Type I pneumococci injected into the leg muscles of otherwise normal mice reached a maximum total population of approximately 106 organisms. In mice rendered severely leucopenic by previous irradiation the maximum bacterial counts recorded were of the order of 109. Since the lesions in the latter animals were relatively acellular, the thousandfold difference in the two experiments represented a rough measure of the antibacterial action of the leucocytic exudate.

The suppressive effect of the leucocytic exudate was shown by histologie studies to involve phagocytosis. The ingestion of pneumococci was clearly demonstrable within the first 12 to 18 hours. Accordingly, it was attributed to surface phagocytosis. In support of this conclusion was the finding that type III pneumococci reached a significantly higher total population in the myositis lesions than did type I. The type III strain used had been previously shown to be resistant to surface phagocytosis during active growth, whereas the type I strain was known to be susceptible throughout its growth phase. Evidence was also presented that the dense leucocytic exudate probably caused in addition a significant degree of bacteriostasis.

When penicillin therapy was begun 9 hours after inoculation, the pneumococci were cleared from the lesions with equal rapidity regardless of the presence or absence of leucocytic exudate. At this early stage the pneumococci were multiplying rapidly in the lesions of both the irradiated and unirradiated mice and therefore were promptly killed by the direct action of the penicillin. When the start of treatment was delayed, however, until 24 hours after inoculation, the bacteria in both sets of lesions had already reached their maximum counts and therefore were presumably resistant to the bactericidal effect of the antibiotic. Under such circumstances the destruction of the bacteria was found to be significantly less prompt in the acellular lesions than in those with a normal cellular exudate.

It is concluded from these findings that, in established pneumococcal myositis in mice, the curative effect of penicillin is due, not to the bactericidal action of the antibiotic alone, but rather to the combined effect of the drug and the cellular defenses of the host. The same conclusion also appears to be applicable to analogous acute infections in man, particularly when they are sufficiently advanced to be definitively diagnosed.

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