Simple mechanistic model of antibiotic use and resistance in a single population. Uncolonized individuals (X) can become colonized by the sensitive bacterial strain (S) or by the resistant strain (R). Sensitive- and resistant-colonized individuals can naturally clear colonization, for example, via host immunity. Antibiotic use leads to more rapid clearance among susceptible-colonized individuals. Compare Fig. 3A from Lipsitch et al.