Phagolysosome

A phagolysosome is a degradative, particle-containing compartment formed as a phagosome matures through interactions with endosomes and lysosomes. After phagocytosis, the engulfed particle can remain recognizable while its surrounding membrane and contents change. The particle's identity therefore does not establish the biochemical state of the whole compartment.^[ref]

A useful experiment followed low-density latex beads taken up by J774 macrophages. The bead-containing compartments were isolated by flotation on a sucrose gradient. In a pulse-chase analysis, a limited uptake period followed by different waiting intervals yielded compartments of different ages. Comparing their proteins and immunogold labeling revealed changing membrane composition during maturation.^[ref]

Normalization matters when interpreting such images. With invented counts, 10 gold labels along $5\mu \mathrm{m}$ of membrane give $2$ labels per micrometre; 20 labels along $20\mu \mathrm{m}$ give only $1$ label per micrometre. The second sample has more total labels but a smaller label density. Comparing membrane lengths prevents the amount of membrane observed from being mistaken for the abundance of its marker.

That measurement concerns localization of a membrane marker. It is not itself a measurement of luminal pH, degradation rate or killing of a microorganism. Those functions need corresponding observations: acquisition of one marker and completion of a degradative process answer different experimental questions.