Effects of gamma and electron beam irradiation on viability and DNA elimination of Staphylococcus aureus

Background: Owing to the inability of real-time polymerase chain reaction (PCR) to distinguish between signals originating from viable cells and DNA released from dead cells, (i.e. it can detect dead cells) a risk exists that DNA from dead cells can result in false-positive PCR signals, so it is important to use a proper method for sterilization to maintain the accuracy of the quantitative real-time PCR assay. Methods and Findings: Gamma and electron beam irradiation were compared for their effects on elimination of amplifiable DNA, an issue of relevance to molecular PCR applications, as an effective decontamination method. Effects of both methods on the viability of Staphylococcus aureus; using quantitative cultures were evaluated. Quantitative real-time PCR based assays of 16S rRNA gene fragments amplified from Staphylococcus aureus were performed. Differences in radiation sensitivity of extracted DNA before irradiation in comparison with DNA residing within viable bacterial cells at the time of irradiation were also evaluated. Viability was abrogated at 2.6kGy and 3kGy and the radiation dose required to produce 1–log10 or 90% reduction in viable microorganisms (D10values) was 0.32 kGy & 1.73 kGy for γand electron beam irradiation respectively. This study showed that gamma irradiation could eliminate free amplifiable DNA at the tested doses (12-25kGy) compared to accelerated electrons which have negligible reduction in quantity with increasing the dose. D10values for amplifiable DNA were significantly higher for DNA extracted from viable bacterial cells irradiated with electron beam than that with gamma radiation (32.17 & 3.99kGy respectively; p = 0). Conclusions: These attributes are important in clinical practice with increasing use of molecular amplification techniques in microbiological diagnostic applications.