The phagocytosis and survival of staphylococci in the presence of rabbit macrophages has been studied quantitatively. The method permitted an independent measurement to be made of intracellular and extracellular bacteria during the course of phagocytosis. It was found that S. aureus was relatively resistant to phagocytosis. In the presence of specific immune serum, however, it was ingested at a rate comparable with that of S. albus in normal serum; under these conditions more than 90 per cent of bacteria of either strain were inactivated within 60 minutes. Since immune serum did not alter the susceptibility of S. aureus to inactivation within macrophages, it is concluded that S. aureus and S. albus are similar in respect to their ability to survive within macrophages.

The inactivation of staphylococci occurred more slowly, and a greater percentage survived incubation for 180 minutes, in macrophages than in polymorphonuclear leucocytes. Moreover, they retained their morphology for many hours within the former cell, but disappeared rapidly from the cytoplasm of granulocytes. It is suggested that the antibacterial mechanisms of the two cell types are fundamentally different.

When studied over a prolonged period the staphylococci which survived early inactivation within macrophages were observed to die slowly over many hours. When they were recovered from cells and exposed to fresh macrophages, however, they were ingested and inactivated as readily as bacteria of the original population. It is suggested, therefore, that the prolonged survival in vitro of some staphylococci is due to variation in the efficiency of the antibacterial mechanism of exudative mononuclear phagocytes.

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