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Molecular basis and ecological relevance of Caulobacter cell filamentation in fresh water habitats
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

All living cells are characterized by a certain cell shape and size. Many bacteria can change these properties depending on the growth conditions. The underlying mechanisms and the ecological relevance of changing cell shape and size remains unclear in most cases. One bacterium that undergoes extensive shape-shifting in response to changing growth conditions is the fresh water bacterium Caulobacter crescentus. When incubated for an extended time in stationary phase, a subpopulation of C. crescentus forms viable filamentous cells with a helical shape. Here, we demonstrate that this stationary phase-induced filamentation results from downregulation of most critical cell cycle regulators and consequently a block of DNA replication and cell division while cell growth and metabolism continue. Our data indicate that this response is triggered by a combination of three stresses caused by prolonged growth in complex medium, namely the depletion of phosphate, alkaline pH and an excess of nitrogen. We find that these conditions are experienced in the summer months during algae blooms near the surface in fresh water lakes, a natural habitat of C. crescentus, suggesting that filamentous growth is a common response of Caulobacter to its natural environment. Finally, our data suggest that cell filamentation under these conditions may confer an adaptive advantage. We demonstrate that when grown in a biofilm, the filamentous cells can reach beyond the surface of the biofilm and potentially access nutrients or release progeny. Altogether our work highlights the ability of bacteria to alter their morphology and suggests how this behavior enables adaptation to changing environments.

National Category
Microbiology
Research subject
Molecular Bioscience
Identifiers
URN: urn:nbn:se:su:diva-158107OAI: oai:DiVA.org:su-158107DiVA, id: diva2:1233180
Available from: 2018-07-16 Created: 2018-07-16 Last updated: 2018-07-27Bibliographically approved
In thesis
1. Coping with Stress: Regulation of the Caulobacter crescentus cell cycle in response to environmental cues
Open this publication in new window or tab >>Coping with Stress: Regulation of the Caulobacter crescentus cell cycle in response to environmental cues
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

All organisms have to respond to environmental changes to maintain cellular and genome integrity. In particular, unicellular organisms like bacteria must be able to analyze their surroundings and rapidly adjust their growth mode and cell cycle program in response to environmental changes, such as changes in nutrient availability, temperature, osmolarity, or pH. Additionally, they have to compete with other species for nutrients and evade possible predators or the immune system. Bacteria exhibit a myriad of sophisticated regulatory pathways that allow them to cope with various kinds of threats and ensure their survival. However, the precise molecular mechanisms underlying these responses remain in many cases incompletely described. This thesis focuses on the mechanisms that adjust growth and cell cycle progression of Caulobacter crescentus under adverse conditions.

In paper I we describe a mechanism by which environmental information is transduced via the membrane-bound cell cycle kinase CckA into the cell division program of C. crescentus. This mechanism ensures rapid dephosphorylation and clearance of the cell cycle master regulator CtrA under salt and ethanol stress. The downregulation of CtrA leads to a cell division block and cell filamentation, which provides a growth advantage under these conditions.

Cell filamentation of C. crescentus can also be observed in the late stationary phase, in which a small subpopulation of cells transforms into helical shaped filaments. In these cells not only CtrA but all major cell cycle regulators are cleared (paper II), leading to a situation in which cells block their cell cycle but continue to grow. We found that a combination of different stresses, namely phosphate starvation, high pH, and excess nitrogen, triggers this response. These stresses can also be observed in C. crescentus’ natural freshwater habitat during algae blooms. Furthermore, our results indicate that filamentous cells are able to reach beyond biofilm surfaces, possibly enabling cells to reach nutrients and to release progeny.

While our studies highlight that cell filamentation is a common bacterial response to stress, some stress conditions, such as acute proteotoxic stress, lead to a growth arrest. In paper III we show that the regulatory interaction between the major chaperone DnaK and the heat shock sigma factor σ32 adjusts growth rate in response to changes of the global protein folding state. We show that high σ32 activity inhibits growth by re-allocating cellular resources from proliferative to maintenance functions. Under stress conditions when σ32 is active, this re-allocation likely helps cells to survive. However, under non-stress conditions unrepressed σ32 activity is detrimental. We demonstrate that in the absence of stress, the DnaK chaperone is absolutely necessary to limit σ32 activity and in this way to allow rapid proliferation.

In summary, the described studies highlight critical pathways that allow C. crescentus to integrate environmental information with cell cycle and growth regulation and shed new light onto the mechanisms by which bacteria adapt to their environment and in this way ensure their survival.

Place, publisher, year, edition, pages
Stockholm: Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, 2018
Keywords
Caulobacter crescentus, bacteria, cell cycle, stress, filamentation, cell morphology, DNA replication, cell division
National Category
Microbiology
Research subject
Molecular Bioscience
Identifiers
urn:nbn:se:su:diva-158108 (URN)978-91-7797-381-2 (ISBN)978-91-7797-380-5 (ISBN)
Public defence
2018-09-21, Vivi Täckholmsalen, NPQ-huset, Svante Arrhenius väg 20 A, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2018-08-29 Created: 2018-07-27 Last updated: 2018-09-27Bibliographically approved

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