Fig. 4. (a) Transverse section through a L. pneumophila bacterium enclosed in a vacuole. The U937 cells were exposed to L. pneumophila for 30 minutes, then washed free of unattached bacteria and incubated for an additional 5.5 hours before fixation. The surface of the phagosome has reduced numbers of attached ER vesicles by this time. In their place are ribosomes that are directly attached to the phagosome. (b) Thin sections through a vacuole in a U937 cell containing two L. pneumophila. This U937 cell had been exposed to L. pneumophila for 30 minutes and after removal of unattached bacteria incubated for 19.5 hours. By this time, the L. pneumophila had replicated in the rough ER – identified as such by ribosomes attached to the membrane of the vacuole.
