Detection of chemical mutagens use of concentration gradient plates in a high capacity screen

A method is described which makes possible the rapid screening of chemicals for mutagenic potential. Ten fold concentration gradients of test compound in agar are prepared in square petri dishes. Ten strains of bacteria including both histidine auxotrophs and tryptophane auxotrophs are then streaked on the surface. In the activated phase of the test an overlay of 9000 g liver homogenate in agar is applied to the gradient plate before the organisms are streaked. After incubation, the plates are scored by observing the concentration range over which the test compound produces mutant colonies in each of the ten organisms. With this technique it is possible to determine on only eight plates the mutagenic potential of a substance against ten different bacterial strains, both with and without microsomal activation and over a 10,000 fold concentration range. The minimum concentration at which growth of the auxotroph is inhibited is also noted. The method has been found to be very useful for preliminary screening of large numbers of chemicals.