Fig 5.

Induction of the Y. pestis PhoP-PhoQ regulatory system during infection of X. cheopis fleas. Relative amounts of phoQ mRNA expressed by Y. pestis KIM6+ in logarithmic-phase (log) and stationary-phase (stat) cultures grown at 21 or 37°C in media containing high (+) or low (−) Mg²⁺ (A) and in fleas 3 h and 2 weeks after infection (B) were determined by quantitative RT-PCR. As a control, relative expression of caf1, a highly expressed gene which is known to be downregulated at 21°C and in the flea, was also determined. The means and SEM from three experiments performed in triplicate are shown. *, P < 0.05 compared to Mg⁺ cultures by one-way analysis of variance and Tukey's multiple comparison test; **, P = 0.0005 compared to the 3-h sample by unpaired t test (two tailed).