Study Maps Varied Anthrax Samples

Detailed DNA analysis can distinguish even very closely related samples of anthrax, scientists were to report today, a finding that could aid investigators seeking to trace the source of last year’s attacks.

The analysis, to be published online today by the journal Science, compared the anthrax strain isolated from one of the Florida anthrax victims with samples obtained from other sources. The study confirmed that the Florida sample was derived from the so-called Ames strain (originally obtained from a Texas cow in 1981), as has previously been reported.

But it also revealed that the Florida sample is not genetically identical to all samples of Ames, narrowing the possible sources of the microbe even further.

The study, which scanned the entire anthrax genome for tiny differences, could be used to create a fine-detail “anthrax encyclopedia” that would catalog DNA variations for all known anthrax samples, the scientists said. This could be very helpful should any future attacks occur, allowing investigators to quickly focus on possible sources of the spores.

Such approaches should be aggressively pursued to catalog not only anthrax but also other bacteria that could be used as bioweapons, said bioterrorism expert Scott Layne, associate professor of epidemiology at the UCLA School of Public Health.

“We have the technology to do this; we’ve got the know-how,” Layne said.

The study was conducted by researchers at The Institute for Genome Research (TIGR) in Rockville, Md., and the laboratory of Paul Keim of Northern Arizona University in Flagstaff. Keim, a world expert on DNA typing of anthrax, is helping the FBI with its criminal investigation.

Keim and colleagues already had developed DNA fingerprinting methods that could distinguish many more distantly related anthrax samples from one another. Such methods were used to confirm that the anthrax used in the attacks was from the Ames strain.

The current study, by analyzing the entire anthrax genome, went further. The scientists compared a sample of the Ames strain obtained from Porton Down in England (under study at TIGR well before last year’s attacks) with the Florida sample.

The two genomes, although virtually identical, were found to differ in a few places. Those differences must have accrued during the two decades since the strain was first isolated.

The scientists then examined five more samples of Ames, four obtained from labs and one from a goat in Texas.

They found, again, that some of the Ames strains were distinct from one another. Others were identical.

The group now plans to sequence 14 different anthrax strains and closely related bacteria to detect similarities and differences, said Timothy Read, assistant investigator at TIGR and lead author of the study.

Then, he said, DNA differences in hundreds of anthrax samples will be determined to build up a comprehensive anthrax catalog.

Some of the differences might offer clues to the microbe’s biology, such as why some strains of anthrax are especially virulent, he said. They might teach scientists much about the historical spread of Bacillus anthracis through the world.

Such a catalog will have limits when it comes to fighting bioterrorism, said Raymond Zilinskas, senior scientist at the Monterey Institute of International Studies.

Precisely typing a sample of anthrax does narrow the possible source, he said. But it does not lead directly to a criminal.

Not all anthrax differs, and it is unclear how many people have samples of any particular strain of the bacterium.

“DNA is not the answer to everything,” Keim said. “Forensics and eventually convictions still require good police work and supporting evidence.”