Robert Austin and colleagues have developed a microfluidic device that moderates the concentration of antibiotics and nutrients in the environment as a means to simulate a more naturally occurring heterogeneous environment for bacterial isolates, and they follow the emergence of drug resistance in Escherichia coli (Science 333, 1764–1767, 2011 ). The authors inoculated the microfluidic device, which includes a 96-well array, with wild-type E. coli and used two syringe pumps at the top and bottom of the array to establish a concentration gradient of ciprofloxacin. Local microenvironments were created within the wells, in which bacterial density was directly correlated with the ciprofloxacin gradient. Drug resistance emerged quickly, with the rate of development a function of the size of the initial inoculum. The authors demonstrate that the rapid emergence of drug resistance was dependent on the ciprofloxacin gradient, suggestingthatthe heterogeneous drug environment allowed for a more rapidemergence of drug resistance than did comparable homogenous drug environments. Whole-genome sequencing of the E. coli strains from multiple experiments identified four SNPs that were fixed in the drug-resistant bacterial populations. The authors suggest likely functional roles for these mutations in the development of drug resistance.