SUMMARY: The growth of strain 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 2 bacteria, or when the total number of bacteria/4 in. plate exceeds 10.

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 1, after twelve bacterial generations on nutrient agar at 37°, the value of 0.74 ± 0.04 mutations/10 bacterial divisions was obtained, and after eighteen bacterial generations, 0.70 ± 0.04.

Resistance to l phage is probably brought about by at least five separate mutations, as shown by tests of resistance to other phages. Different strains of yield different proportions of some of these mutants. Thus strain yielded a lower proportion of the mutant 1 than did strain Furthermore, as different strains of l phage may contain varying amounts of the phage mutant 1, capable of lysing the bacterial mutant /l, but not 1, 5, variations in the relative proportions of 1 and 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.


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