Exploring the survival mechanisms of some functional lactic acid bacteria under stress conditions: morphological changes and cross-protection

Abstract

Three functional lactic acid bacteria, namely Lactococcus lactis 19.3, Leuconostoc citreum P109 and Lactobacillus helveticus 34.9, which have been previously shown to produce bacteriocins, exopolysaccharides, and S-layer proteins, respectively, were used to study the survival mechanisms under stress conditions. Scanning electron microscopy observation revealed several morphological alterations, including elongated, shorter, shrunken, or curved cells, suggesting these may be adaptations meant to enhance cells fitness in stressful environments. All three strains demonstrated good survival rates when exposed to most of the stress conditions, including high temperatures (up to 50 °C), low pH (2.0–3.0), high concentrations of NaCl (up to 10 %) or bile salts (0.4 %), with no clear link to the presence of specific stress genes in their genome. However, L. citreum P109, which contains three stress-related genes (groEL and LBA1272 associated with low pH resistance, and LBA1446 linked to bile salts resistance), exhibited the most significant cross-protective effect. The upregulation of the three evaluated genes under sub-lethal stress conditions may play an important role in this cross-protection.