A Method for Detection of Mutations, Using Streptomycin Dependence in Escherichia Coli.

N studies of induced mutation it is particularly desirable to use well-conI trolled methods for the detection of mutations. MULLER’S introduction of the ClB method for detecting lethal mutations in Drosophila first opened this field of study and much work has been done with this organism in spite of the laborious technical procedures required. Microorganisms offered a simpler material for this kind of work. DEMEREC’S studies ( DEMEREC, WALLACE, WITKIN and BERTANI 1949) of spontaneous and radiation-induced mutation in the streptomycin resistance system in Escherichia coli indicated that a suitable test material for induced mutation might be the back-mutation from streptomycin dependence to nondependence. The present paper is concerned with the development of such a test, based on plating streptomycin-dependent bacteria on agar containing no streptomycin. On this medium, nondependent cells (back-mutants) are able to form visible colonies; whereas the dependent cells divide a few times (residual growth), thus increasing the chances for back-mutation. If all or most back-mutations occur after plating, exposure to a mutagenic agent before plating can be expected to increase the number of back-mutants. We shall examine in this paper the factors that affect the results obtained with this technique and that must be controlled in quantitative studies of induced mutation.