Antibacterial properties of plant extracts and isolated phytocompounds against Listeria monocytogenes and their effects on the activation of the general stress response

Abstract

Plants synthesize secondary metabolites in response to environmental stimuli. Among these metabolites are potent antimicrobial phytochemicals that serve as defense agents against bacterial pathogens. By sharing natural habitats, bacteria must necessarily be able of sensing the presence of plant antibacterial agents to survive and persevere. The ability of the ubiquitous pathogen Listeria monocytogenes to tolerate and survive harsh environmental conditions challenges food producers since it frequently contaminates food production facilities where it utilizes food vehicles to successfully transmit to human hosts and cause foodborne listeriosis. The alternative stress activated sigma factor σB (SigB) plays a key role in L. monocytogenes in sensing detrimental conditions and circumventing cell injury by reprogramming gene expression, expressing stress tolerance factors and ensuring cell survival. Developing a deeper understanding of how this pathogen resists the environmental stress encountered during its life cycle could facilitate the establishment of new and efficient food safety measures. In this study, numerous plant samples constituting a mixture of plant extracts, fractions of active extracts and isolated phytocompounds were evaluated for their antibacterial effects against L. monocytogenes strain EGD-e wild-type and its isogenic ΔsigB mutant. Overall, the majority of the extracts did not affect the growth and survival of these strains, and the ones that did were categorized with either high, medium or low activities, based on their respective minimum inhibitory concentrations (MICs). The most active samples were: the purified hops bitter acids (HBAs) α-acids and β-acids from Humulus lupulus L.; oleanolic acid and hydroxytyrosol from Olea europaea L.; an Eucalyptus globulus L. fraction; two fractions from Salvia officinalis L.; and, an Orthosiphon stamineus Benth. fraction. The results indicated that there were no differences in the MIC values of the respective samples concerning L. monocytogenes wt or ΔsigB. The monitoring in L. monocytogenes of the SigB-mediated general stress response following the exposure to a selection of plant antimicrobial agents revealed that not all agents trigger this response. Those agents that stimulated a strong SigB activity did not affect the cytosolic localization or phosphorylation pattern of the core stressosome protein RsbR1, although the results indicated that SigB location could be affected upon exposure to antibacterial stress from plant agents. Finally, selected plant antimicrobial agents were tested in food matrices, and some agents, including the H. lupulus L. HBAs and S. officinalis L. products were able to maintain their growth inhibitory effects in the studied food systems. Moreover, selected samples were studied for activity against other food pathogens of interest to the food industry, including Bacillus subtilis, Staphylococcus aureus, Bacillus cereus, Escherichia coli and Salmonella enterica serovar Typhimurium. The results indicated that most samples active against L. monocytogenes were also active against the other Gram-positive food pathogens