#microbial growth stages

Microbial Physiology

What is the relationship between growth rate (kg) and death rate (kd) in the stationary phase?

Population dynamics of batch cultures are quantitatively described by balance between formation of new cells and loss of viability. Growth rate constant kg represents frequency of new cell formation per existing cell per unit time via binary fission, while death rate constant kd represents probability per cell per time of losing ability to form colony due to irreversible damage or lysis. In lag ks slight excess over kd but numbers appear unchanged; in log kg far exceeds kd generating rapid increase. As nutrients limit and inhibitory metabolites accumulate, replication slows, cell cycle checkpoints delay division, kd rises due to damage and energy exhaustion. At stationary culture reaches steady state where cells newly formed per hour equal cells dying per hour producing flat viable count plateau despite ongoing microscopic turnover. Mathematically dN/dt equals kg minus kd times N equals zero when kg equals kd. If kg greater than kd numbers keep rising; if less numbers fall as in death phase. Equivalent definition mu net equals mu max minus kd gives zero at stationary where mu max equals kd. This equilibrium underpins chemostat theory where dilution rate balances growth and guides optimal harvest timing before decline in industrial fermentations.

Ref: Lodish et al., Molecular Cell Biology, 8th ed., Chapter 4: Stationary Phase Growth Rate and Death Rate Equality.

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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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Microbial Physiology

Which of the following occurs in the stationary phase of bacterial growth?

When batch cultures exhaust carbon, nitrogen or phosphate and accumulate organic acids like acetate lowering pH, growth rate decelerates and matches death rate producing plateau termed stationary phase. This phase triggers global reprogramming governed by alternative sigma factor sigmaS encoded by rpoS in Escherichia coli and master regulator Spo0A in Bacillus subtilis. Cells activate stringent response via RelA sensing uncharged tRNA and SpoT synthesizing alarmones ppGpp and pppGpp which inhibit stable RNA promoters and redirect transcription to survival genes. Adaptations include highly resistant endospore formation via asymmetric septation, engulfment and cortex synthesis in spore-formers, storage of carbon as glycogen and polyhydroxybutyrate inclusions, synthesis of secondary metabolites including antibiotics, toxins and siderophores to scavenge resources and inhibit competitors, upregulation of catalase KatE and DNA-protecting Dps ferritin. Nutrient levels are low, net growth zero, maximum per-cell metabolic rate occurred earlier in mid-log where ribosomes most active. Hence sporulation and secondary metabolite production typify stationary phase survival strategy not growth.

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Stationary Phase, Sporulation and Secondary Metabolites.

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Microbial Physiology

During the log phase, bacterial cells:

Logarithmic phase also termed exponential phase reflects balanced growth where all cellular constituents increase in constant proportion and cells divide at maximal specific rate mu max set by genotype and environment. Biomass, protein, RNA, DNA and numbers double at exact constant generation time. DNA replication precisely matches division rate, ribosome content peaks up to 70 percent dry weight, peptidoglycan precursor synthesis via Mur enzymes operates maximally, and transcription of glycolysis and TCA cycle remains high while alarmone ppGpp stays low. Plotting log colony forming units versus time yields straight line slope related to mu. Physiologically cells are most uniform, possess thinnest walls, are most sensitive to beta-lactams targeting active wall synthesis and to aminoglycosides requiring active translation, and produce minimal secondary metabolites. They do not sporulate or lyse; those events dominate stationary and death phases where mass increase decouples from division. This uniform high activity distinguishes log phase and explains its use for preparation of industrial inocula, starter cultures, and for studies requiring homogeneous actively growing cells.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 4: Log Phase and Exponential Growth Dynamics.

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