Diauxic shift

A diauxic shift occurs when a microorganism is grown in a batch culture with two substrates. Rather than metabolizing the two available sugars simultaneously, microbial cells commonly consume them in a sequential pattern, resulting in two separate growth phases.

When growing Saccharomyces cerevisiae with glucose and plenty of aeration, the diauxic growth pattern is commonly observed in a batch culture. During the first growth phase, when there is plenty of glucose and oxygen available, the yeast cells prefer glucose fermentation to aerobic respiration (crabtree effect). After glucose is depleted, the fermentation product ethanol is oxidised in a noticeably slower second growth phase, if oxygen is available.

Typical growth curve of yeast with glucose substrate. A graph showing the different growth phases of yeast with the number of yeast cells on the y-axis and time on the x-axis. The y-axis is a log scale. In the first phase, the lag phase, the number of yeast cells inreases only a little. In the second phase, the first exponential or exponental fermentation phase, the number of cells increases the fastest. In the third phase, the second exponential or exponential respiration phase, the yeast still increases exponentially, but less quicky than in the first exponential phase. In the fourth phase, the stationary phase, the number of yeast cells stays the same as the same. In the fifth and final phase, the death phase, the number of cells decreases.

Figure 1: Typical growth curve of yeast in high glucose substrate.