Fig. 5. Community structure in microcosms without plasmid-carrying E. coli at various starting concentrations of antibiotics.

Each panel shows a different antibiotic concentration given as a percentage of the starting concentration in the main experiment (Fig. 1): 12.5% = 0.9 μg/ml piperacillin and 0.18 μg/ml tazobactam, 6.25% = 0.46 μg/ml piperacillin and 0.09 μg/ml tazobactam, 3.125% = 0.23 μg/ml piperacillin and 0.04 μg/ml tazobactam, 0% = No antibiotic. The 100% treatment (7.5 μg/ml piperacillin, 1.5 μg/ml tazobactam) is not shown; we detected negligible growth in this treatment, with 3 out of 4 replicates having no colonies at all.