Burkholderia were first discovered as plant pathogens in 1949 by Walter Burkholder, who identified them as the agent causing onion-skin rot. Later, Burkholderia species were identified as the causative agent of the disease melioidosis, a public health threat, especially in tropical countries like Thailand and in parts of Australia. B. pseudomallei, which causes melioidosis, is classified by the Centers for Disease Control and Prevention as a potential bioterrorist agent.
Other Burkholderia belong to the Burkholderia cepacia complex, a group of related bacteria that are not true pathogens but can cause "opportunistic" or hospital-acquired infections in people with weakened immune systems or with cystic fibrosis. Although some members of the Burkholderia cepacia complex have been used to protect plants from dangerous fungal infections, their potential to cause infection has resulted in severe limits on their use in agriculture.
It wasn't until many decades after Burkholder's discovery that closely related Burkholderia species were found to enter plant roots not as pathogens but as helpful symbionts — generating root nodules in which the bacteria provide nitrogen fertilizer to the plant. Bacteria that cause the formation of these nodules in legumes, such as soybeans, alfalfa and peanuts, are crucial to sustainable agricultural systems, Hirsch said.
Although the nodulating, symbiotic species of Burkholderia are related to the more dangerous species, a detailed analysis of their evolutionary relationships published earlier this year by Hirsch and her colleagues showed that the two groups have a distinct evolutionary lineage.
The harmful Burkholderia species are more resistant to antibiotics than the symbiotic and agricultural strains. In addition to the bioinformatics analysis in the current study, the team analyzed resistance to a panel of common antibiotics, and tested the potential of different Burkholderia species to cause infection in laboratory conditions.
Experiments testing the potential of the four symbiotic species to cause infection in the small nematode worm known as Caenorhabditiselegans and in human cells grown in culture verified the bioinformatics analysis, showing that the bacteria were not harmful.
"We used a variety of detailed experiments to make sure that the symbiotic species are safe to put into farmers' fields and home gardens, just like currently used nitrogen-fixing bacteria," Hirsch said. "Our goal is to have these newly discovered nitrogen-fixing bacteria be used for a more sustainable approach to agriculture in the future."
Co-authors of the PLOS ONE research included Annette Angus and Christina Agapakis, UCLA postdoctoral scholars in Hirsch's laboratory; Stephanie Fong, Paul Yang, Nannie Song and Stephanie Kano, former UCLA undergraduate researchers in Hirsch's laboratory; Shailaja Yerrapragada of the Baylor College of Medicine in Houston; Paulina Estrada–de los Santos of the department of microbiology at Mexico's Instituto Politécnico Nacional, Prolongación de Carpio y Plan de Ayala; Jésus Caballero-Mellado (now deceased) of the Genomic Sciences Center at the National Autonomous University of Mexico; Sergio de Faria of Brazil's Embrapa Agrobiologia; Felix Dakora of the chemistry department of Tshwane University of Technology in South Africa; and George Weinstock of the department of genetics at Washington University School of Medicine in St. Louis.
The research in Hirsch's laboratory is federally funded by the National Science Foundation, by grants from the Shanbrom Family Foundation, a UC Office of the President Presidential Postdoctoral Fellowship, and a L'Oréal USA for Women in Science Fellowship.
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