Time-dose reciprocity mechanism for the inactivation of Escherichia coli using X-ray irradiation.

The time-dose reciprocity has long been a cornerstone in understanding ultraviolet (UV) sterilization. However, recent studies have demonstrated significant deviations from this law, attributed to complex mechanisms involving reactive oxygen species (ROS). This study investigates whether similar deviations occur at much shorter wavelengths of electromagnetic radiation than UV, specifically in the X-ray region, with a focus on the dose-rate dependence of bacterial inactivation. Using Escherichia coli as a model organism, it is found that dose-rate effects were highly dependent on the bacterial growth phase. In the stationary phase, lower dose rates with prolonged irradiation resulted in greater inactivation efficacy. The inactivation ratio obtained by the dose rate of 15.3 mGy/s shows more than 3 times larger than that obtained by the dose rate of 147 mGy/s at the dose of 200 Gy, which is consistent with findings from previous UV studies. On the other hand, in the exponential phase, higher dose rates with shorter irradiation durations were more effective. The inactivation ratio obtained by the dose rate of 147 mGy/s shows 40 times larger than that obtained by the dose rate of 15.3 mGy/s at the dose of 200 Gy. These results can be effectively explained by a stochastic multi-hit model that accounts for three terms of linearly proportional to dose, nonlinearly proportional to dose, and binary fission. This work bridges fundamental physical biology with practical applications, such as gamma sterilization, offering a robust framework for optimizing dose-rate strategies across diverse fields.