Fig. 1.

Natural selection for antibiotic resistance in liquid culture. Susceptible and resistant strains were competed in liquid culture supplemented with carbenicillin (extracellular resistance) or streptomycin (intracellular resistance). Both streptomycin (a) and carbenicillin (b) suppress the growth rate of the susceptible strain (red line shows the susceptible growth in the presence of the resistant strain, black line shows the susceptible growth alone). In contrast, the growth rate of the resistant strain is not altered by antibiotic exposure (c, d, red line shows the resistant growth in the presence of the susceptible strain, grey line shows the resistant growth alone). As a result, all antibiotic concentrations select for the resistant strain (e, f). Please note that we use different axis ranges for the susceptible (a, b) and resistant (c, d) cell division data, because the data have very different ranges. The mean and standard error of two experiments, each with n = 6 for co-cultures and n = 3 for monocultures, are shown (Colour figure online). See also Supplementary Fig. S1