Which statement best describes the susceptibility of Listeria monocytogenes to irradiation with respect to growth phase?

• A. L. monocytogenes is more susceptible during stationary phase.
• B. L. monocytogenes is not affected by growth phase.
• C. Logarithmic phase makes little difference for L. monocytogenes.
• D. Logarithmic phase makes some organisms more susceptible at low doses than stationary, including L. monocytogenes.

Answer: (D)

Growth state affects how cells respond to DNA-damaging irradiation because active processes during rapid growth influence vulnerability. In the logarithmic (exponential) phase, bacteria are actively dividing and synthesizing DNA, RNA, and proteins. When irradiation causes DNA breaks, those breaks disrupt replication and vital functions more readily in cells that are gearing up for division, especially at low doses where repair can’t keep pace with ongoing damage. Stationary-phase cells, by contrast, are less metabolically active and often devote resources to maintenance and stress resistance, which can make them relatively more able to withstand low levels of damage. Listeria monocytogenes, though not a spore-former, can exhibit greater susceptibility during logarithmic growth at low irradiation doses, reflecting this general principle with organism-specific variation. At higher doses, the difference between growth phases tends to diminish as the damage becomes overwhelming.