Diauxie
With diauxie (from Ancient Greek δί αὕξη, di AuXe = "two growth") refers to a two-phase growth of chemotrophic microorganisms in the presence of two different energy sources.
If two energy sources that can be used by the organism are available at the same time, Diauxie only uses one. The metabolism for the breakdown of the second remains switched off until the first is completely consumed. After the first energy source has been depleted, it takes a certain amount of time before the metabolism of the second begins. This results in a brief halt in growth, which is called the lag phase .
At the physiological level, the diauxia is explained by catabolite repression : the energy source that is easier to use because fewer enzymes are needed than the other ones, causes a repression of the formation of those enzymes that are specifically needed to break down the second energy source. Only when the first energy source has been used up is the formation of the enzymes required to break down the second energy source induced (so-called substrate induction). For example, in the bacterium Escherichia coli, the glucose- degrading enzymes are constantly produced and are not subject to substrate induction. The enzymes specifically required to break down lactose , on the other hand, are only formed when the glucose has been used up. Therefore, in most cases, growth with glucose occurs faster than with other sugars (glucose effect). At the genetic level, diauxia is based on both a negative and a positive control. In this context, the lactose operon in the genome of Escherichia coli has been described in great detail.
With lactose as an energy source, the time between two successive divisions ( doubling time ) in Escherichia coli is about twice as long as with glucose. The increase in bacterial mass made possible by lactose is thus significantly slower than that with glucose as an energy source.