Effects of high pressure on survival and metabolic activity of Lactobacillus plantarum TMW1.460

The application of high pressure (HP) for food preservation requires insigh t into mechanisms of HP-mediated cell injury and death, The HP inactivation in model beer of Lactobacillus plantarum TMW1.460, a beer-spoiling organis m, was investigated at pressures ranging from 200 to 600 MPa. Surviving cel ls mere characterized by determination of (i) cell viability and sublethal injury, () membrane permeability to the fluorescent dyes propidium iodide ( PI) and ethidium bromide (EB), (iii) metabolic activity with tetrazolium sa lts, and (iv) the activity of HorA, an ATP binding cassette-type multidrug resistance transporter conferring resistance to hop compounds. HP inactivat ion curves exhibited a shoulder, an exponential inactivation phase, and pro nounced tailing caused by a barotolerant fraction of the population, about 1 in 10(6) cells. During exponential inactivation, more than 99.99% of cell s were sublethally injured; however, no sublethal injury was detected in th e barotolerant fraction of the culture. Sublethally injured cells were meta bolically active, and loss of metabolic activity corresponded to the decrea se of cell viability. Membrane damage measured by PI uptake occurred later than cell death, indicating that dye exclusion may be used as a fail-safe m ethod for preliminary characterization of HP inactivation. An increase of m embrane permeability to EB and a reduction of HorA activity were observed p rior to the loss of cell viability, indicating loss of hop resistance of pr essurized cells. Even mild HP treatments thus abolished the ability of cell s to survive under adverse conditions.