## 3 Results
### Effect of 455 nm LED Treatments on Low-\(\mathrm{a_{w}}\) S. Typhimurium  
To understand the inactivation effect and the underlying antibacterial mechanism of 455 nm light pulses against low-\(\mathrm{a_{w}}\)\(S\). Typhimurium, a treatment dose of \(\sim\)250 J/cm\({}^{2}\) (12 min) at 80% power level and at 4 cm distance from the LED head was selected in this study. We observed that the 455 nm light pulses reduced (\(p\) = 0.0014) the cell counts of low-\(\mathrm{a_{w}}\)\(S\). Typhimurium by 0.859 \(\pm\) 0.201 log (CFU/g) (Table 3). This inactivation efficacy could be attributed to the generation of intracellular ROS as we observed an increase (\(p\) = 0.0072) in the fluorescence values after treatment with the 455 nm LED (Table 3). An increase (\(p\) < 0.0001) in the percentage of the cell population of \(S\). Typhimurium oxidized due to the 455 nm LED treatment by C11 BODIPY\({}^{\mathrm{\mathrm{\textregistered}}\mathrm{\text{\textregistered}}\mathrm{\text{ \textregistered}}\mathrm{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{ \text{\text{\text{\text{\text{\text{\text{\texttexttext  
cells that did not grow on TSBYE after 24 h of [36] and lysis of PI permeant dead cells during storage and treatment. The reduction in the cell counts after the 365 and 395 nm LED treatments observed in this study during this preparation method (Table 1) are similar to the reductions reported in our previous study [30]. Overall, the LED treatments probably do not cause the cell membrane damage in low-a\({}_{\mathrm{w}}\) bacteria. Therefore, the preparation of low-a\({}_{\mathrm{w}}\)_S_. Typhimurium was modified to limit the effect of drying and equilibration periods in the subsequent studies. Low-a\({}_{\mathrm{w}}\)_S_. Typhimurium showed susceptibility to the treatments with the 365 and 395 nm light pulses with an \(\sim\)217 J/cm\({}^{2}\) dose at 80% power level and 4 cm distance from the LED head (Figure 2). In addition, low-a\({}_{\mathrm{w}}\)_S_. Typhimurium showed more sensitivity toward the 365 nm LED treatment as compared with the 395 nm LED treatment with the same dose, power level and distance from the LED head, which was also reported in [PERSON] et al. (2019) [30], and this observation needs further research. Understanding the underlying mechanism in the antibacterial effect of the 365 and 395 nm LED treatments against low-a\({}_{\mathrm{w}}\)_S_. Typhimurium would help in developing these light pulses emitted from the LEDs as a potential decontamination method against foodborne pathogens in low-a\({}_{\mathrm{w}}\) food systems. In this study, carboxy-H2 DCFDA was used as a fluorescent indicator of oxidative stress (hydroxyl, peroxyl and other ROS species) to understand the role of ROS production in the inactivation efficacy of the LED treatments. The intracellular esterases in the live cells cleave the diacetate group of the agent, and in the presence of ROS, the non-fluorescent compound converts to the fluorescent form and emits fluorescence [17; 32], which was detected by a spectrophotometer in this study. Previously, carboxy-H2 DCFDA has been used to study the intracellular oxidative stress in \(S\). Typhimurium ATCC13311 by [PERSON] and [PERSON] (2022) [37]. The LED treatments with the 365 and 395 nm light pulses produced a significant intracellular ROS generation in low-a\({}_{\mathrm{w}}\)_S_. Typhimurium cells (Table 2). Previously, the generation of ROS due to the 365 nm LED treatment in high-moisture conditions was reported [14; 38; 39]. Similarly, 405 nm LED treatment showed a significant increase in the intracellular ROS generation in _Staphylococcus epidermis_ by using carboxy-H2 DCFDA dye as the fluorescent indicator [40]. In high-a\({}_{\mathrm{w}}\) foods such as fresh-cut papaya inoculated with _Salmonella_, the 405 nm LED treatments showed oxidation of the bacterial cells [20]. An increase in the oxidative stress might affect the antioxidant defense system of the cells.