The Ames Test: How Bruce Ames Used Bacteria to Solve the Mystery of the Murderous Mutagen

How do scientists tell whether a chemical is safe or secretly a cancer-causing mutagen? In the early 1970s, biochemist Bruce Ames built a clever test to catch the culprits, and this video follows how the #AmesTest works. It uses mutant strains of the bacterium Salmonella typhimurium, each carrying a mutation in the histidine operon that leaves the cells unable to make their own histidine and therefore unable to grow. When a mutagen reverses that mutation, the bacteria regain the ability to synthesise histidine and form visible colonies, so growth itself becomes a readout for mutation. The video then explains the improvement that made the test matter for people: adding a liver homogenate (the S9 fraction) from rats and humans, which mimics mammalian metabolism and activates otherwise harmless-looking #carcinogens into potent frameshift mutagens. You will see why flat, ring-shaped carcinogens can slip between the bases of deoxyribonucleic acid (DNA) and add or delete base pairs during replication or repair, and how a reactive side chain that bonds covalently to DNA makes a mutagen stronger still. Along the way it introduces real suspects like benzo(a)pyrene, benzidine and 2-aminofluorene, and shows why this reversion assay in #SalmonellaTyphimurium is still trusted in pharmaceutical safety labs today. Creator: Luka Biskupovich References Ames BN, Durston WE, Yamasaki E, Lee FD. 1973. Carcinogens are mutagens: a simple test system combining liver homogenates for activation and bacteria for detection. Proc Natl Acad Sci U S A 70(8):2281–2285. https://doi.org/10.1073/pnas.70.8.2281 Ames BN, Gurney EG, Miller JA, Bartsch H. 1972. Carcinogens as frameshift mutagens: metabolites and derivatives of 2-acetylaminofluorene and other aromatic amine carcinogens. Proc Natl Acad Sci U S A 69(11):3128–3132. https://doi.org/10.1073/pnas.69.11.3128 Ames BN, Lee FD, Durston WE. 1973. An improved bacterial test system for the detection and classification of mutagens and carcinogens. Proc Natl Acad Sci U S A 70(3):782–786. https://doi.org/10.1073/pnas.70.3.782 Ames BN, Sims P, Grover PL. 1972. Epoxides of carcinogenic polycyclic hydrocarbons are frameshift mutagens. Science 176(4030):47–49. https://doi.org/10.1126/science.176.4... Imray FP, Macphee DG. 1976. Spontaneous and induced mutability or frameshift strains of Salmonella typhimurium carrying uvrB and polA mutations. Mutat Res 34(1):35–42. https://doi.org/10.1016/0027-5107(76)... McCann J, Choi E, Yamasaki E, Ames BN. 1975. Detection of carcinogens as mutagens in the Salmonella/microsome test: assay of 300 chemicals. Proc Natl Acad Sci U S A 72(12):5135–5139. https://doi.org/10.1073/pnas.72.12.5135 Streisinger G, Okada Y, Emrich J, Newton J, Tsugita A, Terzaghi E, Inouye M. 1966. Frameshift mutations and the genetic code. Cold Spring Harb Symp Quant Biol 31:77–84. https://doi.org/10.1101/sqb.1966.031....