Fig. 9. U937 cells were infected with L. pneumophila icmW mutants for 45 minutes, then unattached bacteria were washed away and incubation was continued for an additional 45 min before fixation. As is the case with wild-type L. pneumophila, ER vesicles become attached by tiny hairs (arrows in b) to the phagosomal membrane surrounding icmW mutants. Unlike the membrane surrounding wild-type L. pneumophila, which decreases from 70 Å to 60 Å within 15 minutes, there is no observable decrease in the thickness of the membrane surrounding icmW mutants at 1.5 hours (b). (c) and (d) U937 cells were infected with icmW mutants for 45 minutes, then washed and incubated for an additional 7.25 hours. Replicating bacteria could be found at this time and the phagosome membrane surrounding them had attached ER vesicles and mitochondria. However, unlike the situation after 1.5 hours (a), the thickness of the phagosomal membrane surrounding these replicating icmW mutants had decreased from 70 to 60 Å. (d) The membrane surrounding replicating icmW mutants is the same thickness as the ER membrane.