Is natural pesticide too hard on people?

A bacterium discovered in rotting onions might be a great alternative to pesticides--except that some strains occasionally kill people.

The adaptable Burkholderia cepacia is inspiring such a fuss that the American Phytopathological Society is devoting a symposium to the organism Nov. 9 at its annual meeting in Las Vegas. Co-organizer Jennifer Parke of Oregon State University in Corvallis is bringing together an unlikely mix of specialists in plant and human diseases in hopes of sorting out the issues.

Cornell University pathologist Walter Burkholder described the organism in 1950 as the culprit behind sour skin, a disease that reduces onions to slimy mush.

"It's an amazingly versatile organism," Parke says. "They're in soil, pond water--under your fingernails." It's one of the common soil organisms that researchers have been investigating to compete with pathogens, produce natural antibiotics, and perhaps boost a plant's own defenses.

The bacterium, Parke notes, has shown promise in keeping seeds and seedlings of peas, beans, and sweet corn free from rot; protecting cuttings of poinsettias; preserving fruits in storage; and even giving tree seedlings a head start. One company, Stine Microbial Products, has received permission from the Environmental Protection Agency to market B. cepacia. Two more companies, including one that Parke works with, have applications pending.

In Parke's work, survival jumped from about 30 percent to 90 percent in pea seeds treated with B. cepacia. "It's not as effective as a chemical pesticide, but it's darned good," she reports.

In the 1950s, however, a case of endocarditis, or inflamed heart lining, became the first recorded instance of human disease due to B. cepacia. Since the 1980s, lung infections have increased among people with cystic fibrosis. Concern over transmission has disrupted social life in the CF community. As many as 35 percent of people with both CF and the infection develop the rapidly fatal illness known as cepacia syndrome, note Alison Holmes of Imperial College School of Medicine in London and colleagues. In the April-June Emerging Infectious Diseases, they proposed a moratorium on widespread agricultural use of B. cepacia.

Although the bacterium is already, in Parke's words, "on every potato and onion in the grocery store," a contentious issue is how close environmental strains lie to human pathogens. The bacteria fall into at least five subpopulations, or genomovars, explains John Govan of the University of Edinburgh Medical School in Scotland, one of Holmes' coauthors. Strains being developed for agricultural use seem to cluster in genomovar V.

"Genomovar III is probably the real baddie, but that doesn't mean the others are absolutely harmless," he says. Strains from all groups have now been found in the lungs of CF patients. Govan remembers a case where a strain from a mild-mannered group played a role in a severe brain abscess in a patient whose antibiotic bill alone totaled some $17,000.

Regardless of the taxonomy, Govan frets that a friendly bug spread around as a pesticide could easily turn ugly. "My major worry is that this organism has a genome twice the size of E. coli," he says. "The potential for mutation and adaptation is quite considerable."