FIG. 1.

Survival of phoP⁺ and phoP mutant Y. pestis strains in macrophages as determined by CFU assay or fluorescence microscopy. J774A.1 cells were infected with KIM5/GFP or KIM5phoPΔ/GFP at an MOI of 5 and then incubated in the presence of gentamicin as described in Materials and Methods. (A) The infected cells were lysed at different times, and serial dilutions of the lysates were plated to determine the number of intracellular bacteria by a CFU assay. The data are the results of one representative experiment, and values from triplicate wells were averaged. The error bars indicate standard deviations. (B to D) Macrophages were infected with KIM5/GFP for 23 h, exposed to 500 μM IPTG for 1 h to induce GFP expression, and then fixed and stained with an anti-Yersinia antibody and a secondary antibody conjugated to Alexa-594. Fluorescence or phase-contrast microscopy images at a magnification of ×40 were captured using a digital camera. (B and C) Representative images of GFP fluorescence and Alexa-594 fluorescence (anti-Yersinia), respectively. (D) Fluorescence images merged with a phase-contrast image.