Effect of gamma irradiation on viability and DNA of Staphylococcus epidermidis and Escherichia coli

Gamma irradiation is widely used for sterilization; however, its effect on elimination of amplifiable DNA, an issue of relevance to molecular diagnostic approaches, has not been well studied. The effect of gamma irradiation on the viability of Staphylococcus epidermidis and Escherichia coli (using quantitative cultures) and on their DNA (using quantitative 16S rRNA gene PCR) was evaluated. Viability was abrogated at 2(.)8 and 3(.)6 kGy for S. epidermidis and E coli, respectively. The radiation dose required to reduce viable bacteria by one log(10) (D-10 value) was 0(.)31 and 0(.)35 kGy for S. epidermidis and E coli, respectively. D-10 values for amplifiable DNA extracted from bacteria were 2(.)58 and 3(.)09 kGy for S. epidermidis and E coli, respectively, whereas D-10 values for amplifiable DNA were significantly higher for DNA extracted from irradiated viable bacterial cells (22(.)9 and 52(.)6 kGy for S. epidermidis and E coli, respectively; P < 0(.)001). This study showed that gamma irradiation of DNA in viable bacterial cells has little effect on amplifiable DNA, was not able to eliminate amplifiable 16S rRNA genes at a dose of up to 12 kGy and cannot therefore be used for elimination of DNA contamination of PCR reaction components or laboratory equipment when this DNA is present in microbial cells. This finding has practical implications for those using molecular diagnostic techniques in microbiology.