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#nutrient depletion

3 public questions tagged with this topic.

Microbial Physiology

Which phase of bacterial growth follows nutrient depletion and accumulation of toxic waste?

Batch culture progression from exponential to stationary triggered primarily by depletion of limiting substrates and buildup of inhibitory waste. During log phase glucose, ammonium, phosphate abundant, oxygen well dissolved, growth maximal. Cells actively metabolize producing organic acids acetate via overflow metabolism, carbon dioxide, secondary metabolites that acidify medium, increase osmolarity, reduce water activity. Oxygen consumption outpaces transfer creating microaerophilic conditions, electron transport slows, proton motive force drops. Sensors RelA synthesizes ppGpp upon uncharged tRNA, SpoT monitors fatty acid starvation, activating stringent response redirecting transcription from rRNA to stress genes governed by RpoS sigma. Consequently division rate declines, cell size reduces, storage compounds polyphosphate, glycogen accumulate, ppGpp inhibits initiation. When growth limiting threshold reached, culture enters stationary where cryptic growth recycles lysed cells. This nutrient and waste governed transition explains classic growth curve shape observed in closed system without replenishment, distinguishing batch from continuous chemostat where fresh medium maintains log phase indefinitely for industrial production.

Ref: Madigan et al., Brock Biology of Microorganisms, Chapter 6: Stationary phase triggers - Nutrient depletion.

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Microbial Physiology

What is the primary reason for bacterial death in the death phase?

Transition to death phase reflects inability to maintain energy homeostasis and repair. Prolonged incubation exhausts fermentable carbon, electron acceptors, phosphate, amino acids, halting ATP generation via respiration and substrate level phosphorylation. Concomitantly toxic end products accumulate: organic acids acetate and lactate lower pH below 5, alcohols ethanol, ammonia, hydrogen sulfide inhibit cytochrome oxidases, and reactive oxygen species superoxide and H2O2 from autooxidation damage iron sulfur clusters. Oxidative lesions cause DNA strand breaks, protein carbonylation, membrane lipid peroxidation increasing permeability. Proteolysis and autolysis via peptidoglycan hydrolases Atl, LytA release intracellular content. Without ATP, chaperones DnaK GroEL cannot refold, SOS response fails, cells lyse. Survivors enter viable but nonculturable state or sporulate. Decline follows first-order kinetics, exponential decay constant kd reflects medium conditions. Understanding accumulation of wastes explains why dilution into fresh medium resuscitates growth and why industrial fed batch strategies remove or neutralize toxic byproducts to extend productive lifespan and improve yield.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Death phase - Toxic metabolites accumulation.

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Microbial Physiology

In which phase do bacteria begin dying exponentially due to nutrient depletion?

Death or post-stationary decline phase follows stationary when lethal stresses exceed repair capacity. Utilizable substrates fully depleted, waste products such as fermentation acids, alcohols, reactive oxygen species and toxic secondary metabolites accumulate to inhibitory levels, extracellular pH shifts outside optimum disrupting proton motive force and increasing competition for maintenance energy. Proteases Lon and Clp degrade non-essential proteins recycling amino acids, nucleases degrade rRNA, membrane depolarization causes ion leakage. Viability loss typically follows exponential first-order kinetics where death rate constant kd exceeds growth rate constant kg causing logarithmic decline in viable count on semi-log plot. Small subpopulations of persister cells that are dormant and highly tolerant and spores in spore-formers may remain viable long-term and reseed growth when conditions improve. Lag is adaptation with negligible death, log is net increase, stationary is dynamic equilibrium where division equals death, distinguishing death phase as exponential population decrease due to starvation and toxicity important for sterilization modeling, antibiotic persistence and culture stability assessment.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 4: Death Phase and Nutrient Depletion.

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