On phagosome individuality and membrane signalling networks

Cells such as macrophages take up pathogens into specialized membrane organelles (phagosomes) that fuse with other organelles, including lysosomes, in a process termed maturation. The fully matured phagolysosome is a low-pH, hydrolase-rich killing device that some pathogens can bypass. One might expect that phagosomes containing a given type of particle that entered cells simultaneously via the same receptor would behave the same, at least in a single cell. Surprisingly, however, recent data show that phagosomes formed via the same receptors can find themselves in different chemical states even within the same macrophage. Here, I argue that each phagosome is an individual entity whose behaviour depends on a finite number of stable equilibrium states in its membrane signalling networks.