Fig. 1.

The Cas9 regulatory axis is necessary for polymyxin resistance. (A) WT, cas9, or cas9/1103 deletion mutants were grown overnight in broth culture containing the indicated concentration of polymyxin B. Percent growth compared with untreated cultures is plotted (n = 3). (B) 10⁹ cfu of WT, cas9, or cas9/1103 deletion mutants were treated with 800 µg/mL of polymyxin B, and cfu were enumerated at the indicated times to quantify antimicrobial killing. (C and D) WT, cas9, or cas9/1103 deletion mutants were grown to midlog phase, washed, and stained with (C) propidium iodide or (D) ethidium bromide, and fluorescence was measured (n = 3). *P ≤ 0.05; **P ≤ 0.005; ***P ≤ 0.001.