone. By the time they are finished, each milliliter of their victim's blood can seethe with 100,000 microbes.
Lack of further nutrients is the signal for the bacteria to turn back into spores. The carcass rots. Its billions of spores return to the earth. There they can persist for years, even for a century or more, until the soil is disturbed and the murderous cycle repeated.
The details of how a single minuscule anthrax spore can fell an ox is being studied by a select group of researchers, some of them academic biologists interested in how bacteria work, others with a veterinary or military interest in anthrax. Much attention has been paid to the toxin itself, which has long been the target of anthrax vaccines.
To kill their prey, the anthrax bacteria first grow furiously. Once the first handful of bacteria have escaped from the scavenger cell that ingested a spore, their numbers double every half hour, said Dr. Philip C. Hanna, a biologist at the University of Michigan, who studies events in the first few hours after infection.
With the bacterial population exploding, their toxin pours into the bloodstream. Dr. Theresa M. Koehler, of the University of Texas at Houston, has found that the carbon dioxide in the blood is the signal that activates the bacterium's toxin regulation gene. This gene in turn switches on three other genes that produce the three separate components of the anthrax toxin.
The toxin is a cunning and complex piece of biological engineering. It consists of a system for breaking into cells and two agents of havoc. The break-in protein is called protective antigen, a misleading name conferred before its exact role was understood and because it is the protein attacked by the anthrax vaccine. The other components are called edema factor and lethal factor.
Protective antigen is designed to dock onto a receptor protein that studs the surface of many cells, even though macrophages are the principal target. On a cell's surface, the protective antigen proteins seek one another out and click together into a seven-member barrel with a central channel. On top of each protein is a docking site for either edema factor or lethal factor. These factors drop out of the bloodstream and settle on top of any seven-member barrels they find. Charged with their warheads, the barrels then make their host cells swallow them. The region of cell membrane that holds the barrel forms a pocket that first dips into the cell, then pinches off.
The attack complex is now inside the cell, enclosed in a bubble of the cell's own membrane. The seven- member barrel now injects its edema factor and lethal factor proteins through its central channel and into the cell's interior.
Dr. Stephen H. Leppla, who studies anthrax at the National Institutes of Health, found several years ago that edema factor was an enzyme that generated a signal used by cells for their internal communications. The factor creates so much signal that the cell is deranged. The excessive amount of signal "inactivates phagocytes so they can't kill bacteria," Dr. Leppla said, referring to the scavenger cells that sweep the body surfaces clean of spores.
If edema factor helps an anthrax infection begin, by killing off the body's first line of defense, lethal factor administers the coup de grace. It too is an enzyme, and it forces macrophage cells to produce the two powerful agents they use to provoke local inflammation.
"The toxin causes the macrophages to make TNF-alpha, a call to arms to other immune cells, and interleukin-1-beta, which causes fever," said Dr. Terry C. Dixon who studies anthrax at Duke University.
Though the two agents are a natural part of the immune response, an excess of them produces quick death. Rats injected with lethal factor may die in less than 40 minutes from fluid in the lungs, Dr. Leppla said.
With the animal dead, its immune defenses are paralyzed and, from the bacterial perspective, it is just a harmless bag of nutrients. The microbes multiply until there is no more left to eat. Under the stress of sudden famine, they turn back into spores that spill out in the fluids that ooze from the stricken carcass.
How did the anthrax bacterium evolve to be so lethal? "Part of its life cycle involves growing to large numbers in its host and killing it," Dr. Hanna said. "Other bacteria spread from animal to animal. This one doesn't spread, so needs to go back into the soil. Maybe if it wasn't so virulent the body could beat it. It's a very efficient pathogen."
Anthrax is a relatively new disease. From variations in the DNA of different strains, Dr. Paul Keim of Northern Arizona University estimates the anthrax bacterium arose only 10,000 years or so ago. Creatures like cows, goats and bison have long been anthrax's primary target. But when people catch the disease from animals, the spore attacks them in the same way. Dr. Martin Hugh-Jones, an anthrax expert at Louisiana State University in Baton Rouge, said the disease could be eradicated if all cattle were vaccinated and if infected carcasses were burned, not buried.
"We are still hobbled by the Pasteuran belief that anthrax is forever and only control is possible," he said at an international anthrax conference in June. "No. If cases are found promptly, carcasses burned, not buried, and the stock vaccinated for at least three years, the disease will not just be controlled but eradicated."