High pressure inactivation kinetics of Bacillus subtilis cells by a three-state-model considering distributed resistance mechanisms

Inactivation of vegetative Bacillus subtilis ATCC 9372 by high hydrostatic pressure treatment (200-450 MPa) was tested at 20, 30 and 40 degrees C. Time-inactivation curves of the bacteria suspended in Ringer's solution showed sigmoid asymmetric shapes when plotted in logarithmic scale. Kinetic analysis of the survivor data was performed by fitting a two-step-model. It was assumed that during pressure treatment, the bacterial cells pass through a metastable intermediate state which is reached after endogenous homeostatic mechanisms balancing the pressure induced displacements of equilibria can no longer be maintained. Combined pressure-temperature-pH effects may target this state and cause lethal cell damage. Modelling this concept, a distributive function describing the initial transition was used in combination with a first-order reaction which was assumed to govern the irreversible second step. Regressively derived characteristic parameters showed logarithmic-linear behaviour. Applicability of the model on dynamic treatments with constant rate of pressure increase could be proven.