The sensitisation of Escherichia coli towards lysozyme, nisin and lactoperoxidase by high-pressure homogenisation (HPH) at pressures ranging from 100 to 300 MPa was studied. At above 150 MPa, E. coli became sensitive to lysozyme and nisin when these compounds were added before the HPH treatment, but cells treated by HPH remained insensitive for lysozyme and nisin added after that treatment. No sensitisation to the lactoperoxidase enzyme system was observed, and no evidence of cytoplasmic membrane damage could be found in cells surviving HPH treatment using SYTO 9/propidium iodide staining. A previously isolated spontaneous E. coli mutant that is resistant to lysozyme under high hydrostatic pressure, was also found to be resistant to lysozyme upon HPH treatment. Further, lysozyme resistance in this mutant was not due to enhanced levels of periplasmic lysozyme inhibitor (Ivy), since knock-out of the ivy gene did not increase the sensitivity of the mutant to lysozyme upon HPH treatment. These results indicate that HPH treatment at pressures up to 300 MPa, unlike treatment with high hydrostatic pressure, does not cause sublethal injury, and thus does not sensitise bacteria to antimicrobial compounds that disturb cellular homeostasis such as might be the case for the lactoperoxidase system. However, like high hydrostatic pressure treatment, HPH may sensitise E. coli to lysozyme and nisin by inducing a transient permeabilisation of the outer membrane that does not involve a physical disruption and that is immediately repaired after the process. (c) 2005 Elsevier B.V. All rights reserved.