Impact of Water Activity on the Inactivation and Gene Expression of Listeria monocytogenes during Refrigerated Storage of Pressurized Dry-Cured Ham

Listeria monocytogenes population and the expression patterns of three virulence (plcA, hly, and iap) and one stress-related (sigB) genes in dry-cured ham with different water activity (a(w)) values (0.92, 0.88, and 0.84) and treated with high pressure processing (HPP, 450 MPa/10 min and 600 MPa/5 min) were monitored throughout 30 days (d) at 4 °C. The antimicrobial effect of HPP at 600 MPa against L. monocytogenes S4-2 (serotype 1/2b) and S12-1 (serotype 1/2c) was greater in dry-cured ham with a(w) values of 0.92, with reductions of 2.5 and 2.8 log units, respectively. The efficacy of HPP treatments decreased at lower a(w) values. Regarding gene expression, L. monocytogenes strains responded differently to HPP. For strain S4-2, the four target genes were generally overexpressed in dry-cured ham immediately after HPP treatments at the three a(w) values investigated, although the extent of this induction was lower in the samples pressurized at 600 MPa and with a(w) values of 0.84. For strain S12-1, the expression of all target genes was repressed at the three a(w) values investigated. The antimicrobial efficacy of HPP against L. monocytogenes could be compromised by low a(w) values in food products. However, no growth of HPP-survival cells was observed during refrigerated storage in low-a(w) dry-cured ham, and the overexpression of virulence and stress-related genes decreased.