Potassium standard

P. aeruginosa strains were grown overnight in a low K⁺ media (10 mM bis-tris propane pH 7.0, 5 mM succinate, 2 mM MgCl₂, 5 mM (NH₄)₂SO₄, 1 mM Na₂HPO₄, 10 μM Fe(NH₄)SO₄, 0.1% (w/v) tryptone, 0.005% (w/v) yeast extract). Overnight cultures were back diluted 1:100 in low K⁺ media, grown to mid-log phase, pelleted by centrifugation at room temperature, and the OD₆₀₀ set to 2 in a 5 mL volume of low K⁺ medium. Cultures were then induced with 0.02% (w/v) arabinose for the indicated times, cells were pelleted by centrifugation, and the supernatant collected and sterilized through a 0.2 μm cellulose acetate membrane filter (VWR, Radnor, PA). As a positive control for maximal potassium release, cells were lysed by sonication and subsequent boiling. Potassium measurements were performed by inductively coupled plasma-optical emission spectrometry (ICP-OES Optima 8300, Perkin Elmer, Waltham, MA) operating both 766.4 and 404.7 nm emission lines. Data were calibrated with a potassium standard (Sigma-Aldrich).