Revived interest in bacteriophages

Tree life: The entire horse chestnut tree is the environment that shapes the mutual adaptation of Pseudomonas syringae bacteria and their phages, as research from Koskella and colleagues has shown. (Photo: Photolibrary.) In the unlikely setting of Oxford University’s Zoology Department, NERC research fellow Britt Koskella studies horse chestnut trees, their bacterial pathogens and, most importantly, the bacteriophages that infect those bacteria. It’s all ecology, she says, which fits in with the interests of the department. But why horse chestnuts? “A lot of work on phages has been done in water,” Koskella explains, “but in the leaves of the chestnut tree we have a well-defined environment where we can pin down the interactions of phages and their hosts.” The bacterial hosts are Pseudomonas syringae, which cause bleeding canker disease in the trees. Koskella and her colleagues wanted to find out about the length scale on which bacteria and phages adapt to each other. In other words, is their frame of reference the border of the leaf, the entire leaf, or perhaps the entire tree? They found out that the tree is an important unit defining the relationship between phage and host. Phages were highly efficient at infecting bacteria sampled from elsewhere in the same tree, but much less so for bacteria from other trees (Am. Nat. (2011), 177, 440-451). Building on these findings, Koskella is now further exploring the ecology of phages in the chestnut tree to see whether their presence actually helps the tree fight off the disease. She is also interested in agricultural applications of phages, for instance in cultivation of tomatoes. “One of our motivating factors for this work is determining whether phages would be good candidates for control of plant pathogens in naturally variable populations,” Koskella explains. There is, she says, a revived interest in bacteriophages which can build on a long but frequently interrupted history of achievements.