Rapid fluorescence assessment of the viability of stressed Lactococcus lactis

The aim of this study was to establish the use of the fluorescent probes ca rboxyfluorescein (cF) and propidium iodide (PI) for rapid assessment of via bility, using Lactococcus lactis subsp. lactis ML3 exposed to different str ess treatments. The cF labeling indicated the reproductive capacity of mixt ures of nontreated cells and cells killed at 70 degrees C very well. Howeve r, after treatment up to 60 degrees C the fraction of cF-labeled cells rema ined high, whereas the survival decreased for cells treated at above 50 deg rees C and was completely lost for those treated at 60 degrees C. In an ext ended series of experiments, cell suspensions were exposed to heating, free zing, low pH, or bile salts, after which the colony counts, acidification c apacity, glycolytic activity, PI exclusion, cF labeling, and cF efflux were measured and compared. The acidification capacity corresponded with the nu mber of CFU. The glycolytic activity, which is an indicator of vitality, wa s more sensitive to the stress conditions than the reproduction, acidificat ion, and fluorescence parameters. The cF labeling depended on membrane inte grity, as was confirmed by PI exclusion. The fraction of cF-labeled cells w as not a general indicator of reproduction or acidification, nor was PI exc lusion or cF labeling capacity (the internal cF concentration), When the ce lls were labeled by cF, a subsequent lactose-energized efflux assay was nee ded for decisive viability assessment. This novel assay proved to be a good and rapid indicator of the reproduction and acidification capacities of st ressed L. lactis and has potential for physiological research and dairy app lications related to lactic acid bacteria.