FIG. 5.

Differential expression of a promoter within a genetically identical population. Salmonella serovar Typhimurium strain JH3010 (carrying prgH-gfp⁺) was grown overnight in LO medium containing no salt. A culture containing 10⁴ bacteria per ml (final concentration) was then grown in either LO medium (solid line) or LO medium containing 0.3 M NaCl (shaded graph). Samples were collected after 4 h (A), 6 h (B), 8 h (C), 12 h (D), or 24 h (E) of growth with or without salt (F) (dotted line, LO medium; solid line, LO medium containing 0.3 M NaCl), fixed in 4% formalin, and analyzed by flow cytometry. The level of induction was calculated by comparing the fluorescence for the most fluorescent peak at each time point with the fluorescence at the same time obtained when strain JH3010 was grown in LO medium. The percentage of the population in each fluorescence peak is indicated on the graph. To ensure that the differential expression of the prgH-gfp⁺ fusion was genuine, the experiment was repeated 25 times, and the data from a single representative experiment are shown.