#bacterial metabolism

Microbial Physiology

Catabolite repression ensures that bacteria:

Global carbon regulation termed catabolite repression prioritizes substrate yielding highest growth rate and ATP per investment. In enteric bacteria glucose is fastest because it enters glycolysis directly without conversion steps, PTS transport phosphorylates it preserving energy, yields catabolic enzymes already expressed. Using secondary sugars simultaneously would require synthesis many glycosidases, permeases consuming ribosomes and amino acids reducing fitness. Molecular implementation: glucose transport dephosphorylates PtsG EIIA component which inhibits MalT and LacY permeases via indu

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Catabolite repression and carbon prioritization.

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Microbial Physiology

The diauxic growth curve results from the metabolism of:

Historical diauxie experiment performed by Jacques Monod in 1940s used Escherichia coli growing in mixture of glucose and lactose where total sugar limited growth. Growth curve displayed first rapid exponential on glucose, plateau slight lag, second slower exponential on lactose. Reasoning glucose metabolized via Embden Meyerhof pathway directly to pyruvate yielding ATP quickly, while lactose requires uptake via LacY permease and hydrolysis by beta-galactosidase LacZ to glucose plus galactose then Leloir pathway. Regulation ensures lac operon silent during glucose phase due to low cAMP and abs

Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: The lac operon and diauxic glucose-lactose metabolism.

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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

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

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Microbial Physiology

What happens in catabolite repression?

Glucose represents most energetically efficient carbon source giving maximal ATP per bond and direct entry into glycolysis via phosphotransferase system without induction cost. Bacteria evolved catabolite repression as global control suppressing alternative substrate utilization when glucose present to conserve resources. In Escherichia coli mechanism involves drop in second messenger cAMP due to inhibition of adenylate cyclase CyaA by dephosphorylated EIIA-Glc during glucose transport and prevention of active CRP-cAMP complex formation that binds conserved DNA sequence upstream of lac, araBAD

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Catabolite Repression and Glucose Preference in Bacteria.

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Microbial Physiology

In catabolite repression, bacteria:

Catabolite repression represents global regulatory strategy ensuring efficient use of carbon sources by prioritizing best substrate. In presence of rapidly metabolizable glucose, intracellular cyclic AMP cAMP falls dramatically due to inhibition of adenylate cyclase CyaA by dephosphorylated EIIA-Glc component of phosphotransferase system and increased phosphodiesterase activity. Low cAMP prevents formation of active CRP-cAMP complex that binds conserved motif TGTGA upstream of catabolic operons such as lacZYA for lactose, araBAD for arabinose and malEFG for maltose required for alternative sug

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Catabolite Repression and Efficient Carbon Source Prioritization.

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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 lactos

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

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Microbial Physiology

The Thioredoxin system in bacteria is essential for:

Maintenance of intracellular reducing environment and proper thiol-disulfide balance is crucial for protein function, enzyme activity and protection against oxidative damage that would otherwise cause aggregation. The thioredoxin system consists of small 12 kDa redox-active protein thioredoxin TrxA with highly conserved active site motif WCGPC containing two cysteines, flavoprotein thioredoxin reductase TrxB that transfers electrons from NADPH to oxidized thioredoxin via FAD and target disulfide-containing proteins. During oxidative stress or normal oxidative protein maturation in periplasm, i

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 21: Thioredoxin System and Disulfide Bond Reduction.

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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, separ

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

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