Suspensions or biofilms : and other factors that affect disinfectant testing on pathogens

 Disinfectants are very important for the maintenance of proper hygiene in the food industry. In Europe, candidate disinfectants have to be tested on suspended bacteria in so called suspension tests, before they can be approved as disinfectants. In the food industry bacteria usually are attached to surfaces, where they may form biofilms. This mode of growth makes them less susceptible to disinfectants than free-living (suspended) bacteria. Thus, disinfectant testing would greatly profit from a biofilm disinfectant test. The aim of the research described in this thesis was to improve the current European disinfectant tests. To achieve this goal we studied factors that influence the efficacy of disinfectants and alternatives for viability assessment by plate counting. Furthermore, we developed a biofilm disinfectant test.The bacteria used in this study were the biofilm forming pathogens Listeria monocytogenes and Staphylococcus aureus . The disinfectants were benzalkonium chloride (BAC), sodium hypochlorite (NaOCl), hydrogen peroxide, and dodecylbenzene sulphonic acid.The growth phase of cells grown in suspension had a large influence on their susceptibility to disinfectants. S. aureus cells grown according to the prescription of the European suspension test were phenotypically not the most disinfectant resistant cells of all the cell types tested. Thus, in suspension tests, cells that are grown differently and have a higher phenotypic resistance than the cells currently used for disinfectant testing could be used.Fluorescent labeling could be used as a rapid alternative for viability assessment by plate counting, for both free-living cells and biofilm cells, provided the proper fluorescent probes were selected. Thus, for rapid screening of candidate disinfectants, fluorescent probes in combination with flow cytometry may be used instead of plate counting.In the biofilm disinfectant test, S. aureus biofilm formation and biofilm disinfection by BAC and NaOCl were reproducible and a genuine biofilm was produced. To improve disinfectant testing in general, the biofilm disinfectant test developed in this thesis can be added to the set of tests that are used currently. The biofilm test will show which currently used or new disinfectants are the most effective against biofilm cells. In the end, this will contribute to food safety and food quality and the control of cleaning costs in the food industry.