A study of bacterial populations with induced nuclear and cellular divisions 



and several independent lines of evidence suggest that this state arises if the infecting 

 phage particle, or part of it, combines successfully with a bacterial nucleus (Murray, 

 1953; Lederberg, 1953; Jacob and Wollman, 1953; Appleyard, 1953). The prophage 

 state, though very stable, is disrupted occasionally, with the result that unrestricted 

 phage multiplication sets in, and, as mentioned above, the cell eventually lyses. 

 The Ac-pha.ge is a mutant derived from A, and it differs from the parental strain in 

 that it nearly always causes lysis of the cell it infects; this mutant can be used, there- 

 fore, to eliminate non-lysogenic, sensitive bacteria from a mixed population of lyso- 

 genic and non-lysogenic cells (Lwoff, Kaplan and Ritz, 1954). 



The system just described has been used to study the response of a synchronized 

 culture to infection with the /1-phage. Experiments of the following type were carried 

 out at various times during synchronous division: a sample of the culture was 

 diluted 1:10 into broth containing about 100 million yl-particles per ml.; after 

 exactly one minute a further 1 : 50 dilution was made to stop adsorption of phage 

 particles onto bacteria and a sample of the infected and diluted culture was plated 

 together with a large excess of ^4c-phage. All these operations were carried out at 

 25 C. The ^4-phage adsorbs very rapidly onto bacteria; under the conditions just 

 described, one minute is sufficient for about 40 per cent, cf the cells to become in- 

 fected. As there are about 100 phage particles per bacterium during this period the 

 percentage of infected cells is independent of such fluctuations in bacterial concen- 

 tration as may be encountered. On the agar plates the excess of Ac -phage eliminates 

 the non-infected and still sensitive bacteria, while permitting the lysogenized and 

 immune cells to form colonies. 



Figure 2 shows the results obtained by calculating the frequency with which sen- 

 sitive cells become lysogenic when exposed to phage A at various times during cyclic 

 temperature changes. The striking feature of the curve is the rapid increase in 

 frequency of lysogenization, by a factor of approximately two, which regularly 

 occurs some minutes after raising the temperature to 37 C; this is succeeded by a 

 slow decrease which extends over most of the following 25 C. period. Later, when 

 the results of the cytological observations have been presented, the abrupt rise in the 

 lysogenization frequency will be correlated with important changes inside the cells. 

 At this stage we can draw the provisional conclusion, however, that the rises and 

 falls in the frequency curve reflect periodic changes in intracellular conditions which 

 are important in deciding whether a cell is going to lyse or become lysogenic. This 

 can be concluded because control experiments have shown that the percentage of 

 infected cells is constant throughout the division cycle; in other words, the shape of 

 the curve of Figure 2 is the same whether the frequency of lysogenization is expressed 

 as fraction of the total cell count or as fraction of the infected cells only. In Figure 2 

 is drawn also a broken curve showing the increase in total colony count; it is im- 

 portant to note that the increase in frequency of lysogenization is much steeper than 

 the simultaneously occurring increase in colony count which, as usual, begins some 

 minutes before the temperature is raised. 



The abrupt increase in frequency of lysogenization, which appears to be induced 

 by raising the temperature to 37 C, is probably a more direct effect of the tempera- 

 ture change than is the increase in the rate of cell division. It therefore seemed likely 

 that changes in the lysogenization frequency might be induced by a treatment which 



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