Article paru dans New Scientist 29 mai 1999
Red flag for green spray
Debora Mackenzie
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, p7). 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 a 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.