Histone deficient plates are used to calculate the background
mutation rate, which is the control plate for the filter disk
experiment. The more colonies growing on the plate, the better the
mutagen.

The chemical compound to be tested is added to a filter paper
disk and placed on agar plates containing the tester Salmonella
strain; potent mutagens result in MORE colonies forming on
histidine free plates as compared to control plates.

The ratio of bacterial colonies to drug concentration (bc:dc) is
used to calculate the mutagenic index of a chemical compound; high
bc:dc ratios means the drug functions as a growth factor for the
cells.

The Salmonella test strain used in the Ames test contains a
mutation in the histidine biosynthetic pathway, which is used to
identify compounds that induce back mutations to permit bacterial
growth on histidine free plates.

Bruce Ames invented the Ames test to determine which chemical
compounds cause human cancer, however bacterial cells are not liver
cells and so it is not very useful to prevent gout.

The Ames test uses filter disks containing chemical compounds to
quantitate the level at which the compound is lethal to the
Salmonella tester strain; the FEWER colonies on the plate, the more
toxic is the chemical regardless of mutagenic potential.

The Salmonella tester strain is grown in histidine containing
media since it lacks the ability to synthesize its own histidine
for protein synthesis; chemical compounds that mimic histidine lead
to MORE bacterial growth on the plates.

The mechanism of reaction is attack by the 5’OH group of the
pentose on the a-phosphate of an incoming nucleoside
triphosphate.

The process requires a primer.

A specific region of the DNA is recognized and bound by the
polymerase.

The direction of polymerization is 5′ to 3′

The direction of enzyme movement on the template strand is 3′ to
5′

The process includes its own 3′ to 5′ exonuclease proofreading
mechanism.