FIG. 9.

Quantitation of bacteria surrounded by phagosomal membranes. THP-1 cells were infected with the F. tularensis LVS or RCI and prepared for transmission electron microscopy as described in the text. Bacteria were scored as having a phagosomal membrane if at least 50% of their circumference was surrounded by a morphologically discrete membrane bilayer. Eighty to 90% of the bacteria had identifiable membrane bilayers when they were fixed immediately after a 90-min incubation with THP-1 cells (0 h); the percentage dropped to <20% by 14 h after infection. In the first 6 h after infection, 40 to 50% of the RCI phagosomes and 58 to 88% of the LVS phagosomes had densely staining fibrillar coats (data not shown). At 14 h, it was difficult to quantify the fraction of phagosomes with fibrillar coats because very few phagosomal membranes were visible. This experiment was performed twice, with similar results.