Figure 2.

FESEM analysis of HEp-2 cells co-cultured with S. zooepidemicus strain 1-4a for 1.5 h and 3.5 h. Adherent bacteria (A) trigger three different types of invasion. Membrane ruffling and cytoskeletal rearrangements (B), large invaginations in the plasma membrane of the HEp-2 cells (C–D), and engulfment from the middle of the chain causing the cell membrane to form protrusions which overgrow the bacterial chain (^*) (E) were observed after 1.5 h of co-culturing. At the later time point (3.5 h) the bacterial cell surface showed expression of pili-like appendages (arrows) (F). Heat-inactivated strain 1-4a was adherent but showed no invasion (G) as the non-invasive control L. lactis (H) after 3.5 h of co-culturing. Bars represent 1 μm in (A–C) and (E–H) and 0.5 μm in (D).