Diauxic Growth (Diphasic Growth) | Growth of Microorganisms | Microbiology

Diauxic growth is a 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. When E. coli grows in a medium containing both glucose and lactose, it uses glucose preferentially until the glucose is exhausted.

Then after a short lag phase during which bacterium synthesizes the enzymes needed for lactose use, growth resumes with lactose as a carbon source. If this diphasic growth of E. coli is plotted in respect to bacterial density against time, two growth curves follow one after the other intervened by a short lag phase to produce a diauxic or diphasic growth curve (Fig. 19.3).

The enzyme needed for lactose use is β-galactosidase, which splits lactose into glucose and galactose, and the bacterium utilizes glucose for growth. Galactose can also be utilized, but only after it is converted to glucose. It has been demonstrated that E. coli growing in a medium containing both glucose and galactose produces a diauxic (diphasic) growth curve as in case of glucose and lactose.

Similar response has been found in case of other sugars such as arabinose, maltose, sorbitol, etc. when they are used in combination with glucose by E. coli. Each of these sugars is utilized only after glucose has been used up in the growth medium.

The cause of diauxic (diphasic) growth is complex and not completely understood, it is considered that catabolite repression or the glucose effect probably plays a part in it. In catabolite repression of the lac-operon of E. coli, glucose exerts an inhibitory effect on the transcription of the lac genes.

As a result, lactose- utilization enzymes are not synthesized, even if lactose is present in the medium. When glucose is completely consumed by E. coli, the bacterium is now competent to transcribe the lac-operon genes resulting in production of necessary enzymes that help metabolise lactose.