by Debora MacKenzie http://www.newscientist.com/news/news.jsp?id=ns99991490 As anthrax continues to turn up in US postal facilities, and postal workers, evidence is emerging that it is an American product. Not only are the bacteria genetically close to the strain the US used in its own anthrax weapons in the 1960s, but New Scientist can reveal that the spores also seem to have been prepared according to the secret US "weaponisation" recipe. This is troubling, say bioterrorism specialists. While the terrorists behind the anthrax-laced mail US might have got hold of the strain of anthrax in several laboratories around the world, the method the US developed for turning a wet bacterial culture into a dangerous, dry powder is a closely-guarded secret. Its apparent use in the current spate of attacks could mean the secret is out. An alternative is that someone is using anthrax produced by the old US biological weapons programme that ended in 1969 - in which case the scope for further attacks could be limited. Experiments to determine which is true are underway now in the US. Particle size Analysis of the physical form of the anthrax powder used in the attacks has lagged behind the genetic analysis. Bacteria from patients or contaminated surfaces can be multiplied up to provide enough DNA for analysis. But a physical examination requires a sample of the actual powder, and so far, only two are known. One is from the letter opened in Senator Tom Daschle's office in Washington on 15 October, the other from a letter sent to the New York Post. Last week, US Senator Bill Frist announced that the powder in the Daschle letter was in particles 1.5 to 3.0 microns wide, a very narrow size range. The results of the physical analysis of the New York Post letter are not yet known. The actual bacterial spore is ovoid and around half a micron wide. The whole trick to making anthrax weapons, says Ken Alibek, the former deputy head of the Soviet Union's bioweapons programme, is to turn wet cultures of bacteria into dry clumps of spores that are each between one and five microns wide, the optimal size to penetrate a human lung and stay there. But dried spores tend to form larger particles, with a static electric charge that makes them cling doggedly to surfaces rather than floating through the air where they can be inhaled. Fluidising agent The Soviet Union got around this by grinding dried cultures along with chemicals that cause the particles to remain separate. Iraq is the only other state known to have tried making such a weapon, and it dried anthrax cultures along with bentonite, a clay used as a fluidising agent in powders. But last week the White House said there was no bentonite in the Daschle letter. For its weapon, say informed sources, the US added various molecules, including surfactants, to the wet spores so that when they were dried, they broke up into fine particles within a very narrow size range of a few microns. There was no need to grind the powder further. Chemical tests are now being conducted to see if any traces of the US additives are present. Grinding was considered the most likely way for terrorists to create anthrax powders, as the milling machinery is not hard to obtain. But it results in a wider range of particle sizes. Large particles can be filtered out, but smaller ones remain. The Daschle anthrax, say sources, looks instead like it was made according to the US recipe. Anthrax stockpile The question is, when? At its peak, the US bioweapons programme made 900 kilograms of dry anthrax powder per year at a plant in Arkansas. That stockpile was destroyed when the US renounced bioweapons in 1969. But small samples might have been saved without being noticed. Experiments are now underway in the US to determine how many bacterial generations separate the anthrax being used in the attacks from the most closely related strains in a reference collection of anthrax, which includes the US weapons strain. If the number is very small, and the anthrax closely resembles the weapons strain genetically, it could be a leftover from weapons production before 1969. If, however, the bacteria have gone through many cell divisions since the most closely related strain was frozen, they might have been produced more recently. That would mean someone has obtained not only a virulent strain of anthrax, but the know-how to turn it into what was probably the most sophisticated anthrax weapon ever produced. |