*3.6. COL-R Isolates Remain Susceptible to Dual-Inhibitor Antibiotics*

Recently, physicians have started using the newly approved β-lactam/β-lactamase dual-inhibitor antibiotics to treat clinical infections that are resistant to colistin treatment. We investigated the potential for cross-resistance of these COL-R isolates to a novel dualinhibitor antibiotic, ceftazidime–avibactam (CAZ/AVI). Throughout the 36 day experimental evolution under colistin selection, we also tested the MIC of CAZ/AVI for both planktonic and biofilm populations every three days. We found that both planktonic and biofilm lifestyles showed sustained susceptibility to CAZ/AVI, throughout the 36 days of selection, even with substantial COL-R progression (Figure 6).

**Figure 6.** COL-R isolates do not generate cross-resistance to CAZ/AVI. Following 36 days of experimental evolutions of KP ATCC 43816 under colistin selection, cross-resistance to CAZ/AVI was assessed for COL-R planktonic (red) and biofilm (blue) isolates from all populations in all timepoints. The clinical breakpoint for CAZ/AVI is 16 μg/mL, and the resistance MIC area is shown in a gray background. The data points reflect the mean MIC to CAZ/AVI of three replicate populations by lifestyles and colistin treatment conditions. Data are representative of three independent experiments.
