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.