In cell biology, a phagosome is a vesicle formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested. Phagosomes fuse with lysosomes in their maturation process, forming phagolysosomes.
Some bacterial pathogens that enter cells inside phagosomes either reproduce inside of the formed phagolysosome (Coxiella spp.) or escape into the cytoplasm before the phagosome fuses with the lysosome (Rickettsia spp.). Many mycobacteria, including Mycobacterium tuberculosis   and Mycobacteria avium paratuberculosis  , manipulate the host macrophage to prevent nitrous acid-containing lysosomes from fusing with phagosomes and creating mature phagolysosomes. Such incomplete maturation of the phagosome maintains an environment favorable to the pathogens inside it. 
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