A theoretical model for evaluating a specific mode of lethal injury that causes reproductive death in an irradiated microbial population.

When microbial cells exposed to γ-rays and UV light are cultured in a liquid medium, reproductive death is observed. These cells can divide for one to several generations but cannot form visible colonies on an agar medium and are therefore eventually recognized as dead. We call this specific type of lethal injury "ρ injury" and the apparent growth observed "phantom growth". To estimate such a lethal cell population, we propose in this study the use of a double subculture method that combines a derivative of the previously reported growth delay analysis (GDA) method and the conventional colony count method. Furthermore, a novel theoretical model is developed for this purpose.