Phagocytosed bacteria are killed within phagolysosomes by the toxic reactive oxygen intermediates produced during respiratory burst.
Phagocytosis begins when the neutrophil recognizes and attaches to the antigen. Extended pseudopods of the neutrophil engulf the antigen and internalize it to form a phagosome (see Fig. 10.7). Specific and azurophilic granules fuse with the phagosome membrane, and the lysosomal hydrolases of the azurophilic granules digest the foreign material. During phagocytosis, the neutrophil’s glucose and oxygen utilization increases noticeably and is referred to as the respiratory burst. It results in synthesis of several oxygen-containing compounds called reactive oxygen intermediates (ROIs). They include free radicals such as oxygen and hydroxyl radicals that are used in immobilizing and killing live bacteria within the phagolysosomes. By definition, free radicals possess an unpaired electron within their chemical structure, which makes them highly reactive and therefore capable of damaging intracellular molecules, including lipids, proteins, and nucleic acids. The process by which microorganisms are killed within neutrophils is termed oxygen-dependent intracellular killing. In general, two biochemical pathways are involved in this process: the first is the phagocyte oxidase (phox) system that utilizes the phagocyte’s nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex in the phagolysosome membrane; the second is associated with the lysosomal enzyme myeloperoxidase (MPO) found in the azurophilic granules of neutrophils (Fig 10.8).
Within the phagocyte oxidase pathway, or phox system, phagocytosis proceeds by signaling the cell to produce sufficient amounts of NADPH needed to generate superoxide anions. Increased glucose uptake and shunting of NADPH metabolism is achieved via the pentose phosphate pathway (also known as pentose shunt). The cytosolic NADPH  becomes an electron donor: The NADPH oxidase enzyme complex transports electrons across the membrane to molecular O2 inside the phagolysosome to generate the free radical superoxide anions (O–2). These superoxide anions are converted into ROIs. The superoxide dismutase converts superoxide anions to singlet oxygen (1O2) and hydrogen peroxide (H2O2), which further reacts with superoxide anions to produce bactericidal hydroxyl radicals (OH–) (the neutral form of the hydroxyl ion) and more singlet oxygen molecules (see Fig 10.8).
Oxygen-dependent killing with MPO involvement occurs when azurophilic granules containing MPO fuse with phagosomes containing phagocytosed bacteria. During the neutrophil’s respiratory burst, MPO, using heme as a cofactor, catalyzes a reaction that produces hypochlorous acid (HOCl) from hydrogen peroxide (H2O2) and a chloride anion (Cl–). Hypochlorous acid, which is about 1,000 times more effective in bacterial killing than hydrogen peroxide, is further metabolized to a highly toxic hypochlorite OCl– (bleach) and chlorine (Cl2). Some of the hypochloride may spontaneously break down to yield toxic singlet oxygen (1O2) and chloride ions (Cl–) (see Fig. 10.8).
In addition, nitric oxide (NO) and other reactive nitrogen intermediates (RNIs) have also been implicated in the intracellular microbial killing mechanisms. NO has been found in neutrophils; however, it is believed that RNI-mediated killing mechanisms do not appear to have a critical role in humans. The main role of neutrophil-derived NO is to induce vasodilatation, which in turn facilitates the migration of neutrophils from blood vessels to surrounding connective tissue.