Figure 3. Effect of S. marcescens Oral Infection on the Structure of the Midgut.

(A) S. marcescens Db11-GFP is found throughout the digestive tract after ingestion in both wild-type (upper panels) and key flies (lower panels), as observed with a dissecting microscope equipped with epifluorescent illumination. The fluorescent image (FITC channel) has been superimposed onto an oblique transmitted-light brightfield picture (except for the top left panel). Anterior is to the left. The cardia (proventriculus) is indicated by an asterisk. Scale bar is 150 μm.

(B) The bacteria distend the gut lumen (indicated by arrowheads) during the later stages of the infection. Note the thinning of the epithelium in places where the digestive tract is bloated. Scale bar is 150 μm. Intestines of insects feeding on Db11 for 24 h are indistinguishable from those of control insects fed on sugar solution.

(C–G) Confocal optical sections from the anterior part of the midgut of insects feeding on Db11-DsRed, after fixation and FITC-labeled phalloidin staining, that reveals the actin cytoskeleton enriched in the apical region of gut cells. Left panels show a longitudinal section and right panels show the transverse section from the axis displayed in each left panel. Note the thinning of the intestinal epithelium after 72 h (D) or 96 h (E) of infection as compared to the control feeding on sucrose (F) or after 24 h (C) of infection (OD = 0.1). (C–G) Scale bars are 50 μm. (G) Db11-DsRed infection at higher concentration (OD = 0.5) observed after 48 h: a bacterium inside a cell is shown at the center of the main panel and is indicated by circles in the lateral sections.

(H) Immunostainings on frozen sections from the midgut of wild-type Oregon (Or) or key mutant flies fed for 6 h on sucrose (NI: noninfected) or OD = 0.2 Db11-DsRed (Inf: infected). The α-spectrin antibody staining (green) is enriched at the cell periphery. Nucleic acids of the epithelial cell and bacteria (arrows) is shown in red (DAPI staining). Scale bar is 10 μm.