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Applied and Environmental Microbiology, October 1999, p. 4646-4651, Vol. 65, No. 10
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Green Fluorescent Protein-Marked Pseudomonas fluorescens: Localization, Viability, and Activity in the Natural Barley Rhizosphere

Bo Normander,1,* Niels B. Hendriksen,1 and Ole Nybroe2

Department of Marine Ecology and Microbiology, National Environmental Research Institute, DK-4000 Roskilde,1 and Department of Ecology, The Royal Veterinary and Agricultural University, DK-1871 Frederiksberg C,2 Denmark

Received 15 March 1999/Accepted 4 August 1999

The gfp-tagged Pseudomonas fluorescens biocontrol strain DR54-BN14 was introduced into the barley rhizosphere. Confocal laser scanning microscopy revealed that the rhizoplane populations of DR54-BN14 on 3- to 14-day-old roots were able to form microcolonies closely associated with the indigenous bacteria and that a majority of DR54-BN14 cells appeared small and almost coccoid. Information on the viability of the inoculant was provided by a microcolony assay, while measurements of cell volume, the intensity of green fluorescent protein fluorescence, and the ratio of dividing cells to total cells were used as indicators of cellular activity. At a soil moisture close to the water-holding capacity of the soil, the activity parameters suggested that the majority of DR54-BN14 cells were starving in the rhizosphere. Nevertheless, approximately 80% of the population was either culturable or viable but nonculturable during the 3-week incubation period. No impact of root decay on viability was observed, and differences in viability or activity among DR54-BN14 cells located in different regions of the root were not apparent. In dry soil, however, the nonviable state of DR54-BN14 was predominant, suggesting that desiccation is an important abiotic regulator of cell viability.

* Corresponding author. Mailing address: Department of Marine Ecology and Microbiology, National Environmental Research Institute, P.O. Box 358, DK-4000 Roskilde, Denmark. Phone: 45 4630 1244. Fax: 45 4630 1216. E-mail: bn{at}dmu.dk.

Applied and Environmental Microbiology, October 1999, p. 4646-4651, Vol. 65, No. 10
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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