Sequential transition of the injury phenotype, temperature-dependent survival and transcriptional response in Listeria monocytogenes following lethal HO exposure.

The food-borne pathogen Listeria monocytogenes is present persistently in food processing environments, where this bacterium is exposed to various stress factors, including oxidative stress. This study aimed to elucidate the temperature-dependent response of L. monocytogenes to HO exposure and the phenotypic changes in colony formation by HO-treated bacteria. Survival curves indicated an increase in the resistance to HO in L. monocytogenes as the temperature decreased during the stress exposure procedure. Transcriptional induction of genes with key roles in response to HO, including sigB and kat, was observed at 37°C, but not at 20°C, whereas other stress response genes were induced at both temperatures. Following HO exposure, L. monocytogenes produced small colony phenotypes and the colony size decreased in a stress exposure duration-dependent manner. Resuscitated cells with no ability to form colonies in the absence of sodium pyruvate were also found. Our findings show the possibility that a sequential transition in the injury phenotype from small colony phenotype to resuscitated cells occurred during the course of exposure to HO. The higher HO resistance at 20°C than 37°C suggests further investigation of the response to HO exposure under the lower temperatures, including refrigeration temperature, which may contribute to elucidation of bacterial survival over extended time periods in food-processing environments.