Microorganisms can feed on many types of nutrients, including sugars. The most commonly used sugar, and typically the one most efficient for growth, is glucose.
What is Diauxic growth ?
Diauxic growth or Diphasic growth is any bacterial cell growth characterized by cellular growth in two phases, and can be illustrated with a diauxic growth curve.
Diauxic growth, meaning double growth, is caused by the presence of two sugars on a culture growth media, one of which is easier for the target bacterium to metabolize.
The preferred sugar is consumed first, which leads to rapid growth, followed by a lag phase.
During the lag phase the cellular machinery used to metabolize the second sugar is activated and subsequently the second sugar is metabolized.
This can also occur when the bacterium in a closed batch culture consumes most of its nutrients and is entering the stationary phase when new nutrients are suddenly added to the growth media.
The bacterium enters a lag phase where it tries to ingest the food. Once the food starts being utilized, it enters a new log phase showing a second peak on the growth curve.
A diauxic growth curve refers to the growth curve generated by an organism which has two growth peaks
Diauxic Growth Definition
Diauxic Growth is the diphasic (two-phase) growth response seen in a culture of microorganisms making a phenotypic adaptation to the addition of a second substrate.
This phenomenon of diauxic growth shift was discovered and named diauxic in the early 1940s by the French biochemist Jacques Monod and is characterized by a growth phase followed by a lag, after which growth is resumed.
Diauxic Growth Curve
- 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 growth curve.
- 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 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.
Diauxic growth (intro part)