Diauxic growth

While growing these bacteria on various combination of sugars during his doctoral thesis research, Monod observed that often two distinct growth phases are clearly visible in batch culture, as seen in Figure 1.

At the time of the "diauxic shift", there is often a lag period during which cells produce the enzymes needed to metabolize the second sugar.

Monod later put aside his work on diauxic growth and focused on the lac operon model of gene expression, which led to a Nobel prize.

Through evolution, organisms have developed the ability to regulate their genetic control mechanisms so as to only express those genes resulting in the fastest growth rate.

In the case of the baker's or brewer's yeast Saccharomyces cerevisiae growing on glucose with plenty of aeration, the diauxic growth pattern is commonly observed in batch culture.

The intracellular genetic regulatory mechanisms have evolved to enforce this choice, as fermentation provides a faster anabolic growth rate for the yeast cells than the aerobic respiration of glucose, which favors catabolism.

After glucose is depleted, the fermentative product ethanol is oxidised in a noticeably slower second growth phase, if oxygen is available.

During his work on the lac operon of E. coli, Joshua Lederberg isolated β-galactosidase and found it in greater quantities in colonies grown on lactose compared to other sugars.

Thus, if glucose is present, cAMP levels remain low, so CAP is unable to activate transcription of the lac operon, regardless of the presence or absence of lactose.

Upon the exhaustion of the glucose supply, cAMP levels rise, allowing CAP to activate the genes necessary for the metabolism of other food sources, including lactose if it is present.