The bactericidal effect of hydrostatic pressure is reduced when bacteria are suspended in media with high osmolarity. To elucidate mechanisms responsible for the baroprotective effect of ionic and nonionic solutes, Lactococcus lactis was treated with pressures ranging from 200 to 600 MPa in a low-osmolarity buffer or with buffer containing 0.5 M sucrose or 4 M NaCl. Pressure-treated cells were characterized in order to determine viability, the transmembrane difference in pH (⌬pH), and multiple-drug-resistance (MDR) transport activity. Furthermore, pressure effects on the intracellular pH and the fluidity of the membrane were determined during pressure treatment. In the presence of external sucrose and NaCl, high intracellular levels of sucrose and lactose, respectively, were accumulated by L. lactis; 4 M NaCl and, to a lesser extent, 0.5 M sucrose provided protection against pressure-induced cell death. The transmembrane ⌬pH was reversibly dissipated during pressure treatment in any buffer system. Sucrose but not NaCl prevented the irreversible inactivation of enzymes involved in pH homeostasis and MDR transport activity. In the presence 0.5 M sucrose or 4 M NaCl, the fluidity of the cytoplasmic membrane was maintained even at low temperatures and high pressure. These results indicate that disaccharides protect microorganisms against pressure-induced inactivation of vital cellular components. The protective effect of ionic solutes relies on the intracellular accumulation of compatible solutes as a response to the osmotic stress. Thus, ionic solutes provide only asymmetric protection, and baroprotection with ionic solutes requires higher concentrations of the osmolytes than of disaccharides.The accumulation of compatible solutes, such as betaines, proline, and sugars, is a widespread response of microorganisms to elevated environmental osmotic pressures (41). According to a recent definition, a compatible solute is a "cytoplasmic cosolvent (solute) whose levels can be modulated over a broad range without disrupting cellular function" (24, 43). Compatible solutes contribute to the osmotic balance with the extracellular environment, enhance the stability of enzymes, and preserve the integrity of biological membranes (41). The composition of the osmolytes that are accumulated depends on the type of osmotic shock (i.e., ionic stress versus nonionic stress) and the presence of specific compatible solutes in the medium. In addition to their role as osmotic balancers, compatible solutes provide protection against high temperature, freeze-thaw treatment, and drying (41, 43).When the osmotic pressure of the medium is increased, the tolerance of microorganisms to high hydrostatic pressure is enhanced. Baroprotective effects of sodium chloride or sugars have been observed in studies with Escherichia coli, Saccharomyces cerevisiae, Zygosaccharomyces spp., and Rhodoturula rubra (14,17,24,25,36). The influence of salts or sugars on the water activity (a w ) of foods or suspension media does not explain the marked baroprotectiv...

The effect of pressure and temperature on the growth of the mesophilic lactic acid bacteria Lactococcus lactis and Lactobacillus sanfranciscensis was studied. Both strains were piezosensitive. Lb. sanfranciscensis failed to grow at 50 MPa and the growth rate of Lc. lactis at 50 MPa was less than 30% of that at atmospheric pressure. An increase of growth temperature did not improve the piezotolerance of either organism. During growth under high-pressure conditions, the cell morphology was changed, and the cells were elongated as cell division was inhibited. At atmospheric pressure, temperatures above the optimal temperature for growth caused a similar effect on cell morphology and cell division in both bacteria as that observed under high-pressure conditions. The segregation and condensation of chromosomal DNA were observed by DAPI staining and occurred normally at high-pressure conditions independent of changes in cell morphology. Immunofluorescence microscopy of Lc. lactis cells demonstrated an inhibitory effect of high pressure on the formation of the FtsZ ring and this inhibition of the FtsZ ring formation is suggested to contribute to the altered cell morphology and growth inhibition induced by high pressure.