Complex medium

A complex medium is a culture medium containing ingredients whose complete molecular composition is not specified. Common examples include yeast extract and peptone. A recipe can state the mass of such an ingredient precisely while leaving the amounts of its individual molecular constituents incompletely known.^[ref]

That distinction can be expressed quantitatively. In a constructed example, two hypothetical extracts are each present at $10\mathrm{g}{\mathrm{L}}^{-1}$. Suppose a constituent X has mass fraction $0.01$ in the first extract and $0.02$ in the second. Its mass concentration is then respectivelyEqual masses of the whole mixture therefore permit a twofold difference in X. These fractions are invented to illustrate the information missing from an ingredient-level specification; they are not measurements of commercial extracts.

A defined medium makes individual chemical inputs easier to track. A complex medium can instead provide many growth-supporting constituents together, which can be useful when an organism's requirements are incompletely characterized.^[ref] Neither classification by itself establishes the outcome of a bacterial culture experiment.

For an observed change in growth, the useful questions include which ingredient was varied, what was measured about its chemical composition, and whether other conditions were matched. The word complex describes incomplete compositional specification; it does not mean that every measured property of the preparation is unknown.

A peptone is a water-soluble protein hydrolysate used as a nutrient ingredient in a culture medium. Partial hydrolysis produces a mixture of peptides, amino acids and related constituents. Sources and treatments vary: animal, plant or microbial material can be digested by enzymes or treated by acid.^[ref]

The chemistry explains why a peptone is not one pure molecular species. An illustrative linear peptide containing ten amino acid residues has nine inter-residue peptide bonds. Hydrolysing three distinct peptide bonds produces four fragments that still contain ten amino acid residues in total. Their mean length is $10/4=2.5$ residues. The residue inventory is conserved while its distribution among molecules changes.

The same total amino-acid-residue inventory therefore does not fix the amount of free amino acids or the sizes of the remaining peptides. A description by total nitrogen content would likewise omit information about how nitrogen-containing molecules are distributed. Those are different levels of chemical characterization.

A peptone can supply nitrogen-containing nutrients, but its effect on a particular bacterial culture depends on the preparation and biological context. The presence of an accurately weighed ingredient does not make every constituent concentration known. This is why a culture medium containing peptone is commonly a complex medium, rather than a defined medium.^[ref] Matching the name of the ingredient is a starting point for comparison, while matching its relevant measured properties is a more specific experimental requirement.

Yeast extract is the water-soluble material obtained from broken-down yeast cells. In the preparation described by ATCC, Saccharomyces cerevisiae undergoes autolysis: its own enzymes break down cellular material, including proteins into peptides and amino acids. The resulting ingredient can supply nitrogen-containing nutrients, vitamins and other soluble constituents to a culture medium.^[ref]

The word extract matters. The ingredient is a mixture of recovered cellular substances, rather than a count of growing yeast cells or one chemically defined nutrient. Adding yeast extract therefore changes several inputs together. Its known total mass does not specify every individual constituent's concentration.^[ref]

An original accounting example illustrates the distinction. Suppose an analysis of a hypothetical dry extract finds 2 milligrams of nutrient X and 1 milligram of nutrient Y per 100 milligrams of extract. A 5-milligram addition then supplies 0.10 milligrams of X and 0.05 milligrams of Y. Equal additions of total extract would supply different amounts of X if another preparation had a different measured X fraction. These values are invented and do not describe a commercial product.

A defined medium can vary an explicitly specified component while holding the others fixed. With yeast extract, interpreting an observed growth change requires considering the accompanying mixture. Neither the total mass nor one measured constituent establishes that all remaining nutrient inputs were matched. The distinction helps separate a convenient composite ingredient from a chemically isolated experimental variable.