Diauxic growth or diauxie or Diphasic growth is any cell growth characterized by cellular growth in two phases, and can be illustrated with a diauxic growth curve. Diauxic growth, meaning double growth, is caused by the presence of two sugars on a culture growth media, one of which is easier for the targetbacterium to metabolize. The preferred sugar is consumed first, which leads to rapid growth, followed by a lag phase. During the lag phase the cellular machinery used to metabolize the second sugar is activated and subsequently the second sugar is metabolized. This can also occur when the bacterium in a closed batch culture consumes most of its nutrients and is entering the stationary phase when new nutrients are suddenly added to the growth media. The bacterium enters a lag phase where it tries to ingest the food. Once the food starts being utilized, it enters a new log phase showing a second peak on the growth curve. A diauxic growth curve refers to the growth curve generated by an organism which has two growth peaks. The theory behind the diauxic growth curve stems from Jacques Monod's Ph.D. research in 1940. A simple example involves the bacterium Escherichia coli, the best understood bacterium. The bacterium is grown on a growth media containing two types of sugars, one of which is easier to metabolize than the other. First, the bacterium will metabolize all the glucose, and grow at a higher speed. Eventually, when all the glucose has been consumed, the bacterium will begin the process of expressing the genes to metabolize the lactose. This will only occur when all glucose in the media has been consumed. For these reasons, diauxic growth occurs in multiple phases. The first phase is the fast growth phase, since the bacterium is consuming exclusively glucose, and is capable of rapid growth. The second phase is a lag phase while the genes used in lactose metabolism are expressed and observable cell growth stops. This is followed by another growth phase which is slower than the first because of the use of lactose as the primary energy source. The final stage is the saturation phase. This process can also refer to the positive control of the lac operon.
