The behaviour of both Gram-negative and Gram-positive bacteria associated with milk, in response to application of high pressure (HP) combined with nisin, was evaluated. Escherichia coli, Pseudomonas fluorescens, Listeria innocua, and Lactobacillus viridescens grown separately in skim milk were pressure-treated at 250-500 Wa for 5 min at 20 degrees C in the presence of 0, 250, or 500 IU/mI nisin. Combining HP and nisin gave a greater inactivation of all four bacteria than when either was applied individually. A reduction of >= 8 log of L. viridescens and L. innocua in milk was achieved at 500 MPa for 5 min at 20 degrees C, in the presence of 500 IU/ml nisin. The Gram-negative bacteria were more sensitive to HP, either alone or in combination with nisin, than Gram-positive bacteria. Treatment at 400 MPa for 5 min with 500 IU/ml nisin, or at 250 MPa for 5 min with 500 IU/ml nisin, gave a >= 8 log reduction of E. coli and R fluorescens, respectively. By adding nisin to cell suspensions after HP treatment, it was shown that the effect of HP on cells of a single species can vary; some HP-induced sublethal injury is irreversible, allowing the sensitisation of Gram-negative bacteria to nisin post HP treatment. Reversible membrane damage in P. fluorescens caused by HP was demonstrated using fluorescent dye uptake before, after, or during HP treatment. (c) 2005 Elsevier Ltd. All rights reserved.