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#auxotrophs

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Microbial genetics

Auxotrophs differ from prototrophs because auxotrophs

Auxotrophs carry mutation in a gene encoding enzyme in biosynthetic pathway, blocking synthesis of specific growth factor such as histidine, leucine, arginine, or tryptophan. Unlike prototrophs that produce everything, auxotrophs need exogenous supplement of that metabolite added to medium to grow. They are isolated after mutagenesis and used to pinpoint pathway steps and map genes. Reversion to prototrophy via back-mutation or recombination confirms gene identity. Conditional growth requirement makes auxotrophs ideal for selection in conjugation, transformation, and complementation experiments for pathway dissection.

Ref: Hartl & Jones, Genetics, 8th ed., Chapter 6: Auxotrophic Mutants and Nutritional Requirements

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Gene Dissection and Complementation

Complete medium supports growth of

Complete medium provides rich mixture of yeast extract, peptone, all amino acids, vitamins and intermediates that circumvent any biosynthetic defect through external supply. Because required metabolites provided exogenously both prototrophs capable of synthesis and auxotrophs dependent on external supply grow luxuriantly without selection. This contrasts with minimal medium which selects only prototrophs capable of autonomous synthesis. Laboratory maintenance of auxotrophic collections uses complete medium before transferring replicas to minimal plates supplemented singly to identify specific nutritional requirement, standard strategy in Neurospora and E. coli genetics.

Ref: Pierce, Genetics: A Conceptual Approach, 7th ed., Chapter 17: Culture Media for Microbial Genetics

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