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Microbiology, 2021

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11.5 • Mutations 439<br />

Figure 11.24<br />

Identification of auxotrophic mutants, like histidine auxotrophs, is done using replica plating. After mutagenesis, colonies<br />

that grow on nutritionally complete medium but not on medium lacking histidine are identified as histidine auxotrophs.<br />

Figure 11.25<br />

The Ames test is used to identify mutagenic, potentially carcinogenic chemicals. A Salmonella histidine auxotroph is used as<br />

the test strain, exposed to a potential mutagen/carcinogen. The number of reversion mutants capable of growing in the absence of supplied<br />

histidine is counted and compared with the number of natural reversion mutants that arise in the absence of the potential mutagen.<br />

CHECK YOUR UNDERSTANDING<br />

• What mutation is used as an indicator of mutation rate in the Ames test?<br />

• Why can the Ames test work as a test for carcinogenicity?

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