LPS-induced autophagy promoted intracellular bactericidal activity and the co-localization of E. coli with autophagosomes. (A) Bacterial killing assays for E. coli were performed in HMrSV5 cells treated with or without LPS (1 μg/ml, 12 hours). E. coli (ATCC: 25922) (MOI: 20) were incubated with the cells for 60 min (t = 0). The cells were lysed at 30, 60, 90 min later with sterile distilled water and the c.f.u. was counted. Percentage of remaining E.coli (%) = remaining bacteria at each time point / bacteria present at 0 min × 100. Graph represents the mean values ± SD of percentage of remaining E.coli at different time points from n ≥ 3 experiments. (B) HMrSV5 cells were infected with fluorescent E. coli (K-12 strain, green) for 1 hour, washed and incubated for an additional 12 hours in the presence or absence of LPS. Autophagic vacuoles were labeled with MDC (blue). Scale bars: 20 μm. (C) Representative TEM images of E.coli in autophagosomes. Images 1 and 2 show E.coli were engulfed in typical single-membrane phagosomes in control cells. However, more E.coli were harboured in double-membrane autophagosomes in LPS-treated cells (images 3–6). White triangles, E.coli; white arrows, single-membrane compartments; black arrows, double-membrane autophagosomes. Scale bars: image 1 and 2: 0.5 μm; image 3, 4, 5 and 6: 200 nm. (D) The left graph shows quantitation of the co-localization of E. coli with the MDC-labeled autophagosomes in Figure 4B. The right graph indicates the quantitation of 100 internalized E. coli per experimental condition in Figure 4C (mean values ± SD, n ≥ 3). *p < 0.05 (vs. control); **p < 0.01 (vs. control).
Wang et al. BMC Microbiology 2013 13:255 doi:10.1186/1471-2180-13-255