A bacterial strain uses both glucose and lactose. When glucose is depleted, lactose metabolism starts after a lag phase.
Bacteria facing mixture of carbon sources implement economical sequential use, a phenomenon first quantified by Jacques Monod in E. coli. In medium containing glucose and lactose, glucose supports faster growth and higher biomass yield, so it is prioritized. During glucose consumption, phosphotransferase component EIIA-Glc remains dephosphorylated, binding and blocking lactose permease LacY via inducer exclusion, preventing intracellular lactose accumulation. Additionally unphosphorylated EIIA inhibits adenylate cyclase, keeping cAMP low, therefore catabolite activator protein CRP remains inac
Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Microbial Metabolism - Diauxic growth and catabolite repression.
