492 BACTERIOPHAGES 



be spread on Petri dishes 1 ft. in diameter, permitting the sample 

 size to be increased to the order of 10^° bacteria. If the muta- 

 tion is so infrequent that one is not likely to find a mutant in 10^" 

 cells, the sample size can be further increased by using 10 liters of 

 broth culture in which the total bacterial population is of the or- 

 der of 10 1^ cells. 



If a series of independent cultures of strain B is made, starting 

 with an inoculum of a few hundred cells in each culture so that 

 the probability of a mutant cell being present in the inoculum is 

 negligible and after incubation each of these cultures is assayed 

 for mutants, it is found that the proportion of mutants varies 

 widely from culture to culture. In one experiment (Luria and 

 Delbriick, 1943) the proportion of B/1 mutants varied from 0/10^ 

 to 303/10^, with average value 30/10^. The reason for this high 

 variability in the proportion of mutants present in independent 

 cultures is that the probability of a mutation occurring in a bac- 

 terium is the same for each cell division. For mutations which 

 occur early in the growth of a culture, the descendants of the 

 original mutant cell will form a considerable proportion of the 

 population. For mutations which occur late in the growth of the 

 culture, the number of descendant mutants will be small. This 

 variability in the proportion of mutants found in a series of inde- 

 pendent cultures has been used by Luria and Delbriick (1934) to 

 calculate the mutation rate of bacteria. 



Mutational Pattern of Strain B of E. coli 



The mutational pattern of strain B with respect to resistance to 

 the 7 T phages was first worked out by Demerec and Fano (1 945) 

 and further amplified by E. H. Anderson (1946) and Luria 

 (1946). Some of the significant properties of the more useful 

 mutants are listed in Table XXIV. 



Additional mutations of strain B are discussed in connection 

 with host range mutations of the viruses (p. 501). Most of the 

 mutants listed in Table XXIV are resistant by virtue of the fact 

 that they have lost the ability to adsorb the phages to which they 



