BioCoach Activity

Concept 9: Energy Source Preferences of E. coli

Whenever glucose is present, E. coli metabolizes it before using alternative energy sources such as lactose, arabinose, galactose, and maltose.


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Glucose is the preferred and most frequently available energy source for E. coli. The enzymes to metabolize glucose are made constantly by E. coli.

When both glucose and lactose are available, the genes for lactose metabolism are transcribed at low levels.

Only when the supply of glucose has been exhausted does does RNA polymerase start to transcribe the lac genes efficiently, which allows E. coli to metabolize lactose.