The in vivo division and death rates of Salmonella typhimurium in the spleens of naturally resistant and susceptible mice measured by the superinfecting phage technique of Meynell.

Salmonella typhimurium appears to divide faster in the spleen of naturally susceptible BALB/c than in resistant (B10 x A/J)F1 mice. S. typhimurium M526 is an LT2 derivative lysogenic for a non-excluding P22 mutant which allows superinfection with a second, non-replicating, P22 phage so that the proportion of superinfected organisms halves at each division. The true in vivo division and death rates can be calculated from successive determinations of the proportion of superinfected organisms and the viable count. It was found that the division time was 2.86 h in BALB/c and 5.02 h in (B10 x A/J)F1; the death rate was low and actually greater in the susceptible BALB/c strain. These results suggest that the gene controlling in vivo salmonella net growth rate, which is very important in natural resistance to salmonella infection, acts very early by regulating the division rate, perhaps inside macrophages. The actual mechanism remains unknown.