9. A culture medium contains two carbon sources, one is preferred carbon source (glucose) and the second is a non-preferred source (lactose). Which one below is correct regarding the nature of growth curve of E. coli cultured in this medium?
(1) Growth curve will be same as when grown in presence of only glucose.
(2) Growth curve will be same as when grown in presence of only lactose.
(3) A lag phase will be observed between the two exponential phases.
(4) Two lag phases will be observed between the two exponential phases.

 

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E. coli is renowned for its ability to utilize a wide variety of carbon sources, but it does not consume all available sugars at the same rate or in the same order. When presented with a preferred carbon source like glucose and a non-preferred source like lactose, E. coli displays a distinctive growth pattern known as diauxic growth. This article explains the nature of the E. coli growth curve in such mixed-media conditions and reveals the correct answer to the question about its growth dynamics.

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Carbon Source Preference and Catabolite Repression

E. coli’s metabolic machinery is optimized to use glucose first when it is available. This is because glucose is the most energy-efficient carbon source for the bacterium. The presence of glucose suppresses the expression of genes required for the metabolism of alternative sugars, such as lactose, through a regulatory process called catabolite repression.

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What Is Diauxic Growth?

Diauxic growth is a two-phase growth pattern observed when E. coli is cultured in a medium containing both glucose and lactose. Here’s how it unfolds:

1. First Exponential Phase: E. coli rapidly consumes glucose, growing at its maximum rate.

2. Glucose Depletion: Once glucose is exhausted, the cells temporarily stop growing. This pause is known as the lag phase.

3. Second Exponential Phase: After the lag, E. coli resumes growth by metabolizing lactose, but at a slower rate than glucose.

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The Lag Phase Explained

The lag phase between the two exponential growth phases is crucial. During this time, E. coli:

• Adjusts its gene expression profile.

• Induces the lac operon, which encodes proteins necessary for lactose uptake and metabolism.

• Synthesizes new enzymes, including β-galactosidase and permease, to efficiently utilize lactose.

This metabolic switch is not instantaneous, hence the observable lag in the growth curve.

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Growth Curve Characteristics

• With only glucose: E. coli shows a single exponential phase followed by stationary phase.

• With only lactose: E. coli grows more slowly, with a single exponential phase.

• With both glucose and lactose: The growth curve features two distinct exponential phases separated by a lag phase—a hallmark of diauxic growth.

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Correct Answer

Given the options for the nature of the E. coli growth curve in a medium with both glucose (preferred) and lactose (non-preferred):

1. Growth curve will be same as when grown in presence of only glucose.

2. Growth curve will be same as when grown in presence of only lactose.

3. A lag phase will be observed between the two exponential phases.

4. Two lag phases will be observed between the two exponential phases.

The correct answer is: (3) A lag phase will be observed between the two exponential phases.

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Biological Significance

Diauxic growth allows E. coli to maximize its growth efficiency by:

• Prioritizing the most readily metabolizable sugar (glucose).

• Only switching to secondary sugars (lactose) once the preferred source is depleted.

• Minimizing energy waste by tightly regulating enzyme production for alternative sugar metabolism.

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Summary Table: E. coli Growth Curve in Different Carbon Sources

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Frequently Asked Questions

Q: What causes the lag phase during diauxic growth?
A: The lag phase is due to the time required for E. coli to induce the lac operon and synthesize the necessary enzymes for lactose metabolism after glucose is depleted.

Q: Can E. coli metabolize both sugars at the same time?
A: E. coli prefers glucose and represses the metabolism of lactose until glucose is no longer available, resulting in sequential sugar utilization.

Q: Is diauxic growth unique to E. coli?
A: No, diauxic growth is observed in other bacteria and yeast when multiple carbon sources are present, but E. coli’s lac operon is the classic model.

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Conclusion

When E. coli is cultured in a medium containing both glucose and lactose, the growth curve is characterized by two exponential phases separated by a lag phase. This diauxic growth pattern reflects the bacterium’s regulatory mechanisms for optimal energy utilization and adaptation. Understanding this phenomenon is fundamental in molecular biology and microbial physiology.

Keywords: E. coli growth curve, diauxic growth, glucose, lactose, lag phase, exponential phase, catabolite repression, lac operon, carbon source preference, β-galactosidase, bacterial metabolism, gene regulation, sequential sugar utilization, microbial physiology.