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Bug Hunt

The ability of bacteria to evolve has thus far been the bugs' greatest strength, Ed Yong says at Discover's Not Exactly Rocket Science. But two Harvard Medical School researchers may have found a way to use that strength against the bugs. Tami Lieberman and Jean-Baptiste Michel have studied bacterial evolution for many years. They recently ran across an old experiment that led them to sequence microbial genomes as they shift and change within a host, Yong says. After years of looking for the right bacterial infection to follow, the duo found that Children’s Hospital Boston had archived samples from a small 1990s cystic fibrosis epidemic. "This genetic disorder stops people from clearing mucus from their lungs and airways. As a result, otherwise harmless bacteria can cause debilitating and often lethal infections, which last for years or decades," Yong says. "The one that swept through Boston was a single strain of a previously unknown microbe called Burkholderia dolosa. It went on to affect 39 people and the epidemic lasted for 16 years."

The hospital had stored samples from 14 patients, including 'Patient Zero,' giving Michel and Lieberman the chance to study the the bug's evolution, Yong says. In a study published in Nature Genetics, the team reports genomes for each archival sample, and describes how it plotted an evolutionary tree showing how the bug jumped from patient to patient, how it changed, and how it adapted to the antibiotics used to treat the patients. "Some of the samples were twice as resistant to these drugs and others, and they all shared changes in a gene called gyrA," Yong says. "Only a few mutations in this gene conferred resistance, and each strain had acquired them independently."

The team also looked for genes that had independently acquired the same mutations in other patients, as those changes appear critical to the success of an infection, Yong adds. The researchers identified 17 genes that'd built up three or more of the same mutations, indicating that these changes were likely the result of evolutionary pressures. Of those 17, six genes were novel. "No one knows what they do," Yong says. "All we know, thanks to this study, is that they’re involved in infections. By studying this secret six, we can hopefully learn more about how Burkholderia gains a foothold in its hosts, and find new ways of treating it."

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