The radiation resistance of the bioburden from medical devices

An extensive study of the radiation resistance of microbial species constituting the bioburden of a number of different medical devices obtained from Japanese medical device manufacturers has been carried out. A standard protocol for determining radiation resistance was used and validated at the fourteen centres involved in the study. Individual microbial isolates from the bioburden obtained from seven different devices manufactured in these centres were studied. A total of 3742 unselected isolates were obtained, of which 197 failed to survive long enough for subsequent radiation resistance studies. The remainder were subjected to an initial screen test to identify those organisms that were sensitive to the lethal effects of radiation with a D₁₀ of < 1.5kGy. The 465 isolates that survived the screen doses were then tested for survival in an incremental series of radiation doses using methods similar to those of Whitby (1979) and Yan and Tallentire(1995). The isolates from “dry” devices were more resistant than those obtained from the one water filled (“wet”) device studied. The overall distribution of radiation resistance among the isolates was considered to be similar to that forming the “Standard Distribution of Resistance” (SDR) included in the ISO International Standard 11137 “Sterilization of Health Care Products — Requirements for validation and routine control — Radiation sterilization”.