Final answer:
Bacterial growth in an environment with limited glucose initially relies on glucose, but bacteria can switch to lactose when glucose is depleted thanks to regulatory mechanisms of the lac operon. The diauxic growth curve illustrates how bacteria exhibit a two-phase growth pattern when switching from glucose to lactose.
Step-by-step explanation:
When growing bacterial cells in an environment containing only a limited amount of glucose as a nutrient source, bacterial growth is initially dependent on glucose. However, as the glucose is depleted, bacteria such as E. coli can switch to utilizing other sources of energy like lactose thanks to the regulatory mechanisms of the lac operon. This operon is regulated by various factors including the catabolite activator protein (CAP) and the availability of cAMP (cyclic AMP). When glucose levels drop, cAMP begins to accumulate in the cell. This cAMP then binds to CAP, forming a complex that can activate the lac operon, leading to transcription of genes required for lactose metabolism.
The diauxic growth curve seen in studies by Jacob, Monod, and others shows this phenomenon: fast growth on glucose until it is depleted, followed by slower growth on lactose as the bacteria need time to express the appropriate enzymes. When glucose is depleted, the bacterium starts using lactose – this is triggered by the increase in cAMP levels, which then activates transcription of the lac operon.