Bacterial spores sprayed on organic crops as a pesticide may damage the health of people who inadvertently breathe them in. French researchers have found that inhaling the spores can cause lung inflammation, internal bleeding and death in laboratory mice.
Bacillus thuringiensis, or Bt, produces a toxin that kills insects. The dried spores of the bacteria have been used as a pesticide for more than 30 years and are one of the very few insecticides sanctioned for use on organic crops in Europe. Bt is also widely used to combat pest such as the spruce budworm, a caterpillar that attacks trees.
Last year, French scientists isolated a strain of Bt that destroyed tissue in the wounds of a French soldier in Bosnia. The strain, known as H34, also infected wounds in immunosuppressed mice (This Week, 30 May 1998, p 7). Now the same team has found that H34 can kill mice with intact immune systems if they inhale the spores.
Françoise Ramisse of le Bouchet army research laboratories near Paris and her colleagues found that healthy mice inhaling 108 spores of Bt H34 died within eight hours from internal bleeding and tissue damage. Spores from mutants of the same strain which did not produce the insecticide were equally lethal to mice, suggesting that it was not to blame. Ramisse and her colleagues presented their results at a conference in Paris last month.
The researchers think that the symptoms are caused by other toxins. The bacterium's close cousin, Bacillus cereus, produces a toxin that ruptures cell membranes. And in 1991, Japanese researchers showed that B. thuringiensis produces the same toxin. In fact, when the French researchers ran samples from the soldier from Bosnia through an automated medical analyser, it seemed to show that the bacterium was B. cereus. Ramisse suggest that companies producing Bt spores might make them safer by deleting the promoter sequence that activates the gene for the membrane-rupturing toxin.
Although H34 is not used as a pesticide, commercial strains of Bt tested by the researchers also killed some mice or caused lung inflammation when inhaled. The team obtained these strains from Abbott Laboratories, a major supplier of Bt based in Chicago. Ramisse points out that the strains are sprayed on forest pests at concentrations of 1011 spores per square metre-and so might pose a danger to people in the immediate vicinity. But Abbott maintains that Bt is safe. "We stand by our products," says Linda Gretton, a company spokeswoman. The French researchers have not yet tested strains made by other companies.
"I suspect Bt infection is more widespread than we realise," says Ramisse. Recorded infections by Bacillus pathogens are comparatively rare. Known pathogenic species can have very distinctive symptoms. Anthrax, for instance, is caused by B. anthracis. But where such tell-tale signs are absent, Ramisse suspects that doctors often fail to recognise that the bacteria are responsible, dismissing any Bacillus in patients' cultures as contamination. Consequently, the cultures are often discarded. "I wish they would start keeping them so we could check for Bt," she says.
When Bt was sprayed in towns in Oregon in 1991 to combat gypsy moths, the bacterium was found in clinical samples from 55 patients who had been admitted to hospital for a variety of other reasons.
Robert Haward of the Soil Association, which represents Britain's organic farmers, says that they may have to use masks and take more care when spraying the spores on crops.
Author: Debora MacKenzie
New Scientist issue 29th May 1999
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