However, if one is trying to locate the exact laboratory where the anthrax attack strain came from, such distinctions would be more difficult since most laboratories share derivatives of the same strain, according to Salyers.
"Here, natural mutations over the past several decades that occurred in the strain due to numerous passages in the laboratory might be sufficiently abundant to allow the strain obtained from one laboratory to be distinguished from that obtained from another," she says. "But since the differences will probably be few in number, they will be less convincing than for strains isolated in different geographical locations."
The actual degree between strains is another matter of current scientific investigation.
"Scientists believe that B. anthracis mutates very slowly, so it may be difficult to have a differentiation that holds up in court," Salyers says.
There also are errors that occur during the DNA sequencing process (about 0.1 percent at present), but Salyers believes this problem can be solved by resequencing an area more than once.
New tools emerging from the private sector may help answer some of these issues.
An example is the recent delivery of the TIGER biosensor system by Isis Pharmaceuticals to the National Bioforensic Analysis Center (NBFAC), an arm of the Department of Homeland Security's (DHS's) National Biodefense Analysis and Countermeasures Center, located at Fort Detrick, Maryland. TIGER stands for Triangulation Identification for Genetic Evaluation of Risks.
The TIGER technology simultaneously identifies thousands of infectious organisms in a sample without needing to know beforehand what might be present in the sample.
The system does more than identify organisms, however. It may be useful in tracking strain origins.
"Evidence put through the TIGER biosensor system from a biocrime or terrorist attack will allow bioforensic analysts to differentiate closely related infectious agents and their relatedness to other strains and species," says David Ecker, vice president of Isis Pharmaceuticals located in Carlsbad, California.
Ecker says this information or the "biological fingerprint" obtained from the TIGER system will enable highly precise strain identification — critical to bioforensic work and solving crimes involving infectious organisms.
This whole mutation issue has led some scientists to consider molecules other than DNA to track microbial strains, such as those found in traces of the growth media used to cultivate the strain in the lab. It is not yet known whether other growth media molecules can be detected, but if they can, they might differ from one laboratory to another, which would provide an alternate or complimentary roadmap to the source of a strain.
There are other wrinkles in the microbial forensic fabric potentially attractive to defense council.
Other species of Bacillus, such as B. thurengensis (better known as the organic gardener's friend because of its use as a common pesticide), have genome sequences strikingly similar to that of B. anthracis, the cause of anthrax. The main difference between the two species is two plasmids (extrachromosomal segments of DNA that make B. anthracis capable of causing disease, whereas B. thurengensis and most other Bacillus species are innocuous for humans. Some plasmids can be transferred from one bacterium to another, known informally to bacteriologists as bacterial sex.
"No one knows if an innocuous strain of another species could be rendered virulent, but you can imagine ways in which a defense attorney might use this sort of fact to place doubt in the minds of jurors who have no background in biology," Salyers says.
The reverse problem occurs in the cases of some viruses with RNA genomes, such as HIV and influenza, which are mutating so rapidly that it is possible to find different sequence variants in the same person.
"Granted these differences are small in number, but deciding what the 'same' means when comparing the strain from the presumed source with the strain that infected a victim could raise interpretation problems," Salyers says.