A differential killing assay for mutagens and carcinogens based on an improved repair-deficient strain of Escherichia coli.

The lexA gene suppresses the spontaneous inviability of recA strains of Escherichia coli without affecting their repair deficiency. We have taken advantage of this to construct a uvrA recA lexA triple mutant CM871 that combines extreme repair deficiency with near wild-type growth. We have used this strain in conjunction with an isogenic strain WP67 uvrA polA and isogenic wild-type strain WP2 in a differential killing test. Dilute suspensions in buffer of the repair-deficient strains and repair-proficient control are incubated for 2 h at 37 degrees C with test compound in the presence or absence of a S9 metabolising system. Survival is determined by Miles Misra plating. For each strain three 10 microliter spots of each of three concentrations of test compound and of the untreated control are placed on a nutrient agar plate. Following overnight incubation survivors are counted. Results with reference compounds are given and the advantages and disadvantages of the test are discussed.