| Summary: | <i>Listeria monocytogenes</i> population and the expression patterns of three virulence (<i>plcA</i>, <i>hly</i>, and <i>iap</i>) and one stress-related (<i>sigB</i>) genes in dry-cured ham with different water activity (a<sub>w</sub>) 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 <i>L. monocytogenes</i> S4-2 (serotype 1/2b) and S12-1 (serotype 1/2c) was greater in dry-cured ham with a<sub>w</sub> values of 0.92, with reductions of 2.5 and 2.8 log units, respectively. The efficacy of HPP treatments decreased at lower a<sub>w</sub> values. Regarding gene expression, <i>L. monocytogenes</i> 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<sub>w</sub> values investigated, although the extent of this induction was lower in the samples pressurized at 600 MPa and with a<sub>w</sub> values of 0.84. For strain S12-1, the expression of all target genes was repressed at the three a<sub>w</sub> values investigated. The antimicrobial efficacy of HPP against <i>L. monocytogenes</i> could be compromised by low a<sub>w</sub> values in food products. However, no growth of HPP-survival cells was observed during refrigerated storage in low-a<sub>w</sub> dry-cured ham, and the overexpression of virulence and stress-related genes decreased.
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