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Spiral Salmonella assay: Validation against the standard pour‐plate assay

The spiral Ames assay, an automated approach to bacterial mutagenicity testing which simplifies the test procedure and reduces the amount of drug required to generate mutagenic dose‐response information, has been evaluated and validated for routine screening. The spiral plater delivers the Salmonella bacteria, exogenous metabolic activation system and drug to the surface of a rotating agar plate one on top of another in such a way that a uniform density of bacteria is exposed to a logarithmically decreasing volume of drug. Following an incubation of 48 hr at 37°C, the plates are scanned by a laser counter, and the data are subjected to a computerized analysis. Petri plates of 15 cm diameter were used to provide a concentration range of about 250‐fold per plate. The Salmonella were concentrated 20‐fold to increase sensitivity. Thirty‐eight compounds from a variety of chemical classes, including both pharmaceuticals and known mutagens of moderate to strong potency, were tested in both the spiral and the standard pour‐plate assays. There was overall test agreement on positive or negative results for 82% of the compounds tested. When only the results from strains TA98 plus TA100 were considered, the agreement was 87%. When positive results were obtained, the fold increase over vehicle control was on average twice as great for the spiral assay compared to the pour‐plate assay. It was concluded that the two assay procedures generally provided comparable results, with the spiral assay being somewhat more sensitive in terms of dose‐response than the pour‐plate assay. © 1996 Wiley‐Liss, Inc.