#diauxic growth

Microbial Physiology

Diauxic growth in bacteria occurs when:

When culture medium supplies two carbon sources with differing energy yields, growth pattern shows two exponential phases separated by brief lag, termed diauxie. After consumption of preferred substrate usually glucose, cells pause to reprogram transcriptome before second substrate utilization. Molecular basis is carbon catabolite repression. Glucose transport via phosphotransferase system dephosphorylates EIIA-Glc, which blocks secondary sugar permeases via inducer exclusion and inhibits adenylate cyclase lowering cAMP, so catabolic operons like lac, ara, mal remain uninduced because CRP cAMP complex unable to bind DNA. When glucose exhausted, EIIA phosphorylated, exclusion relieved, cAMP rises, CRP activates alternative operons, specific repressors like LacI released by allolactose, enzymes synthesized enabling second growth phase. Process illustrates hierarchical sugar utilization ensuring energy efficient allocation of ribosomes, avoiding simultaneous synthesis of many hydrolases. In nature mixed substrates sequential use shapes competition, cross feeding, niche specialization in gut microbiota where Bacteroides species sequentially degrade glycans and influence community dynamics.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 7: Diauxic growth - Switch between carbon sources.

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Microbial Physiology

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 inactive and cannot enhance lac promoter activity. Lac repressor LacI remains bound to operator, beta-galactosidase LacZ is not synthesized. When glucose exhausts, EIIA becomes phosphorylated, inhibition lifts, cAMP surges, CRP-cAMP complex binds upstream DNA bending it for RNA polymerase, plus allolactose isomer inactivates LacI. A brief lag occurs while LacY and LacZ are synthesized de novo, then second exponential phase proceeds on lactose hydrolyzed to glucose and galactose. This biphasic pattern ensures energy optimization, avoiding costly parallel enzyme production.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Microbial Metabolism - Diauxic growth and catabolite repression.

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Microbial Physiology

Diauxic growth occurs when:

Diauxic growth phenomenon first described by Jacques Monod in Escherichia coli grown on mixture of two sugars illustrates hierarchical carbon utilization strategy to maximize energetic efficiency. Cells first consume preferred carbon source glucose offering highest ATP yield per carbon and fastest uptake via phosphotransferase system, generating metabolic signals via dephosphorylated EIIA-Glc that reduce adenylate cyclase activity and cAMP levels plus cAMP-CRP catabolite activator protein thereby diminishing transcription of secondary catabolic operons needed for alternative sugars like lactose. When glucose is fully exhausted, growth rate temporarily crashes creating plateau known as diauxie lag during which derepression occurs and lac operon encoding lactose permease LacY and beta-galactosidase LacZ is induced enabling import and hydrolysis of lactose to glucose plus galactose. Growth curve thus shows two distinct exponential phases separated by brief lag. Molecular basis centers on PTS regulation, cAMP modulation and inducer exclusion where EIIA-Glc blocks lactose entry while glucose present. Oxygen depletion triggers anaerobic switch, binary fission halt corresponds to stationary entry, ATP absence equals death, none describe biphasic growth which specifically reflects sequential catabolism of two carbon sources.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 14: Diauxic Growth and Carbon Source Shifts.

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Lac Operon

Lag phase in diauxic growth is due to

During diauxic transition, glucose depletion triggers transient growth arrest because cells initially lack sufficient lactose permease and beta-galactosidase to sustain energy generation from lactose. Lag corresponds to interval required for residual cAMP elevation, CAP-cAMP complex binding upstream of promoter, derepression of LacI repressor by allolactose derived from trace lactose entry through basal permease molecules, transcription of lacZYA polycistronic mRNA, translation, membrane insertion of functional LacY transporters, and establishment of positive feedback loop that amplifies intracellular inducer concentration. Once enzymatic capacity accumulates adequately, exponential growth on lactose resumes, so pause reflects induction kinetics rather than cell death or mutation event.

Ref: Journal of Bacteriology General Stress Sigma during diauxic shift – lag reflects lac permease and β-galactosidase induction after glucose exhaustion.

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Lac Operon

Diauxic growth occurs due to presence of

Diauxic growth describes biphasic exponential growth observed by Monod when Escherichia coli is cultured with two fermentable sugars such as glucose plus lactose. Glucose is consumed first, supporting rapid growth while suppressing adenylate cyclase activity, lowering intracellular cAMP levels, preventing CAP-cAMP activation, and causing inducer exclusion where unphosphorylated EIIA-Glc blocks LacY permease function. Lac enzymes remain at basal level. After glucose exhaustion, metabolic adaptation triggers lac operon induction, cAMP rises, and second exponential phase on lactose follows, separated by intermediate lag producing characteristic double sigmoidal optical density growth curve.

Ref: Monod diauxic growth; Journal of Bacteriology: E. coli uses glucose first then lactose, biphasic growth after cAMP-CAP induction.

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