Screening of Carcinogens with the Prophage λclts857 Induction Test¹

The prophage λclts857 induction test with Escherichia coli K12 envA uvrB as the lysogen has been successfully applied to the screening of sparingly water-soluble carcinogens that have been dissolved in dimethyl sulfoxide and metabolically activated with liver enzymes induced either with Aroclor 1254 or phenobarbital. Growth of noninduced lysogenic cells during the test was suppressed with ampicillin, with resultant increase of sensitivity of the test. The maximum inducing activity observed was about 50% of the complete induction level attained with water-soluble carcinogens that did not require metabolic activation. High sensitivity was achieved with the use of the λclts857 prophage strain. In several instances where the Ames Salmonella-microsome test has failed to confirm the carcinogenicity of the respective carcinogens, this induction test has provided a better correlation. Of the carcinogens tested, only one false negative, namely, cyclophosphamide, was encountered. In contrast, the use of the wild-type prophage λ strain resulted in low sensitivity. The adoption of the endolysin assay technique for the assessment of induction has greatly simplified the procedures and has permitted the screening test to be performed quickly and economically.