A "spiral test" applied to bacterial mutagenesis assays

A new procedure (the spiral test) has been set up and validated for the distribution of chemicals in bacterial mutagenesis assays. This method involves the use of a special instrument (spiral plater), which dispenses, along a spiral track, decreasing volumes of liquid samples, from the near centre to the periphery of a rotating agar plate. A gradient of concentration of a compound up to about 1500:1 is thus formed on a single plate. The activity of 18 mutagens of various potencies and chemical classes was checked in the Salmonella/microsome test by dispensing their solutions either on the surface of top agar (method A) or of the minimal-glucose agar medium, before the addition of molten top agar incorporating bacteria and eventually S9 mix (method B). Compared with the spot test, the gradient of concentration of a compound produced by the spiral diluter was much wider and more gradual. Even nondiffusible chemicals (e.g. benzo[a]pyrene and benz[a]anthracene) were efficiently detected in the spiral test, as well as very weak (e.g. mebanazine and trimethylphosphate) or borderline (e.g. perylene, 1,1-dimethylhydrazine and procarbazine) mutagens, which were negative in the spot test. Method B was at least as sensitive as the plate-incorporation test, such a goal being achieved in a single plate instead of in serial plates. Technical problems made method A less sensitive, but it was more efficient in detecting unstable mutagens (e.g. beta-propiolactone). Like the plate test, the spiral test appeared to be suitable for a semi-quantitative assessment of mutagenicity data, and was efficient in demonstrating both the activation of promutagens and the deactivation of some directly acting mutagens. Preliminary assays were also carried out with repair-proficient (WP2) or -deficient (TM1080: lexA-/polA-/R391, and CM871: lexA-/uvrA-/recA-) trp- strains of E. coli.