When the bacteria arrived at the first moderate but lethal dose, they stopped spreading, but only for a while. Eventually, a pocket of bacteria mutated to resist the antibiotic dose, and a bloom of evolved E. coli proceeded beyond the boundary. This kept occurring as new generations of bacteria evolved to survive usually lethal antibiotics, until finally, on day 11, they conquered the highest dose and took over the entire petri dish.
What did this teach the scientists? For one thing, it showed that the most dangerous bacteria isn't always the fastest growing. Some of the bacteria it took to win over the thousandfold antibiotic concentration were stragglers that quietly evolved super-resistance at lower concentrations while other generations were speeding ahead of them. Researchers also think this giant-petri-dish model will be useful in studying what different nutrients and other environmental constraints do to bacterial populations. Learn more about so-called "superbugs" in the videos below.