Ionic strength of the challenge medium influences biofilm susceptibility to positively charged antibiotics.

<p>Bacterial survival was assessed for <i>WT</i> PAO1, PA14, <i>ΔpslAB</i>, and <i>P</i>_BAD-<i>psl</i> after exposure to colistin (A), polymyxin B (B), tobramycin (C), and ciprofloxacin (D) in the presence or without NaCl. By increasing the ionic strength of the challenge medium with NaCl, electrostatic interactions are predicted to be reduced, leading to an increased efficacy of the positively charged antibiotics colistin, polymyxin B, and tobramycin, but not of ciprofloxacin. Error bars represent SD (<i>n</i> = 3).