Effect of various osmolytes on cytoplasmic pH recovery after a rapid pH shift.

<p>Each trace shown is a representative replicate of three biologically independent cultures. Fluorescence intensity was converted to cytoplasmic pH as described in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010078#s4" target="_blank">Materials and Methods</a> with the benzoate and KOH additions not shown. (<b>A</b>, <b>B</b>): <i>E. coli</i> strain MC4100AR Δ<i>tatABCDE</i> TorA-GFPmut3* was resuspended in M63A (5 mM HOMOPIPES; pH 7.5) and subjected to a pH shift to pH 5.5 with 8.5 mM HCl at 0.6 min (arrow). The media contained 100 mM NaCl (blue), 100 mM choline chloride (violet), 100 mM KCl (red), 200 mM sucrose (orange), 200 mM proline (cyan), or no added osmolyte (brown). (<b>C</b>, <b>D</b>): <i>E. coli</i> K-12 strains Frag1 and TK2420 transformed with the GFPmut3b reporter plasmid (pMMB311) were resuspended in M63A (5 mM MOPS, 5 mM MES; pH 7.0) with different osmolytes and subjected to an acid shift to pH 6.0 with approximately 10 mM HCl. Each panel includes Frag1 (black) and TK2420 (brown) cultures in M63A that contains less than 10 mM of both K⁺ and Na⁺. The other conditions included TK2420 with an additional 300 mM KCl (red line), TK2420 with an additional 300 mM NaCl (blue), and TK2420 with 300 mM proline (cyan).