Researchers use next-generation genomic analysis to probe the former USSR's biological weapons program. [CDC]
Researchers use next-generation genomic analysis to probe the former USSR’s biological weapons program. [CDC]

In 1979, nearly 100 inhabitants of the city of Sverdlovsk had suddenly succumbed to virulent anthrax infections. Soviet-era officials denied that the military had any involvement in the catastrophe, even though the town contained a large military facility. The denial was not surprising because a few years prior to the incident the USSR had signed and ratified—along with a number of other nation states, including the U.S.—the Biological Weapons Convention, which prohibited the development, production, and stockpiling of biological and chemical weapons. However, it was widely rumored that the Soviets still had a major biological weapons program in play.

Now, almost 40 years after the Sverdlovsk incident, a new study from investigators at the Translational Genomics Research Institute (TGen) and Northern Arizona University (NAU) was able to assemble the anthrax genome—using deep DNA sequencing methods—from victims of the 1979 outbreak.  

“I have been studying this anthrax outbreak and these specimens for more than 20 years,” explained lead study investigator Paul Keim, Ph.D., director of TGen's Pathogen Genomics Division and director of the Center for Microbial Genetics & Genomics at NAU. “Finally, using genomic technology, we could comprehensively characterize this pathogen genome.”

This new study, which is set to be published in the September issue of mBio, represents a precise and detailed examination of the anthrax strain used in weapons development and includes an anthrax genetic database that puts the weapons strain into a global context. 

“This is the signature agent of the world's largest biological weapons program, and now we have it in our genetic databases,” noted Dr. Keim. “Anywhere this strain shows up again, we will be able to identify it and track it back to its source. This is now an essential part of our forensic arsenal.”

The anthrax bacterium produces small capsules, or spores, that can lie dormant for decades. After settling inside the human lung, for example, it can cause a severe disease that, if not treated with antibiotics, kills 90% of those it infects. Anthrax is found in many parts of the globe and is dispersed through the human movement of animal parts contaminated with spores. Wool and hair from goats and sheep are moved globally as textiles or their precursors. When these originate in anthrax-endemic regions, they can carry the spores, which are long-lived. While this bacterium has little variation from strain to strain, whole-genome sequencing has identified DNA fingerprints that enable molecular epidemiology, tracing it to its source.

When anthrax outbreaks occur, their entire genome profile is now routinely compared to the genetic database to identify possible sources and exclude others. This type of analysis was used by the FBI to track the spores in the 2001 anthrax letter attacks, which infected 22 people and killed 5.

The Soviet Union produced anthrax spores on an industrial scale but repeatedly denied the existence of their biological weapons program. In 1992, an investigative team from the United States led by Harvard biologist Dr. Matt Meselson—noted for his discovery of messenger RNA in 1961—characterized the 1979 Sverdlovsk outbreak by interviewing local physicians, visiting cemeteries, and examining autopsy specimens. This investigation, along with accounts by Ken Alibek, a former Soviet scientist, revealed that the Sverdlovsk anthrax outbreak was due to an industrial accident. A faulty filter at a Soviet spore production facility allowed anthrax spores to drift in a silent plume over the city and into the nearby countryside.

The anthrax genomes were generated from autopsy tissue specimens of two Sverdlovsk anthrax victims. These tissues were moved to the United States with permission of Sverdlovsk pathologists to continue the investigation into the disease outbreak. From these, it was established that the anthrax pathogen was detected in these tissues and the victims died from inhaling the spores.

Consequently, the Sverdlovsk anthrax genome was compared to the global genome database maintained by NAU to identify its close relatives and to look for evidence of genetic engineering. The investigators found that this strain was closely related to other Asian isolates with very few differences to naturally occurring anthrax. There were no signs of genetic engineering.

Dr. Keim remarked that the Soviets had to be very diligent to avoid mutant variants from dominating their production stock. Invariably when wild anthrax strains are grown extensively in the laboratory, they adapt to those conditions and lose their killing power. “The Sverdlovsk strain's genome looked very much like those of wild strains we see across Asia,” he added.

Dr. Meselson, who was not part of the current research article, concluded that “if this strain had been grown repeatedly in the laboratory, it would have mutated into a form that had less virulence and less capacity to cause anthrax. The Soviet scientists must have been very meticulous in their maintenance of the natural form.”