Complex media

Complex media are nutrient mixtures for growing microorganisms whose exact chemical composition is unknown or variable, often because they contain peptones or biological extracts.

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Peptone: A mixture of peptides and amino acids produced by digesting proteins, used as a nutrient source in culture media. Protein digests supply nitrogen and amino acids while making the medium’s exact composition variable.
Lysogeny broth: A nutrient-rich bacterial growth medium commonly made with tryptone, yeast extract, and sodium chloride. Its standard formulation illustrates how complex ingredients support routine laboratory growth.
Minimal medium: A culture medium containing only nutrients required for growth of a microorganism with specified capabilities. Its restricted, known ingredients enable tests that complex mixtures cannot support as cleanly.
Batch-to-batch variation: Differences in composition or performance between separately produced batches of a material. Biological extracts and digests can vary, changing growth even when the recipe is unchanged.
Tryptone: A peptide-rich digest of casein produced with the enzyme trypsin. It is a common peptone ingredient in bacterial media such as LB broth.
Nutrient agar: A general-purpose solid culture medium containing nutrients and agar for growing many nonfastidious bacteria. It uses complex nutrient sources to grow colonies on a solid surface.
Synthetic medium: A culture medium prepared from specified chemical compounds in known concentrations. It allows precise control of nutrient levels, unlike mixtures containing peptones or extracts.
Microbial growth curve: A graph of microbial population or biomass over time under specified culture conditions. Nutrient availability in a complex medium affects growth phases and measured growth rates.
Yeast extract: A soluble preparation of yeast cell contents used to supply nutrients in microbial culture media. Its mixture of vitamins, amino acids, and other compounds helps support growth without a defined formula.
Tryptic soy broth: A general-purpose broth made from enzymatic digests of casein and soybean meal. Its protein digests provide a broad nutrient mixture for cultivating diverse bacteria.