What is Diauxic growth?

When a bacterium is incubated in a medium containing two sugars one monosaccharide and another di or polysaccharide (like glucose and lactose). The bacteria first utilize glucose and complete their lag and log phase. After completion of glucose utilization, they used lactose and produce and another lag phase due to the synthesis of their lactose breaking enzyme. Such types of growth have two lag phases called diauxic growth and the curve is called a diauxic curve.

• The Diauxic growth is also known as the diphasic growth represented by two growth curves intervened by a short lag phase produced by an organism utilizing two different substrates, one of which is glucose.
• The enzyme required for the utilization of lactose is β-galactosidase, which splits lactose into glucose and galactose, and the bacterium utilizes glucose for growth. 
• Galactose can be used, but after it is converted into glucose. 
• It has been confirmed that E. coli growing in a medium carrying both glucose and galactose produces a diauxic (diphasic) growth curve as in the case of glucose and lactose. 
• The same response will be observed in the case of other sugars including arabinose, maltose, sorbitol, etc. When they are utilized in a mixture with glucose by E. coli. each of these sugars is used only after the glucose has been used up in the growth medium.
• The Diauxic growth curve contains two lag phases includes the first lag phase and the Second lag phase.
• The first lag phase occurs due to the adjustment of organisms to the new medium conditions. This was followed by the utilization of glucose.
• The Second lag phase occurs when glucose is depleted from the medium and organisms become prepared to utilize lactose.

Why does diauxic growth occur?

• The process of diauxic growth is complex, it is thought that the catabolite repression or the glucose effect probably plays a part in it. 
• During the catabolite repression of lac- operon of E. coli, glucose first exerts an inhibitory effect on the transcription of the lac genes. As a result of this, the lactose-utilizing enzymes are not synthesized, even if lactose is already in the medium.
• When glucose is completely utilized by E. coli, the bacterium is now competent to transcribe the lac-operon genes resulting in the production of necessary enzymes which are required for the lactose metabolization.

Obtaining Diauxic growth curve

If we put E. coli in a medium with two types of sugars such as lactose and glucose, the glucose can directly metabolize, and lactose needs to be further processed in order to be metabolized by the cell. The cell will first prefer to digest the glucose, which endows the cell with a higher growth rate (making it more fit), and only when glucose is exhausted from the medium will it start to synthesize the machinery needed to transport and digest, and ultimately grow, albeit more slowly, on the second sugar. Thus growth on a mixture of sugars, therefore, exhibits the following phases: (1) fast growth on glucose (2) lag period (3) slower growth on lactose (4) saturation. This experiment was carried out by 
Monod 
in
 the
1940’s
 during
 his
 PhD 
work.

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References

• https://www.slideshare.net/SivaSss1/bacterial-growth-diauxic-growthsynchronous-growth-and-continuous-growth
• https://en.wikipedia.org/wiki/Diauxic_growth
• https://www.accessscience.com/content/diauxic-growth-diauxie/BR0105151
• http://rpdata.caltech.edu/courses/aph162/2009/files_2009/handouts/diauxic_pre-class.pdf
• https://www.biotechfront.com/2021/02/diauxic-growth-of-bacteria.html