Revertant bacterial colony count on histidine-free agar plates
measures
Frameshift and base-pair substitution mutagenic potential (with and without hepatic CYP450 metabolic activation)
format
Agar-plate incorporation or pre-incubation assay with and without rat liver S9 microsomal fraction
pioneer
Bruce N. Ames and colleagues (University of California, Berkeley, early 1970s)
Core Principles & Analysis
Developed in the early 1970s by Bruce Ames and colleagues at UC Berkeley, the assay uses engineered Salmonella typhimurium strains carrying mutations in histidine biosynthesis genes so they cannot grow on histidine-free medium.
When a mutagenic test chemical induces a reverse point mutation or frameshift that restores functional histidine synthesis, revertant bacteria form visible colonies on minimal histidine-depleted agar plates.
Because bacteria lack mammalian cytochrome P450 enzymes that convert pro-mutagens (such as aflatoxin B1 or benzo[a]pyrene) into reactive electrophiles, compounds are tested both with and without a rat liver S9 microsomal homogenate.