A Method for the Measurement of Mutation Rate from Phage Sensitivity to Phage Resistance in Escherichia coli

SUMMARY: The growth of Escherichia coli strain B/r on nutrient agar is the same as in broth with regard to the initial lag period and the rate of increase during the exponential phase. On nutrient agar, growth is finally slowed down either when individual colonies contain more than 219 bacteria, or when the total number of bacteria/4 in. plate exceeds 1010.

The rate of mutation to phage resistance was measured by a modification of Demerec’s method of spraying plate cultures with a phage aerosol, and counting resistant survivals. With the phage T1, after twelve bacterial generations on nutrient agar at 37°, the value of 0·74 ± 0·04 mutations/108 bacterial divisions was obtained, and after eighteen bacterial generations, 0·70 ± 0·04.

Resistance to T1 phage is probably brought about by at least five separate mutations, as shown by tests of resistance to other phages. Different strains of E. coli yield different proportions of some of these mutants. Thus strain B/r yielded a lower proportion of the mutant B/1 than did strain B. Furthermore, as different strains of T1 phage may contain varying amounts of the phage mutant T1h, capable of lysing the bacterial mutant B/1, but not B/1, 5, variations in the relative proportions of B/1 and B/1, 5 may also occur when different samples of phage are used. Consequently, constancyof mutation rate can only be guaranteed for given cultures of both bacteria and phage.