FIG 4.

Sulfate starvation inhibits phospholipase and hemolysin gene expression. (A) The kinetics of sulfate starvation were established by culturing the wild-type strain UMH9 in sulfate-replete M9 medium (+ sulfate) or sulfate-limited medium (− sulfate). Magnesium sulfate was added after growth arrest (7.5 h postinoculation) to sulfate-limited cultures to demonstrate that sulfate was the growth-limiting nutrient under the tested conditions. (B) Strain UMH9 was cultured in sulfate-replete or sulfate-limited medium as described for panel A and used for qRT-PCR gene expression analysis. Arrows indicate the points postinoculation at which aliquots from both cultures were removed for RNA isolation and cDNA synthesis. Growth curves represent the means (± standard deviations) optical densities from triplicate cultures measured in 15-min intervals. (C) Relative expression of the indicated genes was determined by qRT-PCR for cDNA generated from the experiment in panel B. Transcript levels from sulfate-starved bacteria relative to those in sulfate-replete bacteria are presented from 2 and 4 h after inoculation. (D) Gene expression in the S. marcescens fliP mutant for bacteria cultured under sulfate-limited conditions relative to that for sulfate-replete bacteria. All qRT-PCR data are reported as the means (± standard deviations) from triplicate determinations.