Effect of salt on cell viability and membrane integrity of Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium longum as observed by flow cytometry

Abstract

The aim of the current study was to investigate the effect of varying sodium chloride concentrations (0–5%) on viability and membrane integrity of three probiotic bacteria, Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium longum, using conventional technique and flow cytometry. Double staining of cells by carboxyfluorescein diacetate (cFDA) and propidium iodide (PI) enabled to evaluate the effect of NaCl on cell esterase activity and membrane integrity. Observations from conventional culture technique were compared with findings from flow cytometric analysis on the metabolic activities of the cells and a correlation was observed between culturability and dye extrusion ability of L. casei and B. longum. However, a certain population of L. acidophilus was viable as per the plate count method but its efflux activity was compromised. Esterase activity of most bacteria reduced significantly (P < 0.05) during one week storage at NaCl concentrations greater than 3.5%. The study revealed that L. casei was least affected by higher NaCl concentrations among the three probiotic bacteria, as opposed to B. longum where the cF extrusion performance was greatly reduced during 1 wk storage. The metabolic activity and salt resistance of L. casei was found to be highest among the bacteria studied.