The intraleucocytic fate of a variety of P32- and C14-labeled bacteria has been studied in both polymorphonuclear leucocytes and macrophages. Both cell types brought about extensive degradation of bacterial lipids, nucleic acids, and proteins. Intracellular breakdown was primarily dependant upon the composition of the ingested particle rather than on the type or source of the phagocyte. Evidence is presented for the reincorporation of bacterial constituents into leucocyte lipid. More than 50 per cent of the acid-soluble degradation products of P32-labeled bacteria appear as inorganic phosphate. Bacterial RNA is degraded more readily than DNA.

Following phagocytosis, labeled bacteria lose their pool of small molecular weight intermediates. This is followed by the degradation of acid-insoluble constituents. The majority of bacterial breakdown products are then excreted by the leucocyte and appear in the medium.

Heat-killed bacteria were more readily broken down than viable organisms. Only small amounts of C14-labeled bacteria were completely oxidized by leucocytic enzymes to C14O2.

Acid extracts of polymorphonuclear leucocyte granules, which were highly bactericidal, liberated the acid-soluble constituents of labeled bacteria but did not significantly degrade bacterial macromolecules.

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