Diauxic Growth of E. coli on Glucose/Lactose: 04-BS-13, Undated paper, Q9

Question 9: Diauxic Growth of E. coli on Glucose/Lactose (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

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The printed figure shows a classic two-step (diauxic) growth curve with time gridlines at 0, 4, 8, 12, 16, 20, 24, 28 h; the exact crossover points cannot be read precisely from it. The time windows below are read off the curve shape (a first rise-then-plateau, a short lag, then a second rise-then-plateau) using the standard diauxic-growth interpretation; exact hour values could shift slightly, but the qualitative sequence (glucose first, short lag, then lactose) is unambiguous from the classic shape and from the lac operon biology itself. Sub-parts (a) and (b) referenced by the question's own lettering are not printed in the paper (only (c), (d), (e) appear) and are not answered for lack of question text; the figure itself is reconstructed and reproduced below.

Fig. 9: reconstructed diauxic growth curve for E. coli on a glucose/lactose medium — optical density (total growth) vs. time, with the glucose phase, diauxic lag, and lactose phase shaded.

Biological background. When both glucose and lactose are available, E. coli preferentially consumes glucose first. Glucose (via its effect on cAMP levels and via inducer exclusion/CRP-cAMP regulation) keeps the lac operon (lacZ–lacY–lacA, encoding β-galactosidase, lactose permease, and thiogalactoside transacetylase) transcriptionally repressed even in the presence of some lactose, because catabolite repression is active only while glucose remains available (and, independently, the lac repressor LacI keeps the operon off until an inducer, allolactose, accumulates). Growth on glucose alone produces the first exponential phase and plateau seen in the curve. Once glucose is exhausted, catabolite repression is relieved (cAMP rises, CRP–cAMP activates the lac promoter) and, with residual lactose present to act as inducer, transcription of lacZYA begins — but there is a short lag (the "diauxic lag") while the cell synthesizes enough β-galactosidase and permease to resume active growth, now on lactose. This produces the second exponential rise and plateau.

(c) Onset of lacZ/lacY transcription. Transcription of the β-galactosidase (lacZ) and lactose-permease (lacY) genes begins at the point glucose is exhausted and catabolite repression is relieved — i.e. at the start of the diauxic lag, where the first (glucose) growth phase plateaus. Reading the curve: $\boxed{t \approx 8\text{ h}}$ (the end of the first exponential rise / start of the flat "lag" segment before the second rise).
(d) Period lactose is the sole C/E source. This is the second growth phase, after the diauxic lag ends and the second (steeper) rise begins, through to the final plateau: $\boxed{t \approx 11\text{ to }20\text{ h}}$.
(e) Period glucose is the carbon/energy source. This is the first growth phase, from inoculation to the point growth first plateaus (glucose exhaustion): $\boxed{t \approx 0\text{ to }8\text{ h}}$.

Sub-part	Answer
(c) lacZ/lacY transcription onset	≈ 8 h (glucose exhaustion / start of diauxic lag)
(d) lactose as sole C/E source	≈ 11–20 h
(e) glucose as C/E source	≈ 0–8 h

Topic: Diauxic growth and catabolite repression of the lac operon.

Key relations: Growth rate on the preferred substrate (glucose) > growth rate on the secondary substrate (lactose); the diauxic lag duration reflects the time to synthesize sufficient β-galactosidase/permease after derepression.

Why this works: Catabolite repression (via CRP-cAMP) plus inducer exclusion ensures cells do not waste resources expressing lactose-catabolizing enzymes while a better (glucose) carbon source remains, a classic example of gene regulation optimizing resource allocation.

Common pitfall: Confusing the "lag" phase (no net growth, enzyme synthesis under way) with the lactose-utilization phase itself (active growth on lactose, which only begins after the lag ends).

Source: Madigan et al., Brock Biology of Microorganisms, Ch. 8 § diauxic growth and catabolite repression / the lac operon.

Question 9 of 9: Diauxic Growth of E. coli on Glucose/Lactose

Question 9: Diauxic Growth of E. coli on Glucose/Lactose (20 marks)