The rate of bacterial cell decline in the death phase follows:
Loss of viability after stationary eventually outweighs division, producing decline phase where total colony forming units drop. Empirical measurements show semi-log plot of survivors versus time yields linear decay, indicating constant probability of death per unit time per surviving cell, independent of absolute numbers. This defines first order kinetics analogous to chemical decomposition rate minus kd N integrated ln N equals ln No minus kd t. Decimal reduction value D equals ln10 over kd, time required for tenfold drop, used in sterilization validation. Mechanistically stochastic oxidative damage, membrane lesions, enzyme inactivation act independently. Factors elevating kd include high temperature, low pH, antimicrobial presence, oxidative stress. Zero order would imply constant absolute loss irrespective of population, unrealistic when few survivors remain. Enzyme substrate Michaelis Menten kinetics describe metabolic processes, not population death. Recognizing first order nature allows calculation thermal death times for autoclaving, prediction shelf life of stored cultures, and modeling antibiotic bactericidal activity where log linear killing precedes persister tail deviating from simple kinetics.
Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Death kinetics - First-order decline.