Anthrax bacteria likely to be US military strain
The bacteria used for the anthrax attacks in the US is either the strain the US itself used to make anthrax weapons in the 1960s, or close to it. It is not a strain that Iraq, or the former Soviet Union, mass-produced for weapons. There have been charges over the past week that the sophistication of the anthrax suggests that it was produced with the backing of some government, such as Iraq. But neither the strain nor the physical form of the anthrax is particularly sophisticated, say bioweapons specialists.
Last week, Tom Ridge, President Bush's Homeland Security adviser, stated that the anthrax sent to Florida, NBC and Senator Tom Daschle were all the same strain. An FBI spokesman in Florida confirmed that this was the Ames strain.
But there has been confusion over what Ames means. The scientists analysing the anthrax are comparing its DNA with a library of strains collected from all over the world. In this collection, the standard Ames strain is the one the US used when it produced anthrax weapons, a programme which ended in 1969.
To be identified as Ames in the studies currently underway, the anthrax must either be the American military strain or one that's very similar.
Hundreds of strains
It is a good choice for a terrorist. Ames is more likely than other strains of anthrax to cause disease in animals immunised with the standard US anthrax vaccine, which is now being given to US troops. It also has proven virulence and is not traceable to one particular country, says Ken Alibek, former deputy head of the Soviet bioweapons programme.
The Soviets did not mass-produce Ames, says Alibek. Iraq favoured the Vollum strain, which has been identified in samples from its Al Hakam bacterial fermentation plant. The anthrax mass-produced for weapons in the US was destroyed after 1969.
But samples were kept in the US and elsewhere. "The South African collection had hundreds of different strains," Alibek points out. And Wouter Basson, former head of the South African bioweapons programme, reportedly visited Libya after the fall of the apartheid government in 1994.
Not rocket science
As for the size of the anthrax particles used in the attacks, they were reportedly milled down to a few micrometres, optimal for inhalation. This has been cited as evidence of state involvement.
But "you can use readily available equipment to do this," says Alibek. "It isn't rocket science." The attacks have caused relatively few inhalation cases so far, which suggests that the spores were not blended with anti-caking chemicals to promote airborne spread, which Alibek calls the real secret of weaponising anthrax. He suspects the attackers don't have much material to work with.
We could soon know. Paul Keim's team at Northern Arizona University in Flagstaff has pioneered the genetic analysis of anthrax bacilli. Team member Kimothy Smith says they have found that some DNA regions mutate frequently, as often as once in every 1000 cell divisions.
By comparing the amount of mutation, says Smith, "you can say with a high degree of confidence how many bacterial generations separate an unknown strain from closely related reference strains". This can help pinpoint the exact strain the unknown anthrax came from.
It is also a way of counting the number of cell divisions the bacilli have been through since they parted company with the most closely related strain. And a small batch of anthrax will have undergone many fewer cell divisions than a big batch.
So the analysis could reveal whether the anthrax came from a 50-litre fermenter, such as a small-scale terrorist could obtain, or the huge vats of a state-sponsored bioweapons facility.