Effect of high pressure processing on the inactivation and the relative gene transcription patterns of Listeria monocytogenes in dry-cured ham

Abstract The population of Listeria monocytogenes and the expression of five virulence and stress-related genes in pressurized dry-cured ham were monitored throughout storage at 4 oC. L. monocytogenes strains S2 (serotype 1/2a) and S7-2 (serotype 4b) exhibited a moderate resistance to high pressure processing (HPP) at 450 MPa for 10 min or 600 MPa for 5 min. The low water activity (aw) of sliced dry-cured ham (aw=0.88) diminished the efficacy of the treatments, with pathogen reductions lower than 1.5 log units. Regarding gene expression, a strain-dependent difference in gene transcription pattern was observed. All target genes (prfA, plcA, hly, sigB and lmo1421) were hardly downregulated for strain S2 immediately after pressurization, whereas were upregulated for strain S7-2. The initial overexpression exhibited by S7-2 was attenuated during the storage and after 30 d at 4 oC, virulence and stress-related genes were repressed, with fold changes up to 5.3, especially for 600 MPa treated samples.