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Applied and Environmental Microbiology, April 2008, p. 2441-2446, Vol. 74, No. 8
0099-2240/08/$08.00+0     doi:10.1128/AEM.02265-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Optical Tweezers Cause Physiological Damage to Escherichia coli and Listeria Bacteria{triangledown}

M. B. Rasmussen,1 L. B. Oddershede,2 and H. Siegumfeldt1*

Department of Food Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg C, Denmark,1 Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, 2100 Copenhagen, Denmark2

Received 26 September 2007/ Accepted 18 February 2008

We investigated the degree of physiological damage to bacterial cells caused by optical trapping using a 1,064-nm laser. The physiological condition of the cells was determined by their ability to maintain a pH gradient across the cell wall; healthy cells are able to maintain a pH gradient over the cell wall, whereas compromised cells are less efficient, thus giving rise to a diminished pH gradient. The pH gradient was measured by fluorescence ratio imaging microscopy by incorporating a pH-sensitive fluorescent probe, green fluorescent protein or 5(6)-carboxyfluorescein diacetate succinimidyl ester, inside the bacterial cells. We used the gram-negative species Escherichia coli and three gram-positive species, Listeria monocytogenes, Listeria innocua, and Bacillus subtilis. All cells exhibited some degree of physiological damage, but optically trapped E. coli and L. innocua cells and a subpopulation of L. monocytogenes cells, all grown with shaking, showed only a small decrease in pH gradient across the cell wall when trapped by 6 mW of laser power for 60 min. However, another subpopulation of Listeria monocytogenes cells exhibited signs of physiological damage even while trapped at 6 mW, as did B. subtilis cells. Increasing the laser power to 18 mW caused the pH gradient of both Listeria and E. coli cells to decrease within minutes. Moreover, both species of Listeria exhibited more-pronounced physiological damage when grown without shaking than was seen in cells grown with shaking, and the degree of damage is therefore also dependent on the growth conditions.

* Corresponding author. Mailing address: Department of Food Science, University of Copenhagen, Rolighedsvej 30, 1958 Frederiksberg C, Denmark. Phone: 45 35 333286. Fax: 45 35 333214. E-mail: hsi{at}life.ku.dk

{triangledown} Published ahead of print on 29 February 2008.

Applied and Environmental Microbiology, April 2008, p. 2441-2446, Vol. 74, No. 8
0099-2240/08/$08.00+0     doi:10.1128/AEM.02265-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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