DEVELOPMENT OF REPRODUCIBLE TEST INOCULA FOR DISINFECTANT TESTING

For the development and approval of disinfectants, laboratory tests wh ich combine repeatability and reproducibility with relevance to practi cal conditions are required to ensure optimum standards of efficacy un der use conditions. Over the years, although an increasingly rigorous approach has been adopted in devising European and US Standard Test Me thods for disinfectants, which includes specifying all aspects of test methodology, the precision of these methods remains a matter for conc ern. Studies of proposed European test methods indicate that, although some of the variability is methodological in origin, or is derived fr om errors in preparing test solutions and performing the tests, one of the major sources of error is lack of reproducibility in the performa nce of the test inoculum. Results indicate that day to day variability in biocide sensitivity of sequential subcultures arises not only from variations in the phenotype generated fr om the laboratory stock cult ure, but also from lack of standardization of conditions used to harve st and prepare test inocula. Further reductions in reproducibility bet ween test periods both within or between laboratories probably arise f rom alterations in the genotype of the laboratory stock culture or sou rce culture during storage under conditions of refrigeration, freezing or freeze-drying. Methods for production of reproducible inocula are considered and some studies aimed at development of test inocula with more reproducible biocide resistance are described. Copyright (C) 1996 Elsevier Science Ltd