Determination of D10 values of single and mixed cultures of bacteria after gamma irradiation

The D10 value of bacteria represents the absorbed radiation dose required to inactivate 90 % of a viable population or reduce the population by a factor of 10. D10 values of 3 bacterial isolates (Escherichia coli, Staphylococcus aureus and Salmonella parathyphi B) were determined using single and mixed cultures to assess the effect of microbial competition on radiosensitivity. The isolates were inoculated into wakye substrate and exposed to γ-radiation doses of 0, 100, 300, 450, 600, 750, 850 Gy from a 6OCo source at a dose rate of 2.20 kGy/h in air. Enumeration of survivors of the isolates was carried out using serial dilution and pour plate methods. The surviving fraction of isolates decreased with increased irradiation doses. D10 values of E. coli, S. aureus and S. parathyphi B were respectively 0.27, 0.33 and 0.44 kGy when inoculated as single cultures, and 0.24, 0.28 and 0.32 kGy respectively when inoculated as mixed cultures. D10 values were lower for mixed cultures compared to single cultures, which might indicate reduced resistance to γ-radiation as a result of competition among the isolates. Microbiological challenge tests based on the D10 values may result in delivery of higher irradiation doses, but the extra dose could serve as safety margin to enhance the food preservative capacity of radiation processing. (au)