Some call it the “biological Chernobyl.” On 2 April 1979, a plume of anthrax spores was accidentally released from a secret bioweapons facility in the Soviet city of Sverdlovsk. Propelled by a slow wind, the cloud drifted southeast, producing a 50-kilometer trail of disease and death among humans and animals alike. At least 66 people lost their lives, making it the deadliest human outbreak of inhalation anthrax ever.

Now, 37 years later, scientists have managed to isolate the pathogen's DNA from the bodies of two human victims and piece together its entire genome. The study, under review at the journal mBio and released today on the preprint server bioRxiv , answers one of the many remaining questions about the Soviet Union's clandestine biowarfare program by showing that scientists hadn't tinkered with the anthrax strain to make it more resistant to antibiotics or vaccines. If they had, the Soviet bioweapons would have become even more lethal.

This is a perfectly ordinary strain. But that doesn’t mean it wasn’t nasty. It was extracted from people who were killed by it. Matthew Meselson, Harvard University

The study may also prove useful if bioterrorists ever get their hands on anthrax. “It provides us with a molecular fingerprint,” says Roland Grunow, an expert on anthrax at the Robert Koch Institute in Berlin who was not involved in the study. “If this strain ever surfaces again we will be able to tell that it is the strain that was produced in huge amounts in Sverdlovsk.”

Anthrax, caused by the bacterium Bacillus anthracis, kills few people naturally, but is exquisitely suitable for use in bioweapons. That's because the bacterium produces spores: tiny, dry survival capsules that can lie dormant in the soil for many decades. Spores can be weaponized and delivered by the trillions as an invisible, odorless aerosol. After nestling inside the human lung, they can cause a severe infection that, if not treated with antibiotics, kills 90% of those it infects.

When released over, say, a packed sports stadium or a city center, tens of thousands could be exposed, and many would get sick before officials even realized what happened. One study, published in 2006, estimated that releasing just 1 kilogram of anthrax spores in Washington, D.C., would be enough to infect between 4000 and 50,000 people. If too few antibiotics were available within days, or if the microbes were resistant, thousands might die.

Anthrax is a favorite weapon for bioterrorists as well. In June 1993, members of the Japanese cult Aum Shinrikyo sprayed the bacterium from a building in Tokyo; luckily, they made a mistake and used a strain that was innocuous to humans. Shortly after the 11 September 2001 attacks in New York City, anthrax powder was mailed to several politicians and journalists on the U.S. East Coast; 22 people were infected and five died.

During the Cold War, the United States, the United Kingdom, and the Soviet Union all had biowarfare programs. The Biological Weapons Convention, which took effect in 1975, was supposed to end that, but in the Soviet Union, a massive clandestine program continued to produce anthrax spores and several other bioagents.

Questions about the outbreak in Sverdlovsk—which today is called Yekaterinburg and part of Russia—still linger. The Soviet Union at first blamed contaminated meat from an animal outbreak. In 1992, when Boris Yeltsin was president, a team led by Harvard University molecular biologist Matthew Meselson was allowed to visit the area to investigate the incident. In a paper published in Science in 1994, they concluded that the geographic pattern of the outbreak clearly showed that it was caused by an aerosol that had escaped from a facility known as Military Compound 19. “Bad meat does not go in straight lines 50 kilometers long, but wind can do that,” says Meselson, whose wife and fellow team member Jeanne Guillemin wrote a book about the investigation. Many other issues, such as exactly how the cloud was released, remain mysterious.

To fill in a few more pieces of the puzzle, anthrax scientist Paul Keim of Northern Arizona University, Flagstaff, and colleagues attempted to sequence the B. anthracis genome from two samples taken from victims. Russian pathologists who investigated the outbreak as it was occurring had collected the samples and later shared them with Meselson during his trip to the Soviet Union. The tissue had been fixed in formalin and embedded in paraffin, and the DNA was badly degraded, Keim says. Still, the researchers managed to piece together the entire sequence and compare it with hundreds of other anthrax isolates.

The team didn't see any evidence that Soviet engineers had tried to grow a strain that was resistant to drugs or vaccines, or that they had genetically engineered the bacteria in any way. “It is very closely related to other domesticated strains that have been used by the Soviets and Chinese as vaccine strains,” Keim says.

"This is a perfectly ordinary strain,” Meselson agrees. The paper suggests that the Sverdlovsk strain "was one found in the environment which the Russians picked up and used for whatever they were doing there,” he says. “But that doesn’t mean it wasn’t nasty. It was extracted from people who were killed by it.”

It was not unreasonable to suspect that the Soviets would have tried to create a superstrain, Meselson says. "They certainly could have made it resistant to penicillin," he says—such resistant strains even exist in nature. And since 1979, others have engineered B. anthracis to be resistant to antibiotics as well as certain vaccines.

What is most surprising is how few changes there are in the genome, Keim says. Only 13 base pairs differ between the Sverdlovsk strain and its likely common ancestor with the vaccine strains. B. anthracis evolves very slowly; during the many years it spends buried as a spore, its evolution is essentially on hold. But evolution speeds up as generation after generation is grown in the laboratory. Apparently, scientists in Sverdlovsk prevented this from happening, Keim says: “I think that the Soviets were meticulous in maintaining master spore stocks and avoiding extensive lab growth.”

Russia officially agreed (again) to end its bioweapons program in 1992. But there are lingering doubts about whether the country has completely followed through, and security analysts have raised the prospect that Soviet bioweapons could have fallen into the wrong hands. Keim's study should allow scientists to tell whether any future anthrax outbreak came from a leftover Soviet weapon or some other source, Grunow says. “If this strain appears again anywhere in the world, we would be able to identify it unequivocally.”