UV inactivation experiments of microorganisms have been performed and published by various workers for decades. Resulting data even of the same species of microorganisms may show important differences in UV-susceptibility. The reasons for these varying results could be found either in different biological conditions like culturing methods for preparing the test organisms or in technical problems regarding UV-irradiation equipment and dose measurement.

Therefore three groups working on UV inactivation performed a collaborative study to find out which influences could be responsible for varying results in laboratory UV experiments. Each working group had developed a laboratory UV irradiation apparatus, which differed in technical construction and method for UV dose measurement. For our study we used as a test organism spores of Bacillus subtilis ATCC 6633 which were cultured in large quantity, freeze-dried and stored for all following experiments. Thereby we established controlled biological conditions. The first series of experiments in 1992 showed that differences in inactivation curves did occur, related especially to dose distribution in irradiation vessels, in irradiation geometry and in partial shadowing of UV light. Subsequently the irradiation procedure and methods for dose measurement were improved resulting in consistent, reproducible and comparable results. The equation of the regression curve was: log (N/N0) = −0.013 D + 0.18. A 2 log reduction would require a dose of 169 ± 11 J/m2, a 3 log reduction 241 ± 9 J/m2, respectively (level of significance: a = 0.05).

In recent years bioassay methods have been suggested in order to evaluate UV-disinfection plants. Therefore it will be of increasing public health interest to ensure the quality of laboratory UV irradiation devices used for calibration of test-organisms for these bioassays.

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