Brugia Malayi - Clark Science Center - Smith College

Temperature-Dependent Regulation of Fimbrial Phase Variation in E. coli
Da Yae Lee
Source: rehydrate.org
How does temperature act as an environmental cue to elicit genotypic and phenotypic responses in bacteria? This summer, as
part of my continuing research in the White-Ziegler lab on the effect of thermoregulation in Escherichia coli, I examined the
dependence of phase variation [P/V] in its fimbrial operons on the temperature of growth. For E. coli strains capable of intestinal
or extraintestinal infections, fimbriae are essential colonization and virulent factors, aiding in bacterial adhesion to and invasion
of host epithelial cells. 1 As energy-expensive as it is to make fimbriae, fimbrial adhesions are tightly regulated in part by phase
variation, or the reversible switching of phenotypes based on protein expression to generate a heterogenic population. 2
For this study involving transcriptional lac fusions of five E. coli fimbrial operons (fan, pap, daa, sfa and fim), two models of
thermoregulation were tested: does temperature act as a precise switch that turns on or off an operon at a defined degree (˚C),
or does it gradually modulate the amount of fimbrial operon transcripts with changing temperature (observed by a change in the
intensity of blue color of the colonies)? For the experiment, E. coli K-12 strain was grown overnight on five M9 X-gal plates at
37˚C, each plate inoculated with a colony of a different fimbrial operon-lac fusion. Of these control colonies, one darkest blue
colony (blue representing P/V ON; white representing P/V OFF) was streaked onto four new X-gal plates, each then incubated
at a new temperature (30, 29, 27 and 25˚C). Once full-sized colonies had grown, the plates were observed for two notable features:
(1) how does the intensity of blue colonies compare to those on other plates and, (2) what is the temperature (if at all) at which
the operon completely turns off?
The fan operon, which is known not to be phase variable, served as the negative control. Across all temperatures, all fan-lac
colonies of the clonal population were blue. At 25˚C, however, all colonies were white, indicating the gene was turned off. Of
the four known phase-varying operons—pap, daa, sfa, and fim—pap- and daa-lac expressing colonies responded to decreasing
temperature with a decreasing intensity of blue, whereas sfa and fim-lac expressing colonies did not show a sign of intensity
change. A common feature was all four phase-varying fimbrial genes seemed to turn off completely by 25˚C, producing all-white
colonies. The results suggest that P/V is turned OFF somewhere between 27 and 25˚C. As for the two models of temperature
regulation proposed, both appear to be at work but affecting different genes. Further studies will attempt to define the critical
temperature at which fimbrial operons are turned off. Also genotypic data using qRT-PCR and microarrays will be used to confirm
these phenotypic results, to describe how temperature modulates the shift from a less virulent (without fimbriae) to virulent (with
fimbriae) lifestyle. (Supported by the National Institute of Health)
Advisor: Christine White-Ziegler
References:
1 Hacker, J. 1992. Role of fimbrial adhesins in the pathogenesis of Escherichia coli infections. Canadian Journal of Microbiology, 38(7): 720-7.
2 Van der Woude, M. W., Bäumler A.J. 2004. Phase and antigenic variation in bacteria. Clinical Microbiology Review, 17 (3): 581-611.
2012
36