This is a time-lapse video of live Drosophila blood cells expressing Rab5-GFP and exposed to mCherry-expressing E. coli. The cell at the top has engulfed (phagocytosed) a bacterium, and over time, we see the recruitment to and then loss of Rab5 from the phagosome. A red bacterium also associates transiently with the cell in the middle, but is not engulfed (phagocytosed), and drifts away. The video was taken over the course of one hour.
The blood cells are wild-type (not mutant), and we can compare the dynamics of recruitment of Rab5 and other markers of phagosome maturation stages in these wild-type cells with cells from mutants in which phagosome maturation fails. By identifying the stage at which phagosome maturation is blocked in such mutants, we can infer the stage at which the affected gene normally functions.
Video credit: Mobina Roshandell.