Microbiological aspects relating to the choice of radiation sterilization dose

✍ Scribed by James L. Whitby

• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 384 KB
• Volume: 42
• Category: Article
• ISSN: 0969-806X
• DOI: 10.1016/0969-806x(93)90327-q

✦ Synopsis

Ionizing radiation is widely used for sterilization of single-use medical devices, and it is for its lethal effect on microbial life that radiation is employed for this purpose. Unlike larger, more complex organisms where radiation causes lethal damage long before every cell has been killed, microbes are capable of regenerating from a single surviving cell. Moreover, the individual cells of some microbial species are highly resistant to the lethal effects of radiation exceeding the resistance of other biological species by orders of magnitude (1). Thus it is necessary to employ radiation doses far in excess of those lethal to man or animals to ensure that no viable microbe remains to repopulate the device or the patient at the time that the device is used.

Radiation exerts its effect by producing ionizations which lethally alter essential structures or functions in the cell. Ionizations also induce the production of short-lived free radicals that also induce lethal changes. The sensitivity of microbial species to these changes varies markedly but since the changes are induced within the cell, each cell has to sustain the necessary amount of lethal damage before death will occur. Therefore, it is to be expected that the death of cells will occur in the exponential mode (2) the surviving fraction decreasing with increasing dose. Where an arithmetic graphic plot is used to express percentage, microbial survivors against radiation dose, the line becomes asymptotic to the x axis (dose) with increasing dose (3,4).

Using a semi-logarithmic plot, curves of three types may be generated (3,4) (Fig. 1) of which type A, a simple exponential curve with a constant slope over the whole dose range, and Type C, where an initial shoulder is followed by a constant slope, are those usually seen. Curves of Type B, which are concave, are only seen in mixed bacterial populations of differing radiation resistance.