Impact of pulsed light on cellular activity of Salmonella enterica

Aims The objective of this study was a comprehensive characterization of physiological changes of Salmonella enterica induced by intense broad spectrum pulsed light (PL). After exposing the bacteria to this non-thermal decontamination technology on a gel surface, multiple viability parameters beyond culturability were assessed. Methods and Results By applying flow cytometry, a luciferin-luciferase bioluminescence assay and a microplate assay to measure the current redox activity, the impact of pulsed light on the membrane potential, membrane integrity, esterase activity, efflux pump activity, expression of the green fluorescent protein (GFP), respiration activity and ATP-content of Salmonella enterica ATCC BAA-1045 was determined. These culture independent methods for assessing the bacterial activity were compared to the ability to grow on tryptic soy agar. It is shown that this strain is rather sensitive to PL considering colony count reductions, while on the other hand unculturable bacteria still exhibit significant cellular energetic functions. However, this residual activity after PL exposure significantly decreases during sample storage in buffer for 24 h. This study also shows that the GFP expression of PL treated cells which have rendered unculturable is severely reduced. Conclusions This study reveals that although not all cellular functions of Salmonella enterica are immediately shut down after PL exposure, the synthesis of new GFP is strongly reduced and affected to a similar extent as the culturability. Significance and impact of the study It is shown for the first time, that even there is significant bacterial activity measurable after PL exposure, it is likely that non-growing pathogenic bacteria like Salmonella enterica are unable to express proteins, which is of great importance regarding their pathogenicity.