GROWTH-RATE PARADOX OF SALMONELLA-TYPHIMURIUM WITHIN HOST MACROPHAGES

The growth rate of Salmonella typhimurium U937 within host macrophages was estimated by two independent methods. The extent to which ribosom al protein L12 is acetylated to produce ribosomal protein L7 changes m arkedly with the growth rate. By this measure, the intracellular bacte ria appeared to be growing rapidly. Measurements of viable bacteria, h owever, indicated that the bacteria were growing slowly. A solution of this apparent growth rate paradox was sought by treating U937 cells i nfected with S. typhimurium X3306 with ampicillin or chloramphenicol t o help determine the number of bacteria that were actively growing and dividing in the intracellular condition. Use of these antibiotics sho wed that by 2 h after invasion, the intracellular bacteria consisted o f at least two populations, one static and the other rapidly dividing. This finding implies that previously described changes in the gene ex pression of S. typhimurium are important for the survival and/or multi plication of the bacteria within the macrophage.