COMPARISON OF 3 LABORATORY DEVICES FOR UV-INACTIVATION OF MICROORGANISMS

UV inactivation experiments of microorganisms have been performed and published by various workers for decades. Resulting data even of the s ame species of microorganisms may show important differences in UV-sus ceptibility. The reasons for these varying results could be found eith er in different biological conditions like culturing methods for prepa ring the test organisms or in technical problems regarding UV-irradiat ion equipment and dose measurement. Therefore three groups working on UV inactivation performed a collaborative study to find out which infl uences could be responsible for varying results in laboratory UV exper iments. Each working group had developed a laboratory UV irradiation a pparatus, which differed in technical construction and method for UV d ose measurement. For our study we used as a test organism spore's of b acillus subtilis ATCC 6633 which were cultured in large quantity, free ze-dried and stored for all following experiments. Thereby we establis hed controlled biological conditions. The first series of experiments in 1992 showed that differences in inactivation curves did occur, rela ted especially to dose distribution in irradiation vessels, in irradia tion geometry and in partial shadowing of UV light. Subsequently the i rradiation procedure and methods for dose measurement were improved re sulting in consistent, reproducible and comparable results. The equati on of the regression curve was: log (N/N-O) = -0.013 D + 0.18. A 2 log reduction would require a dose of 169 +/- 11 J/m(2). a 3 log reductio n 241 +/- 9 J/m(2), respectively (level of significance: sigma = 0.05) . In recent years bioassay methods have been suggested in order to eva luate UV-disinfection plants. Therefore it will be of increasing publi c health interest to ensure the quality of laboratory UV irradiation d evices used for calibration of test-organisms for these bioassays.