The Journal of experimental medicine
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The results of the experimental analysis reported in this and the two preceding papers (10, 11) indicate that in murine pneumococcal infections penicillin per se destroys the invading organisms only in those parts of the lesions where the bacteria are multiplying rapidly and are thus maximally susceptible to the bactericidal action of the drug. In areas where the bacterial growth rate is slowed, either because the pneumococci have reached a maximum population density, or because the accumulated exudate affords a relatively poor medium for rapid growth, the destructive effect of the antibiotic is greatly diminished. In such portions of the lessions the cellular defenses of the host are observed to play a major role in eliminating the bacteria. ⋯ They pertain only to the action of penicillin in acute infections caused by penicillin-sensitive bacteria which act in the host as extracellular parasites (16). The most common human infections included in this category are those caused by pneumococci and Group A beta hemolytic streptococci.(7) Whether they apply also to infections due to penicillin-sensitive staphylococci may be questioned because of recent evidence that certain pathogenic strains will survive phagocytosis (27). In diseases such as tuberculosis, brucellosis, and typhoid fever, which are treated with antibiotics having properties different from those of penicillin (28) and which are caused by bacteria capable of intracellular parasitism (28), factors other than those considered in the present analysis must certainly be involved in the curative effect of antimicrobial therapy.