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Applied and Environmental Microbiology, August 1999, p. 3681-3689, Vol. 65, No. 8
Department of Food Technology and Nutritional
Sciences, Wageningen University and Research Centre, Wageningen, The
Netherlands
Received 17 December 1998/Accepted 11 May 1999
The aim of this study was to establish the use of the fluorescent
probes carboxyfluorescein (cF) and propidium iodide (PI) for rapid
assessment of viability, using Lactococcus lactis subsp. lactis ML3 exposed to different stress treatments. The cF
labeling indicated the reproductive capacity of mixtures of nontreated cells and cells killed at 70°C very well. However, after treatment up
to 60°C the fraction of cF-labeled cells remained high, whereas the
survival decreased for cells treated at above 50°C and was completely
lost for those treated at 60°C. In an extended series of experiments,
cell suspensions were exposed to heating, freezing, low pH, or bile
salts, after which the colony counts, acidification capacity,
glycolytic activity, PI exclusion, cF labeling, and cF efflux were
measured and compared. The acidification capacity corresponded with the
number of CFU. The glycolytic activity, which is an indicator of
vitality, was more sensitive to the stress conditions than the
reproduction, acidification, and fluorescence parameters. The cF
labeling depended on membrane integrity, as was confirmed by PI
exclusion. The fraction of cF-labeled cells was not a general indicator
of reproduction or acidification, nor was PI exclusion or cF labeling
capacity (the internal cF concentration). When the cells were labeled
by cF, a subsequent lactose-energized efflux assay was needed for
decisive viability assessment. This novel assay proved to be a good and
rapid indicator of the reproduction and acidification capacities of
stressed L. lactis and has potential for physiological
research and dairy applications related to lactic acid bacteria.
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Rapid Fluorescence Assessment of the Viability of
Stressed Lactococcus lactis
*
Corresponding author. Mailing address: Laboratory for
Food Microbiology, Wageningen University and Research Centre, P.O. Box 8129, 6700 EV Wageningen, The Netherlands. Phone: 31 317 484981. Fax:
31 317 484893. E-mail:
Tjakko.Abee{at}micro.fdsci.wau.nl.
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