Stochasticity in gene expression and in other cellular components results in substantial variation within clonal cell populations. Such variation has been speculated to be advantageous in fluctuating environments, where it allows subsets of a population to be well-adapted to unpredictable changes in environment. In a constant environment, however, variation would result in cells being non-optimal.
We investigate what the effect of noise in gene expression is on fitness in a constant environment. We address this issue by means of time-lapse microscopy on individual E. coli cells.
We also look at the effect of noise in the situation where cells need to adapt to a new environment.
Persons Involved:
- Daan Kiviet
- Sarah Boulineau
Fimbriae are thread-like polymers displayed in large amounts on the bacterial surface and used by many pathogens to attach to receptors on host tissue surfaces in an event of infection.
In E. coli, the expression of type 1 fimbriae is believed to act as virulence factor in urinary tract infections. Despite the advantages on cell survival, fimbriae also activate inflamatory response of the host. Hence there is a trade-off between virulence and host immune response to a piliated bacterial population. The expression of E.coli fimbriae is controlled by a 314 bp invertible DNA segment in front of fimA gene, coding for the major structural subunit of fimbriae. This invertible element contains a promoter which switches on the transcription of fimbrial structural genes in one direction, but not the other. The fim inversion requires a recombinase, either fimE (on-to-off inversion only) or fimB (both directions).
Previously, studies in bulk have shown that frequency of switching is affected by environmental conditions, such as nutrition and temperature. In wild-type E.coli, FimE mediated on-to-off switching is much faster (at least 0,01 per cell per generation) than FimB-promoted switching (10-3 to 10-4). Besides its role as a recombinase, it has been speculated that FimE also acts as a memory to know how long the switch has been in the on position. This is possible because FimE production increases over time while the switch is in the on position.
Our aim is to study Fim switch behaviour in single cell level in real time. We inserted GFP into chromosomal fimA gene as a marker for the fim switch activity. Then, we follow growth of cells into microcolonies, using automated phase contrast and fluorescence microscopy, observing the switching events.
Publications:
- "Direct observation of type 1 fimbrial switching" - EMBO Reports 10:527-532 (2009)