Monitoring of infections. Fluorescence and phase microscopy to monitor infections at 4 h post-infection for (a) mock infection, (b) wild-type L. monocytogenes 10403S, and isogenic mutants deficient in (c) actA or (d) prfA. Filamentous actin was stained with rhodamine phalloidin. Yellow arrows in (b) show comet tails behind wild-type L. monocytogenes (10403S). Purple arrows in (c) indicate intracellular colonies formed by actA (DP-L1942). prfA (DP-L1075) bacteria (d) are seen individually as they do not escape the endosome or replicate after being internalized. Scale bar is 10 μm.