MATRIX-DEPENDENT INACTIVATION OF Salmonella Typhimurium BY GLOW DISCHARGE COLD PLASMA: COMPARISON BETWEEN WATERCRESS AND MICROBIAL SUSPENSION

Watercress (Nasturtium officinale R.Br.) is consumed raw, making microbial decontamination challenging due to the limited applicability of thermal treatments. This study evaluated the matrix-dependent efficacy of cold glow discharge plasma against Salmonella enterica subsp. enterica serovar Typhimurium ATCC 14028, comparing bacterial inactivation on watercress leaves versus in microbial suspension (in the absence of the plant matrix). Sanitized watercress leaves were inoculated with S. Typhimurium and treated with glow discharge cold plasma using synthetic air as the working gas at flow rates of 10, 20, and 30 mL/min and exposure times of 10,15, and 20 min. Treatment codes indicate gas flow rate (F: 10, 20 and 30 mL/min) and exposure time (T: 10, 15 and 20 min). Microbial suspensions were processed under the same plasma conditions to assess the direct antimicrobial effect without matrix interference. Treated watercress samples were stored at 4 °C and evaluated after 24, 72, and 120 h by microbiological enumeration. Scanning electron microscopy was used to assess plasma-induced morphological damage to the bacterial cells. In microbial suspension, plasma treatment resulted in no detectable viable colonies under any of the tested conditions, demonstrating the high direct antimicrobial efficacy of glow discharge plasma when cells were fully exposed to reactive species. In contrast, S. Typhimurium reductions on watercress were more variable and depended on gas flow rate, exposure time, and storage period. After 24 h, the greatest reductions on watercress were observed for the F30T20 and F20T15 conditions, reaching approximately 4 log CFU/g. After 120 h of refrigerated storage, the F10T10, F10T15 and F20T10 conditions resulted in counts below the limit of detection. These differences indicate that the watercress matrix partially protected the bacterial cells, likely due to its irregular surface, protected microenvironments, and physicochemical barriers that limit direct interaction between the plasma and the cell. Scanning electron microscopy confirmed plasma-induced cellular damage, including surface collapse, membrane rupture, and structural fissures. Overall, cold glow-discharge plasma demonstrated strong antimicrobial potential against S.Typhimurium, but its efficacy was clearly modulated by the presence of the plant matrix.