High-pressure transient sensitization of Escherichia coli to lysozyme and nisin by disruption of outer-membrane permeability

Escherichia coli MG1655 suspensions in 10 mM phosphate buffer (pH 7.0) were subjected to high pressures in the range of 180 to 320 MPa for 15 min. Cell death war evident at 220 MPa and increased exponentially with pressure. Surviving populations were sublethally injured, as demonstrated by their reduced ability to form colonies on violet red bile glucose agar, a selective growth medium containing crystal violet and bile salts. During exposure to high pressure (>180 MPa), cells were sensitive to lysozyme, nisin, and ethylenediaminetetraacetic acid (EDTA), as was apparent from an increased lethality of pressure in the presence of these agents. Sublethal injury in the surviving population was lower in the presence of nisin and lysozyme, but higher in the presence of EDTA. Combinations of EDTA with nisin or lysozyme present during pressure treatment increased lethality in an additive manner. However, the addition of lysozyme, nisin and/or EDTA to pressurized cell suspensions immediately after pressure treatment did not cause any viable count reduction. Finally, we observed leakage of the periplasmic enzyme p-lactamase from an ampicillin-resistant recombinant E. coli MG1655 under high pressure. These results suggest that high pressure transiently disrupts the permeability of the E. coli outer membrane for water-soluble proteins.