A joint research group from Nagoya City University and Shizuoka University has revealed, for the first time in the world, that the kill rate of Escherichia coli by X-ray irradiation varies depending on whether the bacteria is placed in a nutrient-deficient or nutrient-rich environment.
Radiation sterilization technology is an essential technique for disinfecting and sterilizing medical instruments, medicines, food materials, etc. by destroying the DNA and cell structures of bacteria and microorganisms. Conventionally, it has been thought that with radiation sterilization technology, the sterilization rate is the same if the exposure dose (radiation dose per unit time [dose rate] x exposure time) is the same.
However, in the latest experiment using E. coli, it was demonstrated that this theory does not hold true. Even if the irradiation dose is the same, in an environment where E. coli is lacking in nutrients, sterilization is more effective when it is performed for a long period of time at a low dose rate (low-intensity X-rays), whereas in an environment where E. coli is nutrient-rich, sterilization is more effective when it is performed for a short period of time at a high dose rate (high-intensity X-rays).
This new X-ray sterilization mechanism can now be theoretically explained by analysis using stochastic differential equations, a cutting-edge mathematical technique. In addition to the previously thought bactericidal effect proportional to the dose rate and the bacterial growth effect (appearing in a nutrient-rich environment), the generation and disappearance process of reactive oxygen species within bacteria can be described by introducing a parameter nonlinearly proportional to the dose rate (an effect that weakens with higher dose rates) into the stochastic differential equation. X-ray irradiation generates reactive oxygen species within bacteria, which destroys bacterial DNA and lipid layers, but when X-rays are irradiated at a high dose rate, a large amount of reactive oxygen species is generated, which combine with each other to become harmless, weakening the sterilization effect.
The X-ray sterilization results revealed by this study, which overturn the conventional theory, were also reported in 2022 for ultraviolet sterilization.Although the sterilization mechanisms are different, it is now possible to treat them in a unified manner using an analytical method that uses stochastic differential equations.
This result is expected to be useful for setting optimal sterilization conditions in radiation sterilization technology and for accurately calculating optimal radiation exposure conditions, and to contribute to the realization of radiation therapy that is more reliable and gentler on the body than ever before.
Publication information: [Scientific reports] Time-dose reciprocity mechanism for the inactivation of Escherichia coli using X-ray irradiation
