FIG. 4.

Effect of light on the localization of Salmonella in leaf tissue. (A) Incidence of S. enterica serovar Typhimurium (STm) on leaf surface and in internal tissue. Preexposure refers to experiments in which the whole lettuce head was preexposed to light (100 μE m⁻² s⁻¹) for 30 min before the leaves were cut and taken for internalization assay performed in the dark. Each experiment was performed in triplicate and repeated at least twice at different days. Different letters indicate significant difference (P < 0.05) between the means of surface (capital letters) and internal (lowercase letters) fields harboring bacteria by analysis of variance by the Tukey-Kramer multiple-comparison test. (B) Confocal microscopy images showing GFP-tagged bacteria residing on the surface of the leaf and in internal leaf tissues following 2-h internalization assay. Internal leaf tissue images are composed of a stack of fluorescent images taken every 1.2 μm to a depth of 100 μm along a z section of the same field. All images were overlaid with DIC images. Note Salmonella aggregation near and within stomata (indicated by white arrows) under illumination, but not in the dark. Bars, 50 μm.