Reduction of PCR-amplifiable DNA by ethylene oxide treatment of forensic consumables

A reliable method to provide molecular biology products free of contaminating DNA is of forensic interest. Ethylene oxide (EO) treatment has been demonstrated as an effective method in published studies. This study aimed to address some additional experiments that are closer to forensic practice. In the first part of this study, different consumables such as cotton swabs, latex gloves and micro test tubes were spiked with saliva, blood and skin cells to mimic a real-life contamination scenario. EO treatment was performed for a period of 3, 5, 7, and 10 h, respectively. For comparison, gamma and electron beam treatment was applied. In the second part of this study, a cell culture line (K562) was used to apply defined cell counts on cotton swabs followed by EO treatment for 3 and 5 h. After extraction of samples, the DNA content was quantified using a real-time PCR based system. STR analysis was performed using a latest generation STR kit to meet current sensitivity limits. A good correlation of real-time PCR results and STR results was observed. This work confirmed the findings of earlier studies showing that chemical EO treatment is much more successful in reducing the amount of PCR-amplifiable DNA than ionising radiation. Furthermore, the efficacy of EO treatment is affected by the nature of the samples. DNA in saliva was more susceptible to damage by EO gas than DNA in blood. Our results show, that accessibility of the sample to EO gas has a strong influence on the method's efficiency. While treatment of samples on cotton swabs packed into gas-permeable bags was very successful, samples inside a closed micro test tube were resistant to the same treatment conditions. Our work with defined K562 cell numbers and multi-copy quantitative PCR could show that a 5 h EO treatment results in a 105 fold reduction of PCR-amplifiable DNA. Corresponding STR-PCR results also show only sporadic allele calls in the Mini-loci range, providing a reliable interpretation of forensic analysis. Finally, we do recommend an EO treatment of forensic consumables and a multi-copy quantitative PCR approach to establish reliable treatment conditions.