Flow cytometry assessment of Lactobacillus rhamnosus GG (ATCC 53103) response to non-electrolytes stress

Purpose - Lactobacillus rhamnosus GG, a probiotic of human origin, known to have health beneficial effects can be exposed to osmotic stress when applied in food production as important quantities of sugars are added to the food product. The aim of this study is to assess the mode of action of non-electrolytes stress on its viability. Design/methodology/approach - Investigations were carried out on stationary phase cells treated with 0-1.5M sugars, by means of flow cytometric method (FCM) and plate enumeration method. Osmotically induced changes of microbial carboxyfluorescein (cF)-accumulation capacity and propidium iodide-exclusion were monitored. The ability of the cells to extrude intracellularly accumulated cF upon glucose energization was ascertained as an additional vitality marker, in which the kinetics of dye extrusion were taken into consideration as well. Sugar analysis by HPLC was also carried out. Findings - The results of FCM analysis revealed that with sucrose, only cells treated at 1.5M experienced membrane perturbation but there was a preservation of membrane integrity and enzymatic activity. There was no loss of viability as shown by plate counts. In contrast, the majority of trehalose-treated cells had low extent of cF-accumulation. For these samples a slight loss of viability was recorded on plating (logN/N-o similar to -0.45). At 0.6 M, cells had similar extrusion ability as the control cells upon glucose energization. However, 20 per cent of sucrose-treated cells and 80 per cent of trehalose-treated cells extruded the dye in the first 10 min. Originality/value - This finding pointed out the importance of trehalose to enhance the dye extrusion activity, which is regarded as an analogue of the capability of cells to extrude toxic compounds. Sugars exert different effects on the physiological and metabolic status of LGG but none caused a significant viability loss. LGG can be a choice probiotic bacterium in sugar-rich food production e.g. candies, marmalade etc., in which exposure to high osmotic pressure is be expected.