Figure 1.

Expression of phoP::GFP by Salmonella Typhimurium in media of varying [Mg²⁺] and pH. Salmonella Typhimurium 14028 phoQ⁺ and the mutant phoQ101::MudJ phoQ^- were transformed with the plasmid pMLZ205 that encodes phoP::GFP. A small inoculum was grown in medium M9 supplemented with the indicated MgCl₂ concentration and adjusted to the specified pH. After 5 h of culture, the bacteria were harvested and fluorescence was analyzed by flow cytometry. (A) Representative histograms of one experiment; (B) quantitation of total fluorescence from histograms like those shown in A (TF, mean fluorescence multiplied by the % of GFP-positive bacteria). Where indicated by asterisks, bacterial viability was compromised and fluorescence was not quantified. Similar results were obtained in three independent experiments. (C) Time course of GFP expression in phoP::GFP Salmonella suspended in M9 minimal medium with 0.01 mm MgCl₂ at pH 6.0. Aliquots of the suspension were withdrawn at the indicated times and GFP fluorescence and optical density at 600 nm were measured as described in Materials and Methods. Data are representative of three similar experiments.