This figure shows a schematic representation of how natural selection selects for antibiotic-resistant bacteria. The left hand side of the figure shows four susceptible bacteria cells represented as blue ovals. Following antibiotic treatment these cells die (represented as transparent blue ovals). The right hand side of the figure shows a population of bacteria in which one cell (in green) is resistant to antibiotics. Following treatment susceptible cells die however the resistant cell survives and replicates leading to a population of resistant (green cells) that do not die following antibiotic treatment.