Figure 1.

Effects of antibiotic-biocide combinations on the relative fitness of Pseudomonas aeruginosa MPAO1. Relative fitness was measured as the growth rate in the absence of antimicrobial (0), in the presence of antibiotic (A), in the presence of biocide (B), and in the presence of a combination of antibiotic with biocide (AB) normalized by the growth rate in the absence of antimicrobial. Concentrations of antibiotics were 0.06 μg ml⁻¹ for meropenem; 0.6 μg ml⁻¹ for ciprofloxacin; and 0.18 μg ml⁻¹ for gentamicin. The concentrations of the biocides (in μg ml⁻¹) are depicted in each panel. The dashed lines depict the combined effect of both antibiotic and biocide predicted under the Bliss independence model. The predicted fitness under combinatorial treatment is zero in panels in which the dashed line is absent. Combinations that show a higher measured fitness in the presence of the combination as compared to the predicted fitness are typically antagonistic (shown as red bars). Combinations that show a lower measured fitness in the presence of the combination as compared to the predicted fitness are synergistic (dashed line displayed, but measured fitness in combination typically equals zero). Additive combinations are shown as gray bars. Significance was tested with a one-sample t-test on the Bliss interaction score as described in the main text (see Figure 2). The bars represent the median and the white circles represent individual measurements of seven biological replicates (in some treatments one or two replicates had to be omitted due to difficulties in fitting the growth curves).