For almost 50 years, the fight against bacteria that cause everything from pneumonia to strep throat has been a crude war of attrition conducted on the molecular level.

Antibiotics are developed, and within years, sometimes months, bacteria develop ingenious methods to resist them. New drugs are developed, and new defense systems arise.Now the battlefield is changing, for new strains of bacteria are emerging that are using a little-understood defense system that is chilling in its implications. The bacteria are incorporating DNA, the genetic blueprint of life, from unknown sources to remake themselves into cells that are incredibly resistant to almost all antibiotics.

"The mechanism of trading (DNA) is amazing," Alexander Tomasz told writers gathered for a conference sponsored by the Council for the Advancement of Science Writing.

Because the bacterium has fundamentally changed itself by taking in the unknown strand of DNA, it is not merely mutating, as has been the case in the past. It is becoming something new.

Tomasz is the head of the Rockefeller University's laboratory of microbiology and specializes in what he described as the armaments race between science and bacterial infections.

"We don't know yet who the maker of this DNA sequence is," he said, referring to other cells in the body where the DNA apparently comes from. But the new DNA-altered bacteria are "seeping out of hospitals" around the world and getting into the healthy population.

When confronted with the super-resistant bacteria, Tomasz said, "There is a chill that goes down the spines of doctors."

Several strains of highly resistant bacteria associated with pneumonia have been discovered in Hungary and may be common in much of Eastern Europe, he said. A similar strain also has been found in some groups of American Indians.

The difference between the new superstrains of bacteria and the more common strains is best described via the war analogy, he said.

In the past, bacteria developed chemical shields against the antibiotic arrows of doctors. Penicillin and other antibiotics ran into the shields as they tried to attach themselves to the surface of the bacteria. Another defense, Tomasz said, was that of a bazooka against a tank. The bacteria would actually send out chemicals to destroy the incoming antibiotic.

Other bacteria would allow the antibiotic into a cell, but would pump it back out before it could do any damage.

Each defense led to the development of a new offensive weapon, and about 45,000 antibiotic drugs have been developed to overcome the wide assortment of bacterial defenses.