(A, C) and after 12 days of increasing concentrations (B, D). Empty and filled symbols (3 days, left; and 12 days, right) represent CAZ or IMI MIC after 3 and 12 days of evolution. Blue symbols evolved from B ancestors were isolated from prior biofilm selection; red squares were evolved from P ancestors with a prior history in planktonic culture. Some symbols representing identical data points are jittered to be visible. MICs were measured in triplicate and shown± SEM. All populations increased CAZ resistance at day 3 (nested one-way ANOVA, Tukey’s multiple comparison tests MIC day 0 vs. MIC day 3, p=0.0080 q = 4.428, df = 51) and at the end of the experiment (nested one-way ANOVA Tukey’s multiple comparison tests MIC 0 vs. MIC day 12, p≤0.0001, q = 11.12, df = 51). All populations increased IMI resistance at day 12 but not at early timepoints (day 3) (nested one-way ANOVA Tukey’s multiple comparison tests MIC at day 0 vs. MIC at day 12, p<0.0001, q = 9.519, df = 51; MIC at day 0 vs. MIC at day 3, p=0.3524, q = 1.969, df = 51). (E) Absolute and relative contributions of each evolutionary force. Error bars indicate 95% confidence intervals. Asterisks denote p<0.05.