#minimal medium

Microbial genetics

Complete medium differs from minimal medium by

Complete medium is enriched with yeast extract, peptone, casein hydrolysate, vitamins, and all amino acids, providing preformed metabolites that bypass biosynthetic needs. Minimal medium lacks these additives, forcing cells to synthesize their own. Adding supplements to minimal converts it to supplemented or complete medium that supports both prototrophs and auxotrophs. Classic Luria-Bertani, nutrient agar, or YPD are examples of complete media. The contrast allows replica plating screens where colonies grown on complete medium are tested for failure on minimal, identifying auxotrophic mutations.

Ref: Hartl & Jones, Genetics, 8th ed., Chapter 6: Culture Media Definitions

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Microbial genetics

Minimal medium supports the growth of

Minimal medium contains only essential inorganic salts, nitrogen source, carbon source such as glucose, and trace elements, without amino acids, vitamins, or nucleotides. Only organisms possessing intact biosynthetic apparatus for all growth factors can proliferate, meaning prototrophs. Auxotrophs lacking even one pathway fail to grow unless specific supplement restores requirement. Selection for prototrophic recombinants after conjugation or transformation uses minimal medium to eliminate auxotrophic parents. Therefore minimal medium distinguishes metabolic competence and allows direct scoring of wild-type alleles complementing mutations.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 6: Minimal vs Complete Medium

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Gene Dissection and Complementation

Minimal medium contains

Minimal medium deliberately excludes complex nutrients containing only simple carbon source like glucose or sucrose supplying energy, inorganic salts supplying nitrogen, phosphate and sulfate, trace elements and single vitamin biotin for Neurospora. No amino acids, purines or yeast extract added forcing organism to manufacture all macromolecular precursors endogenously via its own enzymes. This selective medium supports prototrophs but reveals auxotrophs through growth failure, enabling screens for nutritional mutants and ordering of biosynthetic steps via supplementation tests after mutagenesis in microbial genetics experiments.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 7: Minimal versus Complete Medium Design

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Gene Dissection and Complementation

Auxotrophic mutants fail to grow on minimal medium because they

Auxotrophic mutants fail on minimal medium because structural gene encoding enzyme for specific anabolic step carries lesion abolishing catalytic activity. Without functional enzyme pathway halts, intracellular pool of needed amino acid or vitamin depletes and protein synthesis stops preventing colony formation. Wild-type prototroph synthesises metabolite endogenously whereas mutant requires supplementation bypassing block. Beadle and Tatum isolated such mutants to prove each gene controls one reaction, with auxotrophy revealing gene-controlled biochemical lesion rather than defects in DNA replication or cell division machinery itself.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 14: Auxotrophs Pathway Blocks Enzyme Loss

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