Stress responsive biomarkers for Lactobacillus plantarum robustness

Background. Lactobacillus plantarum is one of the most versatile Lactic Acid Bacteria (LAB) that is encountered in many environmental niches, including dairy, meat and vegetable, in which it is used as starter culture for fermentation processes. Moreover, it has a demonstrated ability to survive gastric transit, can colonize the intestinal tract of human and other mammals and is commonly used in the formulation of functional probiotic foods for its health-promoting effects in consumers. L. plantarum, like other microorganisms, must face adverse environmental conditions during steps of technological processing, storage and consumption that include changes of temperature, pH, solute concentrations, nutrients and oxygen level, which can reduce its growth or survival potential. In order to adapt to environmental stresses and then stabilize their survival potential and their metabolic activity, these microorganisms have developed sophisticated mechanisms to sense changes and trigger a cascade of alterations in gene expression and protein activity. Wine may be considered one of the most stressful environments (ethanol and acidity) in which several LAB have developed different mechanisms expressing their optimal biological role. Oenococcus oeni and Lactobacillus species are the main LAB associated with grape must and wine. Wine LAB are responsible for malolactic fermentation (MLF) during which the conversion of malic acid into lactic acid, and the metabolic activities of the associated LAB can lead to the production of compounds that can impact wine aroma and flavor. Aims. In this study, in order to identify suitable molecular markers capable of improving L. plantarum robustness, six L. plantarum strains, autochthonous and commercials, have been investigated in response to stress conditions. To assess the effects of these strains on the sensorial quality of wine, volatile organic compounds (VOCs) released during fermentation in the headspace of food matrix have been characterized. Materials and methods. Gene expression of six L. plantarum strains, grown at low pH and in presence of oxidizing factors, such as diamide and hydrogen peroxide, has been monitored by real-time PCR. Moreover, VOCs released by strains inoculated in fresh must (of Apulian grape) in two different moment (co-inoculum and sequential inoculum) have been monitored through PTR-MS in collaboration with Edmund Mach Foundation (FEM) in San Michele all’Adige (Italy). Results. The results suggest that the expression of the genes analyzed in stress conditions is strains-specific and a combination of hsp2 and hsp3 genes as stress responsive markers, may be suitable to identify the most resistant L. plantarum strains, to low pH value and oxidative stress. A total of 50 VOCs were identified. The concentration of most of the volatile compounds was influenced by inoculated strain. The similarities and differences expressed in the volatile profiles of wines reveal the potential of individual strains inoculated as starter cultures to produce wines of a certain character.