Increased Survival of Lactococcus lactis Strains Subjected to Freeze-Drying after Cultivation in an Acid Medium: Involvement of Membrane Fluidity
1University of Burgundy, AgroSup Dijon, PAM UMR A 02.102, F-21000 Dijon, France
2Indigo Therapeutics, 5 rue Salneuve, 75017 Paris, France
Received: 26 November 2020
Accepted: 29 September 2021
freeze-drying, viability, membrane fluidity, acid pre-stress, Lactococcus lactis
Research background. Freeze-drying is the most widely used dehydration process in the food industry for the stabilization of bacteria. Studies have shown the effectiveness of an acid pre-stress in increasing the resistance of lactic acid bacteria strains to freeze-drying. Adaptation of bacteria to an acid stress is based on maintaining the properties of the plasma membrane. Indeed, the fatty acid composition of lactic acid bacteria membrane is often changed after an acid pre-stress. However, few studies have measured membrane fluidity after an acid stress realized during lactic acid bacteria strains cultivation.
Experimental approach. In order to use two pH profiles, the strains Lactococcus lactis NCDO 712 and NZ9000 were cultivated in two media, without any pH control. The two pH profiles obtained were representative of initial media composition, media buffering properties, and strains metabolism. Absorbance at 600 nm and pH were measured during bacterial cultivation. Then, the two strains were freeze-dried and their survival rates determined. Membrane fluidity was evaluated by fluorescence anisotropy measurements using a spectrofluorometer.
Results and conclusions. Cultivation under a more acidic condition significantly increased both strains survival to freeze-drying (p<0.05, ANOVA). Moreover, in both strains of L. lactis, a more acidic condition during cultivation significantly increased membrane fluidity (p<0.05, ANOVA). Our results revealed that cultivation in such condition, fluidifies the membrane and allows a better survival to freeze-drying for the two strains of L. lactis. A more fluid membrane can facilitate membrane deformation and lateral reorganization of membrane components, critical for the maintenance of cellular integrity during dehydration and rehydration.
Novelty and scientific contribution. A better understanding of the involvement of membrane properties, especially of membrane fluidity, in bacterial resistance to dehydration is provided in this study.