The different pathways of spore germination and inactivation of Bacillus subtilis under high pressure and elevated temperatures

Abstract High pressure combined with elevated temperatures could produce commercially sterile and shelf stable low acid foods. Depending on the applied temperature and pressure level, bacterial endospores pass through different physiological pathways, which can induce germination or a subsequent inactivation. To improve the understanding of spore inactivation Bacillus subtilis spores and isogenic strains, which lack the gens for encoding the germinant receptors, were pressure treated. Therefore, the strains FB114 and FB115 and its wild type strain ( PS832 ) were processed from 1 s up to 24 h in a pressure-temperature range of 200 – 1000 MPa and 30 - 80 °C under isothermal and isobaric conditions during dwell time in pressure stable ACES buffer solution. Afterwards, aliquots of each sample were mild heat treated and the released amount of DPA was quantified by HPLC, to estimate the amount of germinated spores. Further, two model approaches were tested to derive a global model from the kinetic data. It was confirmed for both FB-strains, that spore germination above 500 MPa is possible without germinant receptors and even at 300 MPa germination of these spores was detected. Furthermore, no spore inactivation occurred for pressures below 700 MPa and temperatures up to 50 °C. Whereas the behavior of PS832 was contrary, here an inactivation of more than 3 log 10 -cycles was detected at 300 MPa after 30 min dwell time at 40 °C and no inactivation was observed at 550 MPa, 37 °C, 120 min. Possibly, spores with nutrient receptors could proceed through stage II of germination at low pressure levels, whereas this germination step is retarded at higher pressures. Moreover, above 300 MPa both germination mechanisms interfere with each other, whereas above 500 MPa the opening of the Ca 2+ -DPA-channels is dominant. For all strains a strong acceleration of germination with increasing pressure and temperature was detected, whereas above a certain threshold pressure (p > 600 MPa) the temperature played the dominant role for spore inactivation.