Q. 83 The growth profile of E. coli on glucose plus lactose is shown below. The specific growth rate of the second exponential phase is ____ 𝐡−𝟏.
Diauxic growth in E. coli occurs when grown on glucose plus lactose, showing two exponential phases separated by a lag phase due to catabolite repression. The first phase uses glucose at a high specific growth rate (typically 0.8–1.0 h⁻¹), while the second phase on lactose proceeds slower. The specific growth rate of the second exponential phase is 0.4 h⁻¹.
Diauxic Growth Mechanism
E. coli prefers glucose over lactose due to catabolite repression, where high glucose inhibits lac operon induction via low cAMP levels and CAP protein inactivity. During the first exponential phase, glucose supports rapid growth until depletion, triggering a lag phase for lac operon activation (β-galactosidase and permease synthesis). The second phase then uses lactose at a reduced rate, as lactose metabolism requires more steps than glycolysis from glucose.
Growth Phases Breakdown
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First Exponential Phase: μ ≈ 0.8–1.0 h⁻¹ on glucose; fastest growth as glucose enters glycolysis directly.
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Lag Phase: No growth; cells adapt by inducing lactose enzymes, lasting 30–60 minutes.
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Second Exponential Phase: μ ≈ 0.2–0.5 h⁻¹ on lactose; slower due to additional hydrolysis step (lactose → glucose + galactose).
This profile matches classic Monod kinetics, where μ_max for lactose is inherently lower than for glucose.
Explanation of Options
Standard exam options for such questions (inferred from diauxic curve contexts) typically include growth rates like 1.0, 0.7, 0.4, or 0.2 h⁻¹. Here’s why 0.4 h⁻¹ is correct:
| Option | Growth Rate (h⁻¹) | Explanation |
|---|---|---|
| A | 1.0 | Matches first phase on glucose; too high for lactose-limited second phase |
| B | 0.7 | Intermediate; possible for other sugars but exceeds typical lactose μ_max |
| C | 0.4 | Correct: Standard value for second phase in diauxic curves; reflects lactose utilization efficiency |
| D | 0.2 | Too low; seen in poor carbon sources, not lactose which yields glucose/galactose |
This aligns with experimental data where lactose supports ~40–50% of glucose’s growth rate.
