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Journal of Bacteriology, December 1999, p. 7470-7478, Vol. 181, No. 24
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
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
-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.
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