Confocal micrographs of GFP-EcO157 cells colonizing romaine lettuce leaf tissue (cultivar Green Towers). (A) Sparse colonization of healthy tissue by EcO157 with greater proliferation around a stomate (long arrow). (B) High density of EcO157 cells in the necrotized tissue due to infection with the plant pathogen B. lactucae, and association of the human pathogen with the oomycete hyphae (short arrows). The large masses of GFP-cells are evidenced by the green fluorescent signal. The yellow signal results from the GFP fluorescence of the bacterial cells co-localizing with the red autofluorescence of the leaf or of the oomycete. (C) Single optical scan longitudinally across a hypha revealing that the GFP-EcO157 cells invaded the oomycete tissue either actively, or passively via damages to the hyphal wall. In contrast, the hypha to the right remained uncolonized. In this image, the signal acquired in the red and far red was pseudo-colored blue. The left and middle images are pseudo-3D projections of multiple optical slices in the z range. Bars, 20 μm.