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Journal of Bacteriology, December 1999, p. 7470-7478, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Type 1 Fimbriation and Phase Switching in a Natural Escherichia coli fimB Null Strain, Nissle 1917

Bodil Stentebjerg-Olesen,1 Trinad Chakraborty,2 and Per Klemm1,*

Department of Microbiology, Technical University of Denmark, DK-2800 Lyngby, Denmark,1 and Institute for Medical Microbiology, Justus-Liebig Universität, D-35392 Giessen, Germany2

Received 23 August 1999/Accepted 27 September 1999

Escherichia coli Nissle 1917 has been used as a probiotic against intestinal disorders for many decades. It is a good colonizer of the human gut and has been reported to be able to express type 1 fimbriae. Type 1 fimbriae are surface organelles which mediate alpha -D-mannose-sensitive binding to various host cell surfaces. The expression is phase variable, and two tyrosine recombinases, FimB and FimE, mediate the inversion of the fimbrial phase switch. Current evidence suggests that FimB can carry out recombination in both directions, whereas FimE-catalyzed switching is on to off only. We show here that under liquid shaking growth conditions, Nissle 1917 did not express type 1 fimbriae, due to a truncation of the fimB gene by an 1,885-bp insertion element. Despite its fimB null status, Nissle 1917 was still capable of off-to-on switching of the phase switch and expressing type 1 fimbriae when grown under static conditions. This phase switching was not catalyzed by FimE, by truncated FimB, or by information residing within the insertion element. No further copies of fimB seemed to be present on the chromosome of Nissle 1917, suggesting that another tyrosine recombinase in Nissle 1917 is responsible for the low-frequency off-to-on inversion of the phase switch that is strongly favored under static growth conditions. This is the first report documenting the non-FimB- or non-FimE-catalyzed inversion of the fim switch.


* Corresponding author. Mailing address: Department of Microbiology, Bldg. 301, Technical University of Denmark, DK-2800 Lyngby, Denmark. Phone: 45 45 25 25 06. Fax: 45 45 93 28 09. E-mail: impk{at}pop.dtu.dk.


Journal of Bacteriology, December 1999, p. 7470-7478, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.



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