Figure 2.

Proposed model for intracellular killing of bacteria by phagocyte and bacterial defense strategies against phagocytosis. Bacteria could be engulfed by a phagocyte into the phagosome (1), followed by fusion with a lysozyme to form a phagolysosome (2), being killed by varied strategies like pH decrease, enzymes (solid blue oval) release (2a), and production of antimicrobial NO by iNOS (2b). We propose that the bacteria containing ADI pathway genes may employ this pathway to defend these killing strategies in the following ways: firstly, the production of ammonia could probably raise the cytoplasmic pH, thereby inhibiting the formation of phagolysosome (3); secondly, the ADI pathway competes with iNOS for the common substrate (arginine), thereby reducing NO production (4); thirdly, arginine depletion would also activate the autophagy and/or apoptosis pathways, like that in cancer cells (Figure 3), to induce programmed cell death and release bacteria (5).