Inactivation of Group I and Group II Clostridium botulinum spores by ultraviolet irradiation in water

Spores of Clostridium botulinum are widely distributed in the environment, including in foods. Prevention of foodborne botulism relies on the inhibition of spore germination and subsequent growth and toxin production, or the destruction of viable spores in food and beverages. This study examined the lethality of 254 nm UV radiation (UV-C) to spores of Group I and Group II C. botulinum. Spores of C. botulinum were inactivated by UV-C, with doses required for incremental log reduction (D₁₀) values calculated using linear regression ranging from 2.87 to 3.70 mJ/cm² for Group I strains and 4.46 to 6.15 mJ/cm² for Group II strains. The measured D₁₀ value for spores of C. sporogenes ATCC 19404 was 8.27 mJ/cm² indicating it was more resistant than the strains of C. botulinum used in this study. Calculation of dose per log using a Weibull model resulted in higher D₁₀ values of 6.67 to 8.81 mJ/cm² for Group I strains and 9.24 to 10.7 mJ/cm² for Group II strains. Spores of C. sporogenes possessed a D₁₀ value of 14.4 mJ/cm². The higher values for the Weibull model indicate the Weibull model to be more conservative as a result as it factors in the lag prior to inactivation and the tailing observed with very low numbers of survivors. Spores of both Group I and Group II C. botulinum strains tended to form large aggregates, visible with phase contrast microscopy, that resulted in severe tailing. Disruption of aggregates by ultrasonication was necessary to obtain linear destruction curves extending beyond 5 log reduction. All strains from Group I and Group II required <55 mJ/cm² to achieve 5 log inactivation. The strain of C. sporogenes used in this work can therefore be a conservative non-pathogenic surrogate, having higher UV-C resistance than the C. botulinum strains used in this study. Overall, this study is the first detailed study to demonstrate UV-C as an effective treatment method to inactivate C. botulinum spores in a suspending medium. In addition, the study paves the way for further studies towards the applications of this technology to inactivate C. botulinum spores in beverages or other liquids.