= 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.<Bacteriology Culture Guide by ATCC>[Growth Media, Bacterial Culture Media: biological extracts and peptone; Glossary, Complex medium]{ref}
That distinction can be expressed quantitatively. In a constructed example, two hypothetical extracts are each present at $10\,\mathrm{g\,L^{-1}}$. Suppose a constituent X has <mass fraction> $0.01$ in the first extract and $0.02$ in the second. Its <mass concentration> is then respectively
$$
(10\,\mathrm{g\,L^{-1}})(0.01)=0.10\,\mathrm{g\,L^{-1}},\qquad
(10\,\mathrm{g\,L^{-1}})(0.02)=0.20\,\mathrm{g\,L^{-1}}.
$$
Equal 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.<Bacteriology Culture Guide by ATCC>[Growth Media, Bacterial Culture Media: range of growth factors supplied by complex media]{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.
Back to article page
