Division and Death Rates of Salmonella typhimurium Inside Macrophages: Use of Penicillin as a Probe

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Abstract
In mouse peritoneal macrophages infected in vitro with S. typhimurium the number of viable bacteria and the number of stainable bacteria detected by light microscopy both increased at similar rates with a doubling time of more than 1 h. Antibiotics were not present; instead extracellular bacteria were removed by frequently rinsing the cells. The bacterial doubling time in the same medium in the absence of macrophages was about 20 min. Penicillin added to macrophage monolayers rapidly entered the macrophages, reaching a diffusion equilibrium. The penicillin-induced bacterial death rate appeared to depend on the bacterial division rate and on the penicillin concentration. These properties of penicillin were used to monitor intracellular bacterial division and death rates. The results indicated that intracellular killing, with the disappearance of stainable bacteria, did not contribute to the extended doubling time in macrophages. The intracellular environment of the bacteria was probably growth inhibitory but not bactericidal.

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