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Appl Environ Microbiol, June 1998, p. 2072-2078, Vol. 64, No. 6
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Construction of an Efficient Biologically Contained Pseudomonas putida Strain and Its Survival in Outdoor Assays

Lázaro Molina,1 Cayo Ramos,2,3 María-Carmen Ronchel,2 Søren Molin,3 and Juan L. Ramos2,*

GX-Biosystems España, Granada,1 and Department of Biochemistry and Molecular and Cellular Biology of Plants, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, 18008 Granada,2 Spain, and Department of Microbiology, Technical University of Denmark, Lyngby, Denmark3

Received 10 November 1997/Accepted 17 March 1998

Active biological containment systems consist of two components, a killing element designed to induce cell death and a control element which modulates the expression of the killing function. We constructed a mini-Tn5 transposon bearing a fusion of the Plac promoter to the gef killing gene and a fusion of the Pm promoter to the lacI gene plus the positive regulator of the Pm promoter, the xylS gene. This mini-Tn5 transposon was transferred to the chromosome of Pseudomonas putida CMC4, and in culture this strain survived in the presence of 3-methylbenzoate (an XylS effector) and committed suicide in the absence of this aromatic compound. The rate of killing escape was on the order of 10-8 per cell and per generation. This contained strain and an uncontained control strain were used in outdoor tests performed in the spring-summer and autumn-winter periods to determine their survival in planted and unplanted soils with and without 3-methylbenzoate. In unplanted soils the numbers of both the contained strain and the uncontained strain per gram of soil tended to decrease, but the numbers of the contained strain decreased faster in soils without 3-methylbenzoate. The decrease in the number of CFU per gram of soil was faster in the spring-summer period than in the autumn-winter period. In planted soils survival in the rhizosphere and survival in bulk soil were studied. In the rhizosphere the uncontained control strain tended to become established at levels on the order of 105 to 106 CFU/g of soil regardless of the presence of 3-methylbenzoate. In the bulk soil the numbers of bacterial cells were 2 to 3 orders of magnitude lower. In planted soils the contained strain tended to disappear, but this tendency was more pronounced in the absence of 3-methylbenzoate and occurred faster in the summer assay than in the winter assay. We found no evidence of dispersal of the test strains outside the experimental plots.

* Corresponding author. Mailing address: CSIC-Estación Experimental del Zaidín, Apdo. Correos 419, E-18008 Granada, Spain. Phone: 34-58-121011. Fax: 34-58-129600. E-mail: jlramos{at}eez.csic.es.

Appl Environ Microbiol, June 1998, p. 2072-2078, Vol. 64, No. 6
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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