Giant Petri Dish Displays Evolution in Space and Time. (video)

For 10 days, the researchers imaged the E. coli every 10 minutes as the microbes expanded across the plate, and saw that the bacteria paused briefly at the boundaries of increasingly stringent antibiotic concentrations until a mutant struck out into the higher-drug territory. By challenging the bacteria with differing doses of antibiotic in the first step of the gradient, the team demonstrated that E. coli evolve higher resistance more quickly if they first encounter an intermediate, rather than a high, concentration of antibiotic.

Using the easy-to-see evolutionary trajectory of the bacteria as a guide, the researchers isolated and sequenced the charge-leading mutants. They found adaptive mutations in the gene for the proofreading enzyme DNA polymerase III, the target genes of the antibiotics, and in unexpected genes such as those coding for a phosphate transporter and a kinase that don’t have a known function in establishing resistance, hinting at alternative pathways that could arise.

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