Silicon highlights remaining questions over anthrax investigation

Did Bruce Ivins weaponize deadly spores?

Amber Dance

The deadly anthrax bacterium.MedicalRF.com / Alamy

Nearly two months after the suicide of scientist Bruce Ivins — whom the US Federal Bureau of Investigation (FBI) claims was solely responsible for mailing a series of letters laced with anthrax in 2001 — questions still remain over whether he was actually able to produce those anthrax spores.

Scientists initially believed that the spores had been weaponized – modified to make them disperse more easily and penetrate tissue more deeply.

But one of the scientists who first drew that conclusion has now changed his mind. Nature finds out why he thinks he got it wrong; why it matters for those trying to tie up the Ivins case; and what it means for the chances of a similar attack happening in the future.

What was the initial evidence that the 2001 spores were weaponized?

The powder was described as being 'weightless' and 'smoke-like'. One of the first scientists to work on it was Peter Jahrling, then a virologist at the US Army Medical Research Institute of Infectious Diseases in Frederick, Maryland. He recalls that he couldn't even weigh out a fraction of it: "It literally jumped off the spatula and was repelled by the weighing paper; it was like nothing I had ever seen before." Under an electron microscope, Jahrling and a colleague observed black dots that they speculated might be particles of silicon dioxide, or silica. Materials analysis by the Armed Forces Institute of Pathology in Washington DC confirmed that the sample contained both silicon and oxygen, and many assumed that the elements were combined as silica.

Why would a silicon compound increase the spores' virulence?

Spores are sticky, and tend to clump together. One method of weaponizing the spores is to coat them with something that interrupts the weak van der Waals interactions between each particle. Tiny particles of silica would do the trick, allowing the spores to float individually through the air. Silicon and oxygen can also form polymers called siloxanes, and such compounds are used to make inhaled medicines more dispersible.

Why have Jahrling and others changed their minds?

In 2002, as part of the FBI investigation, scientists at Sandia National Laboratories in Albuquerque, New Mexico, used electron microscopy to analyse the composition of the spores. The results were finally made public last month. They found silicon and oxygen in the spore coat, but not on the most external layer, the exosporium. The location of the silicon, the FBI says, suggests that it was incorporated naturally into the structures during growth, not added as a final coating to weaponize them.

But other experts disagree with the conclusion. "I don't think the guys at Sandia understand that the exosporium is not some kind of brick wall," says Stuart Jacobsen, a research chemist based in Dallas, Texas, who is an expert on the preparation and properties of fine-grained powders and has followed the case closely. "It's more like a chain-link fence." Decades ago, a study found that the exosporium is porous to various small molecules¹.

How might silicon get into the spores naturally?

The FBI points to experiments from 1990, in which scientists found some silicon in naturally grown spores². Another hypothesis, suggested by Serguei Popov, an anthrax researcher at George Mason University, Manassas, Virginia, is that the bacteria picked up a bit of silicon from anti-foaming agents while they were being grown. To produce the large number of spores in the contaminated envelope analysed - one trillion spores per gram — Popov infers that the perpetrator must have had to grow a large-scale bacterial culture in a fermenter, where silicon-containing anti-foaming agents are commonly used.

But this would add a relatively low concentration of silicon to the spores. Deliberate weaponization would probably leave much higher amounts. At a House of Representatives Judiciary Committee hearing on 16 September, Representative Jerrold Nadler (Dem.) quizzed FBI director Robert Mueller about the quantity of silicon present in the spores. Mueller did not answer the question.

Why does this matter to the investigation?

Many scientists involved in the investigation still wonder: could Bruce Ivins, in a few late-night lab sessions and in secret, have engineered the powder in the letters? An answer to this would help to determine whether Ivins was guilty, and if so, whether he needed assistance from other parties. It would also help defence experts to assess how easy it would be for others to mount a similar attack in the future.

"I don't think it would be horrendously difficult to make stuff that was moderate quality," Peter Setlow, a microbiologist at the University of Connecticut Health Center in Farmington, told Nature in August. But Jacobsen and others say the fact that the FBI has apparently failed to produce a powder to match the attack material suggests it must be very difficult to make. That might put it beyond Ivins's reach.

If the spores could not be made by a single scientist in a few evenings, that would suggest the spores came from elsewhere – possibly from a state-organized programme.

What happens next?

"The truth will come out when all the data are revealed," Jahrling says. But there is no indication from the FBI that more data are forthcoming anytime soon. Until they are provided, there will continue to be suspicions and speculations about the silicon in the spores.

• References

  1. Gerhardt, P. & Black, S. H.. J. Bact. 82, 750–759 (1961).
  2. Stewart, M. et al. J. Bact. 143, 481–491 (1990).