An enrichment is a growth condition that favors growth of a mutant over the parent strain. Enrichments often work by adding a compound that kills a proportion (but not all) of the growing cells in a population. The most common enrichment for mutants in bacteria uses penicillin, which kills growing cells (by inhibiting cell wall biosynthesis) but does not kill nongrowing cells. For example, a penicillin enrichment can be used to isolate histidine auxotrophs (mutants that grow on minimal medium with added histidine cannot grow on minimal medium without histidine) from a population of prototrophs (parental cells that can grow on minimal medium with or without histidine).
Note that penicillin only kills about 99% of the His+ cells during each round of enrichment. The remaining His+ cells are sensitive to penicillin but simply survived because they were not actively dividing during the enrichment.
A potential problem with this approach is that the many of mutants obtained from each enrichment experiment will be derived from a single original mutant (i.e. the mutants will be siblings). Because subsequent analysis of the mutants may be laborous and you do not learn anything additional by analyzing identical siblings, it is a good idea to only analyze one mutant with a particular phenotype from each enrichment. Thus, multiple, separate enrichments must be done to obtain several different alleles.
Penicillin specifically inhibits synthesis of the peptidoglycan layer of the cell wall of bacteria, so this approach will not work with Archae or Eukaryotes. However, similar approaches have been developed that take advantage of unique features of other organisms.
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Last modified July 15, 2002