The MPN method is indispensable for evaluating spore inactivation by high hydrostatistic pressure

The MPN method is indispensable for evaluating spore inactivation by high-pressure In microbiological kinetic studies, bacterial endospores are generally enumerated by plate count. Incubation times are short, so spores with long lag times for germination are not detected. This may result in an overestimation of the number of inactivated spores. In this study, a modified MPN method was developed for evaluating high-pressure spore inactivation. In this modified method, spores were diluted before treatment rather than after treatment. Additional advantages were that post-process contamination could not occur and treated spores were not exposed to oxygen. Results were compared with standard plate counts. Clostridium sporogenes PA3679 spores were high-pressure treated under adiabatic-isobaric conditions (initial temperature 70ºC, applied pressure 800 MPa, single pulse) in a batch system. The stabilization of the MPN during incubation took longer than standard incubation times for plate counts (up to 50 days, compared to 5 days for plate counts). The MPN score after incubation was considerably higher than the plate count immediately after the treatment. These results indicate that some surviving spores that were detected by the modified MPN method were not detected by plate count. This may be caused by the longer lag times that sub-lethally damaged spores need for germination. It is our opinion that the (modified) MPN method is indispensable for evaluating spore inactivation by high-pressure.