Diauxic growth is the diphasic (two-phase) growth response seen in a culture of microorganisms making a phenotypic adaptation to the addition of a second substrate. This phenomenon was discovered and named diauxie in the early 1940s by the French biochemist Jacques Monod and is characterized by a growth phase followed by a lag, after which growth is resumed. See also: Bacterial growth; Culture

Microorganisms can feed on many types of nutrients, including sugars. The most commonly used sugar, and typically the one most efficient for growth, is glucose, and microorganisms will concentrate their cellular resources in order to make appropriate enzymes to metabolize this sugar substrate. For example, when microorganisms such as bacterial cells are in their growth phase, they will metabolize the glucose in their environment prior to using any other available sugar (for example, lactose or maltose). In fact, the presence of glucose actively inhibits alternative forms of metabolism: Only after they have depleted the glucose will the bacteria synthesize the enzymes needed to metabolize other sugars. Because the enzymes are not always active or present, they are said to be inducible. (This change in growth behavior is called a phenotypic adaptation rather than an evolutionary adaptation because it depends on a latent genetic capacity rather than a new genetic trait.) Thus, in diauxie, the bacterial cells use the available sugars sequentially, not simultaneously, which results in separate growth phases. See also: Bacteria; Bacterial physiology and metabolism; Energy metabolism; Enzyme; Enzyme inhibition; Glucose; Sugar

This sequential pattern is more favorable in an evolutionary sense because the cells will be at an advantage if they can concentrate fully on the use of whichever nutrient provides the fastest rate of growth. However, there is a lag period (the diauxic lag, shift, or delay) that occurs while the cell switches its metabolism from glucose to another sugar, thereby creating the typical pattern of diphasic or diauxic growth.