Fig. 1

Localization and survival of B. bifidum S17 in MCTS. HT-29 MCTS were incubated with B. bifidum S17/pVG-GFP (a S17, green) or B. bifidum S17/pVG-mCherry (b, c S17, red) for 48 h. Cryosections of MCTS were stained for hypoxia (a Hypoxyprobe-Red549, red) or apoptosis (b TUNEL, green). Nuclei were stained with Hoechst. Images were acquired with a ×10 (in a, b scale bars 100 µm; all three fluorescence channels are shown plus the merged image in the lower right panel) or ×40 (in c scale bar 20 µm) objective. The inset in the upper right corner of c is a digital zoom on the image part marked with a white box. d Viable bacteria of B. bifidum S17 WT in HT-29 MCTS were determined at different time points after inoculation. As controls (medium), bacteria were incubated under identical conditions in the absence of MCTS. For each strain, time-point and condition, six tumors were infected and combined for preparation of tumor lysates. Values are colony-forming units (CFU) per mL lysate and are mean ± standard error of the mean (SEM) of three tumor lysates infected with independent bacterial cultures. Statistical analysis was performed for each time-point using Student’s t test (***P < 0.001)