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

The antimicrobial effects of 405nm 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 405nm light for inactivation of Escherichia coli and Listeria monocytogenes under sub-lethally stressed environmental conditions. Bacteria were exposed to 405nm light from a light emitting diode (LED) array under various temperature, salt (NaCl) and acid conditions to determine if bacterial susceptibility to 405nm light inactivation is affected when exposed under these conditions. Non-stressed bacterial populations (105CFU/mL) were exposed to increasing doses of 405nm light (~70mW/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 (pH3, 3.5 and 7) and salt concentrations (0%, 0.8%, 10% and 15% NaCl), and simultaneously exposed to 405nm 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 405nm light inactivation for both bacterial species was achieved when light exposure was combined with sub-lethal acid stress conditions at pH3. This was demonstrated by a 5-log10 reduction of E. coli following a 405nm light dose of 84J/cm2 compared to 378J/cm2 for non-stressed populations (77% reduction in dose) and by a 5-log10 reduction of L. monocytogenes achieved with a dose of 42J/cm2 which corresponded to 50% of the dose required for the equivalent reduction of non-stressed populations. This acid-enhanced 405nm 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 405nm light inactivation, thereby providing a synergistic inactivation effect, findings which increase the potential of this new light-based decontamination technology for food related applications.