| Summary: | The stress response of 11 strains of <i>Listeria monocytogenes</i> to oxidative stress was studied. The strains included ST1, ST5, ST7, ST6, ST9, ST87, ST199 and ST321 and were isolated from diverse food processing environments (a meat factory, a dairy plant and a seafood company) and sample types (floor, wall, drain, boxes, food products and water machine). Isolates were exposed to two oxidizing agents: 13.8 mM cumene hydroperoxide (CHP) and 100 mM hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) at 10 °C and 37 °C. Temperature affected the oxidative stress response as cells treated at 10 °C survived better than those treated at 37 °C. H<sub>2</sub>O<sub>2</sub> at 37 °C was the condition tested resulting in poorest <i>L. monocytogenes</i> survival. Strains belonging to STs of Lineage I (ST5, ST6, ST87, ST1) were more resistant to oxidative stress than those of Lineage II (ST7, ST9, ST199 and ST321), with the exception of ST7 that showed tolerance to H<sub>2</sub>O<sub>2</sub> at 10 °C. Isolates of each ST5 and ST9 from different food industry origins showed differences in oxidative stress response. The gene expression of two relevant virulence (<i>hly</i>) and stress (<i>clp</i>C) genes was studied in representative isolates in the stressful conditions. <i>hly</i> and <i>clp</i>C were upregulated during oxidative stress at low temperature. Our results indicate that conditions prevalent in food industries may allow <i>L. monocytogenes</i> to develop survival strategies: these include activating molecular mechanisms based on cross protection that can promote virulence, possibly increasing the risk of virulent strains persisting in food processing plants.
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