SUMMARY: The rate of division of lysogenic bacteria in infected animals can be estimated by superinfecting the bacteria with a suitable mutant of the prophage before inoculation. Since, as shown by other workers, the mutant phage is usually stable and does not multiply during bacterial growth, the proportion of bacteria carrying the mutant steadily falls in a predictable manner in each bacterial generation. Comparison of the proportions of superinfected bacteria present at the start and at the end of a known period therefore gives the number of bacterial generations occurring in a known time. It is assumed that there is no selection either for or against superinfected bacteria.

Mice were inoculated with strain K12, lysogenized by phage λb and superinfected by phage λhc. The proportion of superinfected bacteria in the spleen did not usually change in the 30 hr. following inoculation. Hence, this strain of bacteria cannot produce viable progeny in the spleen.

The behaviour of the superinfecting phage differed in two ways from that described by previous workers. First, only part of the adsorbed phage was recovered after induction by ultraviolet radiation. Second, the proportion of bacteria which yielded superinfecting phage sometimes fell very rapidly when superinfected overnight cultures were diluted in a medium which allowed bacterial multiplication. This phenomenon is referred to as ‘unstable superinfection’.


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