A University of Toronto geneticist has discovered a process that clarifies the relationship between bacterial pathogens and their plant hosts, which could eventually help in the battle against infectious disease.
"When we understand how these natural interactions work, we can control the bacteria either by traditional plant breeding methods or by engineering plants to resist the bacteria," says Professor David Guttman of the Department of Botany. "In the long run, we may be able to extend this research to identify elements of bacterial pathogens of animals such as Salmonella or E. coli." His study was published in the March 1 issue of Science.
In the article, Guttman describes the development of a functional screen, a process that uses the natural interaction between a bacteria and its host to identify bacterial protein called type III effectors. These effectors are the bacteria's primary weapons used to infect host cells and cause disease.
In the past, researchers had to "trick" the bacteria or rely on indirect means to identify the type III effectors used in pathogenesis. "Up until now, there hasn't been a direct system - such as the functional screen - for looking at the genes used in the natural interaction between a bacteria and its host," says Guttman, lead researcher of the study.
To make the screen, Guttman and his team inserted half of an identified effector randomly into the genome of the bacteria, in this case the plant pathogen Pseudomonas syringae. About 75,000 samples of the bacteria were then injected into plants. If the plants gave the appropriate response, Guttman could then trace the type III effector back to the original bacterial genome and eventually identify the new disease-associated gene.
Only 25 of the 75,000 samples gave the desired response. Of these, Guttman identified 13 effectors for the pathogen. If the screen were to be repeated enough times, every type III effector in the bacterial genome would likely be identified, he says.
By combining the information recovered from the functional screen with computer analyses, Guttman and his colleagues at the University of Chicago identified a possible 38 type III effectors for the pathogen. With this screen, Guttman has doubled the number of effectors known in the plant pathogen world. In addition, he has identified in P. syringae more type III effectors than in any other animal or plant pathogen so far. "The new screening process has opened up a tremendous pool of resources to study and understand the whole process of pathogenesis," he says.
With his research, Guttman hopes to discover why certain bacterial strains can survive on some hosts but not on others. "This has direct relevance to emerging infectious disease, agriculture and the development of pathogen resistance in plants."
Funding for this research was provided by the National Institutes of Health, the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.
U of T Public Affairs.
Professor David Guttman
Department of Botany