A Dose-Response Model and D10-Value for Mycobacterium tuberculosis Exposed to Dosimetrically Verified Ionizing Radiation

Abstract

Techniques for pathogen inactivation have been employed by laboratories to help ease the financial, physical, and health strains associated with (A)BSL-3 work. Exposure to radiation is the most common and useful of these methods to inactivate pathogens grown in large-scale culture. While robust protocols exist for radiation exposure techniques, there are variances in methods used to determine the radiation dose and dose rate required to inactivate pathogens. Furthermore, previous studies often do not include radiation dosimetry verification or address corresponding dosimetry uncertainties for dose response-assays. Accordingly, this study was conducted with the purpose of completing a dosimetry assessment of the radiation field within the sample chamber of a sealed source irradiator, to subsequently determine the radiation dose required to inactivate pathogenic cultures. Physical dosimetry techniques (Fricke dosimetry, ion chamber measurements, and measurements with thermoluminescent dosimeters) were used to measure dose rate and rate variances within the sample chamber. By comparing the variances between the dosimetry methodologies and measurements, an estimated dose rate within the sample chamber was determined. The results of the dosimetry evaluation were used to determine the radiation dose samples of Mycobacterium tuberculosis received, to accurately associate biological markers of inactivation to specific doses of ionizing radiation. A D10 value and dose-response curve were developed to describe the inactivation of Mtb from increasing doses of ionizing radiation. The D10 value is experimentally relevant for comparative analysis and potentially provides a biological baseline for inactivation verification. This methodology can also easily be translated to other pathogen models. Importance This work set out to give us a better understanding of how much radiation is required to inactivate Mycobacterium tuberculosis, the bacteria that causes tuberculosis disease. Radiation dose from a source is not something that can just be inputted, it must be calculated, so we also determined the approximate dose from the source to address ambiguities that had previously existed while inactivating microbes. We were able to generate an accurate description of inactivation of Mycobacterium tuberculosis by correlating it with a value representing 90% death of the treated cells. We also unexpectedly discovered that very low levels of radiation increase certain activity within the cell. This is important because it allows us to better understand how radiation kills Mycobacterium tuberculosis, and gives us a value to compare to other organisms. It also offers other researchers a method to use under their own specific conditions.