Synergistic actions of nisin, sublethal ultrahigh pressure, and reduced temperature on bacteria and yeast

Nisin in combination with ultrahigh-pressure treatment (UHP) showed strong synergistic effects against Lactobacillus plantarum and Escherichia coli at reduced temperatures (<15 degrees C). The strongest inactivation effects m ere observed when nisin was present during pressure treatment and in the re covery medium. Elimination (>6-log reductions) of L. plantarum was achieved at 10 degrees C with synergistic combinations of 0.5 mu g of nisin per mi at 150 MPa and 0.1 mu g of nisin per mi at 200 MPa for 10 min. Additive eff ects of nisin and UHP accounted for only 1.2- and 3.7-log reductions, respe ctively. Elimination was also achieved for E. coli at 10 degrees C with nis in present at 2 mu g/ml, and 10 min of pressure at 200 MPa, whereas the add itive effect accounted for only 2.6-log reductions. Slight effects were obs erved even against the yeast Saccharomyces cerevisiae with nisin present at 5 mu g/ml and with 200 MPa of pressure. Combining nisin, UHP, and lowered temperature may allow considerable reduction in time and/or pressure of UHP treatments. Kill can be complete without the frequently encountered surviv al tails in UHP processing. The slightly enhanced synergistic kill with UHP at reduced temperatures was also observed for other antimicrobials,, the s ynthetic peptides MB21 and histatin 5. The postulated mode of action was th at the reduced temperature and the binding of peptides to the membrane incr eased the efficacy of UHP treatment. The increases in fatty acid saturation or diphosphatidylglycerol content and the lysylphosphatidyl content of the cytoplasm membrane of L. plantarum were correlated,vith increased suscepti bility to UHP and nisin, respectively.