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Molecular Basis and Ecological Relevance of Caulobacter Cell Filamentation in Freshwater Habitats
Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute. Stockholm University, Science for Life Laboratory (SciLifeLab).ORCID iD: 0000-0001-9150-3217
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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2019 (English)In: mBio, ISSN 2161-2129, E-ISSN 2150-7511, Vol. 10, no 4, article id e01557-19Article in journal (Refereed) Published
Abstract [en]

All living cells are characterized by certain cell shapes and sizes. Many bacteria can change these properties depending on the growth conditions. The underlying mechanisms and the ecological relevance of changing cell shape and size remain unclear in most cases. One bacterium that undergoes extensive shape-shifting in response to changing growth conditions is the freshwater 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 demonstrated that this stationary-phase-induced filamentation results from downregulation of most critical cell cycle regulators and a consequent 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 ammonium. We found that these conditions are experienced in the summer months during algal blooms near the surface in freshwater lakes, a natural habitat of C. crescentus, suggesting that filamentous growth is a common response of C. crescentus to its environment. Finally, 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 might enable adaptation to changing environments.

Place, publisher, year, edition, pages
2019. Vol. 10, no 4, article id e01557-19
National Category
Microbiology
Research subject
Molecular Bioscience
Identifiers
URN: urn:nbn:se:su:diva-172586DOI: 10.1128/mBio.01557-19OAI: oai:DiVA.org:su-172586DiVA, id: diva2:1348459
Available from: 2019-09-04 Created: 2019-09-04 Last updated: 2019-09-04Bibliographically approved
In thesis
1. Regulation of the bacterial cell cycle in response to starvation
Open this publication in new window or tab >>Regulation of the bacterial cell cycle in response to starvation
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Bacteria have adapted to diverse environments, which are often unpredictable and constantly changing. In order to survive, bacteria need to make the most of nutrients while they are available, while being prepared to rapidly change their behaviour when conditions take a turn for the worse. One of the most central processes that must be regulated to ensure survival when conditions change is the cell cycle, the succession of DNA replication, chromosome segregation and cell division connecting growth and proliferation.

In this thesis, we investigate how environmental information, specifically nutrient availability, is used to modulate cell cycle progression. In Paper I, we uncover a mechanism used by Caulobacter crescentus to arrest DNA replication in response to nutrient depletion. We find that the essential replication initiator protein DnaA is eliminated under these conditions, and determine that this occurs by a mechanism based on constant degradation of DnaA by the protease Lon. This constant degradation is coupled with regulated translation of the dnaA mRNA to decrease DnaA synthesis as nutrient levels decrease. We found that this regulated translation of dnaA depends on its long 5′ untranslated region.

The replication initiator DnaA is conserved in almost all bacteria, and although some aspects of its regulation are maintained, others work differently in distantly related bacteria. In Paper II, we investigate how the enteric bacterium Escherichia coli regulates DNA replication at the onset of the stationary phase. We found that although DnaA is eliminated as growth slows, this downregulation is not required to arrest replication. We also found that the signalling molecule ppGpp, which is produced in response to starvation, is required for the elimination of DnaA at entry to stationary phase. High ppGpp levels lead to a block of replication initiation, however we found that chromosome content is still dramatically reduced at the onset of stationary phase in the absence of ppGpp, indicating that a ppGpp-independent mechanism is involved.

While bacteria are usually studied over short timeframes and under optimal conditions in the laboratory, in nature, bacteria are often found in environments where only very slow growth is possible. In Paper III, we investigate a change in morphology observed to occur in a small subpopulation of cells in cultures of C. crescentus after extended incubation in the stationary phase. These cells form long, helical filaments. We determined that the filamentation arises as a result of a block of DNA replication and cell division while growth continues, and that this can be induced by a combination of conditions in the medium: low phosphate, high pH and excess ammonium. We find that these conditions occur in freshwater lakes during persistent algal blooms in the summer months, indicating that this response might occur in the wild.

In summary, this thesis provides new insight into the mechanisms bacteria use to adapt their cell cycle, and specifically, DNA replication to changes in their environment, how bacteria are able to change their morphology by disrupting the coupling between growth and the cell cycle, and investigates how this morphological plasticity may be advantageous in natural environments.

Place, publisher, year, edition, pages
Stockholm: Department of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, 2019. p. 64
National Category
Biological Sciences
Research subject
Molecular Bioscience
Identifiers
urn:nbn:se:su:diva-172600 (URN)978-91-7797-823-7 (ISBN)978-91-7797-824-4 (ISBN)
Public defence
2019-10-18, Vivi Täckholmsalen (Q-salen), NPQ-huset, Svante Arrhenius väg 20, 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: 2019-09-25 Created: 2019-09-04 Last updated: 2019-09-17Bibliographically approved
2. 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: 2019-09-04Bibliographically approved

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