Enhanced inactivation of Escherichia coli and Listeria monocytogenes by exposure to 405 nm light under sub-lethal temperature, salt and acid stress conditions

The antimicrobial effects of 405 nm light have generated interest in its use as an emerging disinfection technology with potential food-related applications. The aim of this study was to assess the bactericidal efficacy of 405 nm light for inactivation of Escherichia coli and Listeria monocytogenes under sub-lethally stressed environmental conditions. Bacteria were exposed to 405 nm light from a light emitting diode (LED) array under various temperature, salt (NaCl) and acid conditions to determine if bacterial susceptibility to 405 nm light inactivation is affected when exposed under these conditions. Non-stressed bacterial populations (105 CFU/mL) were exposed to increasing doses of 405 nm light (~ 70 mW/cm2) and the inactivation results were compared with those generated under stress conditions. Bacteria were held at various temperatures (4 °C, 22 °C and 45 °C), acid concentrations (pH 3, 3.5 and 7) and salt concentrations (0%, 0.8%, 10% and 15% NaCl), and simultaneously exposed to 405 nm light. Enhanced inactivation of both E. coli and L. monocytogenes was achieved when light exposure was combined with each of the sub-lethal stresses, with significantly increased inactivation rates compared to non-stressed populations (P ≤ 0.05). One exception was with L. monocytogenes when light-exposed in the presence of 15% salt, as this combination reduced bacterial inactivation. The greatest enhancement of 405 nm light inactivation for both bacterial species was achieved when light exposure was combined with sub-lethal acid stress conditions at pH 3. This was demonstrated by a 5-log10 reduction of E. coli following a 405 nm light dose of 84 J/cm2 compared to 378 J/cm2 for non-stressed populations (77% reduction in dose) and by a 5-log10 reduction of L. monocytogenes achieved with a dose of 42 J/cm2 which corresponded to 50% of the dose required for the equivalent reduction of non-stressed populations. This acid-enhanced 405 nm light inactivation effect was demonstrated with E. coli and L. monocytogenes when dispersed in liquid suspension and when deposited on a test surface. Overall, results from this study have shown that sub-lethally stressed bacteria have increased susceptibility to 405 nm light inactivation, thereby providing a synergistic inactivation effect, findings which increase the potential of this new light-based decontamination technology for food related applications