APS/Carin Cain

Figure 1: Illustration of the stepwise buildup of antibiotic resistance in a population of bacteria in the presence of a gradient of antibiotic. Initially, the bacteria are confined to regions of low drug concentration, where the nonmutant “wild-type” strain (WT) can survive and grow. After some generations, a slightly more resistant mutant (MT$1$) arises at the boundary of the region with high bacterial population density (dashed region), where it has a high chance to reproduce and become the dominant bacterial strain. This mutant strain is able to invade the previously unoccupied region of higher antibiotics concentration until a new steady state is reached. The process repeats: A new mutation occurs in the “selective window” close to the wave front of the expansion of the MT$1$ population and establishes the next clone of more resistant bacteria (MT$2$). This process can quickly and efficiently generate drug resistance in situations where a homogeneous drug concentration would wipe out the bacterial infection.