Effect of extracellular serum in the stimulation of intracellular killing of streptococci by human monocytes

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Abstract
The intracellular killing of S. pyogenes, S. faecalis and S. pneumoniae by human monocytes is stimulated by the extracellular presence of heat-stable and heat-labile serum factors. A similar kind of stimulation of monocytes was described in respect to catalase-positive microorganisms. Killing of these bacteria was negligible in the absence of extracellular serum factors; a large proportion of the ingested catalase-negative bacteria were killed in the absence of such extracellular stimuli. Monocytes from patients with chronic granulomatous disease, which are unable to kill Staphylococcus aureus even in the presence of extracellular serum, killed S. pyogenes equally effectively whether serum was present or absent. This index proved to be the same as that for killing by monocytes of healthy subjects in the absence of serum. Catalase-negative microorganisms possess some kind of suicide mechanism that leads to the death of these bacteria after their ingestion by monocytes in the absence of an extracellular stimulus. The mechanism by which extracellular serum stimulates intracellular killing probably involves enzymes of the O2-dependent bacterial mechanisms of the monocytes.