FIG. 2.

Colocalization of cell death with adherent EPEC. HeLa cells were left uninfected (A), infected with nonadherent strain E. coli HB101 (B), or infected with EPEC strain B171-8 (C and D) for 2 h. Bacteria had been fluorescently labeled by subculturing in the presence of acridine orange; after infection, HeLa cells were stained with propidium iodide for an additional 30 min as described in Materials and Methods. Original magnifications: A through C, ×600 (under oil); D, ×1,000. Propidium iodide-stained HeLa cell nuclei are 16 to 20 μm in size and fluoresce reddish orange (black arrows on white background), and clumps of adherent EPEC appear greenish yellow (yellow arrowheads). The apparent size of an individual bacterial cell (E. coli B171-8) is 1.8 μm (propidium iodide stains the central nucleoid, not the cell membrane or wall).