A "spiral test" applied to bacterial mutagenesis assays

Area of Science:

• Microbiology
• Toxicology
• Genetics

Background:

• Bacterial mutagenesis assays are crucial for assessing chemical genotoxicity.
• Traditional methods like the spot test have limitations in detecting weak or non-diffusible mutagens and require multiple plates.
• A need exists for a more sensitive, efficient, and quantitative method for mutagenicity testing.

Purpose of the Study:

• To introduce and validate a new procedure, the spiral test, for chemical distribution in bacterial mutagenesis assays.
• To compare the efficacy of the spiral test with conventional methods, such as the spot test and plate-incorporation test.
• To evaluate the spiral test's ability to detect a wide range of mutagens, including weak, non-diffusible, and unstable compounds.

Main Methods:

• The spiral test utilizes a spiral plater instrument to dispense decreasing volumes of chemical solutions onto a rotating agar plate, creating a concentration gradient.
• The activity of 18 mutagens was assessed using the Salmonella/microsome test with two methods: dispensing on top agar (Method A) or on minimal-glucose agar before top agar addition (Method B).
• Preliminary assays were also conducted with repair-proficient and -deficient strains of Escherichia coli.

Main Results:

• The spiral test generated a significantly wider and more gradual concentration gradient compared to the spot test.
• It successfully detected non-diffusible mutagens (e.g., benzo[a]pyrene) and weak or borderline mutagens (e.g., mebanazine, procarbazine) that were missed by the spot test.
• Method B demonstrated sensitivity comparable to the plate-incorporation test but on a single plate, while Method A was more efficient for unstable mutagens.
• The spiral test effectively showed promutagen activation and direct mutagen deactivation.

Conclusions:

• The spiral test is a validated and effective method for chemical distribution in bacterial mutagenesis assays.
• It offers superior sensitivity and efficiency over the spot test, enabling detection of a broader spectrum of mutagens.
• The spiral test is suitable for semi-quantitative mutagenicity assessment and can be adapted for various bacterial strains and testing conditions.