Product formation can be characterized in relation to growth, being growth (primary metabolites) or nongrowth (secondary metabolites) associated. Examples of growth-associated products are direct catabolic products ofthe carbon substrate, such as ethanol and citric or acetic acid. Nongrowth-associated products, comprising many antibiotics, are metabolites that are not necessary for cell growth and typically are only produced during slow or stationary growth phases. Some products, such as xanthan gum and lactic acid, are mixed growth associated in that they are produced during slow and stationary growth phases. Numerous models of product formation have been proposed, taking into account variables such as hyphal morphology, cell age, surface area, metabolic carbon flux, and plasmid copy number. A simple model expresses the growth dependence of qP as (9)
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