#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 checkpo

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-

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 resi

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 vers

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

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