Practice question

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.