Sodium chloride induced stress responses in dairy probiotic bacteria

Abstract

The study focused on possible mechanisms of salt stress injury and responses of selected probiotic bacteria to salt stress. The effects of varying NaCl/KCl concentrations on viability, membrane integrity and metabolic activity of Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium longum were studied using flow cytometry. Furthermore, the changes in surface functional groups, morphology and membrane fatty acid composition of these bacteria were investigated using Fourier-transform infrared (FTIR) spectroscopy, transmission electron microscopy and gas chromatography. The effect of NaCl/KCl stress on the in vitro adhesion ability of stressed bacteria to Caco-2 cells was also evaluated. Cell viability as evaluated using conventional culture technique was compared with the findings from flow cytometric analysis on metabolic activities of the cells and it was revealed that there was a correlation between culturability and dye extrusion ability of Lb. casei and B. longum. However, a certain population of Lb. acidophilus was viable as per the plate count method but the efflux activity was compromised. The metabolic activity of Lb. casei was found to be highest among the 3-probiotic bacteria. The FTIR spectral analysis also highlighted the shifts that occurred mainly in the amide regions upon increasing the NaCl concentration. Significant changes in the morphology of all bacteria were observed at higher salt concentrations. Shrinkage of the cytoplasmic content and irregularities in cell wall on exposure to high NaCl concentrations (10% w/v) were observed in all bacteria. Membrane fatty acid composition was affected by salt stress, and the ratio of unsaturated to saturated fatty acids was altered on exposure to NaCl stress. Adhesion ability of stressed bacteria to Caco-2 cell lines was also reduced at higher NaCl concentrations (10% w/v). Comparing the responses among the selected bacteria, Lb. casei appeared to be most robust to NaCl stress. Overall, the study revealed the impact of salt stress on membrane characteristics and adhesion capability of selected probiotic bacteria.