The species studied was Escherichia coli, or E. coli, a bacterium commonly found in the lower intestine and harmless except for certain strains that cause food-poisoning sickness and death in humans. The UW researchers evolved hundreds of populations of E.coli under environments made ever more stressful by the addition of an antibiotic that cripples and kills the bacterium. The antibiotic was ramped up at gradual, moderate and rapid rates.
Mutations at known genes confer protection to the drug. Researchers examined these genes in surviving populations from gradual- and moderate-rate environments, and found multiple mutations.
Using genetic engineering, the scientists pulled out each mutation to see what protectiveness it provided on its own. They found some were only advantageous at the lower concentration of the drug and unable to save the population at the highest concentrations. But those mutations "predispose the lineage to gain other mutations that allow it to escape extinction at high stress," the authors wrote.
"That two-step path leading to the double mutant is not available if a population is immersed abruptly into the high-concentration environment," Kerr said. For populations in that situation, there were only single mutations that gave protection against the antibiotic.
"The rate of environmental deterioration can qualitatively affect evolutionary trajectories," the authors wrote. "In our system, we find that rapid environmental change closes off paths that are accessible under gradual change."
The work was funded by the National Science Foundation, including money through the consortium known as the Beacon Center for the Study of Evolution in Action, and UW Royalty Research Funds.
The findings have implications for those concerned about antibiotic-resistant organisms as well as those considering
|Contact: Sandra Hines|
University of Washington