Peptone (source code)
= Peptone
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.<Base Ingredients of Microbiology Media by Merck>[Proteins: protein hydrolysates, amino acids and peptones]{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>.<Bacteriology Culture Guide by ATCC>[Growth Media, Bacterial Culture Media: peptone and chemically incompletely specified ingredients]{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.
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