Heavy M. paratuberculosis infection of J774 macrophages. Macrophages were infected at a 30:1 ratio and processed for transmission electron microscopy at 0 (A), 24 (B), 48 (C), and 72 (D) hours. Long arrows in A identify extracellular M. paratuberculosis whereas short arrows in A and B show intracellular M. paratuberculosis within vacuoles, presumably phagosomes. Membrane protrusions indicative of actin rearrangements involving phagocytosis can be seen in the top center of panel A. The inset (E) shows the intracellular detail of selected bacilli. Arrows in (D) identify membrane blebs. Scale bars = 1 μm.