Understanding Listeria monocytogenes Behaviour That Triggers Survival Under Severe Acidity

Foodborne diseases are a constant threat to public health and an important onus for the socioeconomic status world- wide. Listeria monocytogenes is the causative agent of listeriosis, the fifth most commonly reported zoonosis in humans in European Union nowadays, with a progressive rise of outbreaks and an elevated number of deaths. This foodborne pathogen is ubiquitous and survives in a wide range of strong stresses, like refrigeration temperatures and harsh food processing conditions, with the ability to create reservoirs and persist for years. Purpose: This study aimed to investigate the mechanisms that allow Listeria monocytogenes to persist in acidic conditions, obtain acid resistance, and thus sustain viability upon severe acidic treatment. The growth of one L. monocytogenes strain, isolated from a human skin lesion, was monitored through conventional culturing methods, measuring of optical density, and pH values. Three were the in vitro cultures, two submitted to citric acid to reach pH values of 5.5 and 5.2, as well as a control culture without any pH adjustment. The differences in robustness at various phases of growth were quantified by exposure the cultures in different growth phases to an acidic shock at pH 2 for 30 min. Low pH-adapted cultures showed an increasing resistance in comparison with the control culture and the robustness was physiologically expressed at the stationary phase. The strain showed during the adaptation at pH 5.2 a biphasic growth curve, where more resistance was reported at the two stationary phases, contrariwise to the two exponential phases. The same behaviour was exhibited in cultures grown at both 10°C and 37°C under anaerobic conditions. Coupled to the physiological observations described above, RNAseq was performed to delineate the transcriptomic response of the strain. This information should shed light to molecular mechanisms involved.