M Demerec
1951 Genetics  
NIQUE opportunities for the study of mutations, both spontaneous and U induced, have been offered by what we may call the " phage-resistance system '? of bacterial changes from sensitivity to resistance to bacteriophages. In studying this system it is easy to work with large numbers of bacteria (1 x 108 per Petri dish) ; and if sufficient quantities of phage are added, all sensitive bacteria will be eliminated whereas the resistant will survive and form colonies. A spontaneous mutation rate of
more » ... bout 1 x can easily be measured by this method, and induction of mutations at a rate as low as 1 x can be detected. It has been used in a considerable amount of research dealing with spontaneous mutability and mutability induced by X-rays, ultraviolet rays, and chemicals. In fact, for several years it was the only known method of obtaining quantitative measurements of low mutation rates. For some time I have been on the lookout for another system, capable of being studied by similar quantitative methods, that would serve to check and confirm results obtained with the phage-resistance system. W e now have found that mutations resulting in resistance to and dependence on streptomycin, and back-mutations to nondependence, have certain advantages over phage-resistance mutatipns for use in quantitative studies of mutability. I will describe here the results of several studies of spontaneous and induced mutability based on this streptomycin-resistance system. Summaries of these studies have been published in the Carnegie Institution of Washington Year Books Nos. 48 and 49 (DEMEREC, WALLACE, WITKIN and BERTANI 1949; DEMEREC et al. 1950). MATERIAL AND NOMENCLATURE The experiments were made with Strain B / r of Escherichia. coli, which is a radiation-resistant mutant of strain B (WITKIN 1947) . Its resistance to radiation was a distinct advantage in the irradiation experiments, for it permitted treatment with higher doses. In one experiment we used strain B/r/6, a mutant of B / r that is resistant to phage 6 of the T series. Several other phages of the T series were used in tests made to-detect and eliminate possible contaminations.
doi:10.1093/genetics/36.6.585 fatcat:fibviknz2jg6ddtdkggtmkclsi