Biology subjects

Berne, C.

Publications and source records attributed to Berne, C..

5 recordsLinked to original sources

bioRxiv2023

The HmrABCX pathway regulates the transition between motile and sessile lifestyles in Caulobacter crescentus by a HfiA-independent mechanism

Through its cell cycle, the bacterium Caulobacter crescentus switches from a motile, free-living state, to a sessile surface-attached cell. During this coordinated process, cells undergo irreversible morphological changes, such as shedding of their polar flagellum and synthesis of an adhesive holdfast at the same pole. In this work, we used genetic screens to identify genes involved in the regulation of the motile to sessile lifestyle transition. We identified a predicted hybrid histidine kinase that inhibits biofilm formation and activates the motile lifestyle: HmrA (Holdfast and motility regulator A). Genetic screens and genomic localization led to the identification of additional genes that regulate the proportion of cells harboring an active flagellum or a holdfast and that form a putative phosphorelay pathway with HmrA. Further genetic analysis indicates that the Hmr pathway is independent of the holdfast synthesis regulator HfiA and may impact c-di-GMP synthesis through the diguanylate cyclase DgcB pathway. Finally, we provide evidence that the Hmr pathway is involved in the regulation of sessile-to-motile lifestyle as a function of environmental stresses, namely excess copper and non-optimal temperatures. IMPORTANCEComplex communities attached to a surface, or biofilms, represent the major lifestyle of bacteria in the environment. Such a sessile state enables its inhabitants to be more resistant to adverse environmental conditions. Thus, having a deeper understanding of the underlying mechanisms that regulate the transition between the motile and the sessile states could help design strategies to improve biofilms when they are beneficial or impede them when they are detrimental. For C. crescentus motile cells, the transition to the sessile lifestyle is irreversible, and this decision is regulated at several levels. In this work, we describe a putative phosphorelay that promotes the motile lifestyle and inhibits biofilm formation, providing new insights into the control of adhesin production that leads to the formation of biofilms.

microbiology↗

bioRxiv2022

eDNA-stimulated cell dispersion from Caulobacter crescentus biofilms upon oxygen limitation is dependent on a toxin-antitoxin system.

In their natural environment, most bacteria preferentially live as complex surface-attached multicellular colonies called biofilms. Biofilms begin with a few cells adhering to a surface, where they multiply to form a mature colony. When conditions deteriorate, cells can leave the biofilm. This dispersion is thought to be an important process that modifies the overall biofilm architecture and that promotes colonization of new environments. In Caulobacter crescentus biofilms, extracellular DNA (eDNA) is released upon cell death and prevents newborn cells from joining the established biofilm. Thus, eDNA promotes the dispersal of newborn cells and the subsequent colonization of new environments. These observations suggest that eDNA is a cue for sensing detrimental environmental conditions in the biofilm. Here we show that the toxin-antitoxin ParDE4 stimulates cell death in areas of a biofilm with decreased O2 availability. In conditions where O2 availability is low, eDNA concentration is correlated with cell death. Cell dispersal away from biofilms is decreased when parDE4 is deleted, probably due to the lower local eDNA concentration. Expression of parDE4 is positively regulated by O2 and the expression of this operon is decreased in biofilms where O2 availability is low. Thus, PCD by an O2-regulated toxin-antitoxin system stimulates dispersal away from areas of a biofilm with decreased O2 availability and favors colonization of a new, more hospitable environment.

microbiology↗

bioRxiv2019

The two chemotaxis in Caulobacter crescentus operons play different roles in chemotaxis and biofilm regulation

The holdfast polysaccharide adhesin is crucial for irreversible cell adhesion and biofilm formation in Caulobacter crescentus. Holdfast production is tightly controlled via developmental regulators, and environmental and physical signals. Here we identified a novel mechanism of holdfast production regulation that involves chemotaxis proteins. We characterized the two identified chemotaxis operons of C. crescentus and showed that only the previously characterized, major operon is involved in chemotactic response towards different carbon sources. However, both chemotaxis operons encoded in the C. crescentus genome play a role in biofilm formation and holdfast production, by regulating the expression of hfiA, the gene encoding the holdfast inhibitor HfiA. We show that CheA and CheB proteins act in an antagonistic manner: while the two CheA proteins negatively regulate hfiA expression, the CheB proteins are positive regulators, thus providing a modulation of holdfast synthesis and surface attachment. IMPORTANCEChemosensory pathways are major signal transduction mechanisms in bacteria. These systems are involved in chemotaxis and other cell responses to environment conditions, such as production of adhesins that enable irreversible adhesion to a surface and surface colonization. The C. crescentus genome encodes two complete chemotaxis operons. Here we characterized the second, novel chemotaxis-like operon. While only the major chemotaxis operon is involved in chemotaxis, both chemotaxis systems modulate C. crescentus adhesion by controlling expression of the holdfast synthesis inhibitor, HfiA. Thus, we identified a new level in holdfast regulation, providing new insights into the control of adhesin production that leads to the formation of biofilms.

microbiology↗

bioRxiv2019

Comparative analysis of ionic strength tolerance between freshwater and marine Caulobacterales adhesins

Bacterial adhesion is affected by environmental factors, such as ionic strength, pH, temperature, and shear forces, and therefore marine bacteria must have developed holdfasts with different composition and structures than their freshwater counterparts to adapt to their natural environment. The dimorphic -proteobacterium Hirschia baltica is a marine budding bacterium in the Caulobacterales clade. H. baltica uses a polar adhesin, the holdfast, located at the cell pole opposite the reproductive stalk for surface attachment and cell-cell adhesion. The holdfast adhesin has been best characterized in Caulobacter crescentus, a freshwater member of the Caulobacterales, and little is known about holdfast composition and properties in marine Caulobacterales. Here we use H. baltica as a model to characterize holdfast properties in marine Caulobacterales. We show that freshwater and marine Caulobacterales use similar genes in holdfast biogenesis and that these genes are highly conserved among the two genera. We also determine that H. baltica produces larger holdfast than C. crescentus and that those holdfasts have a different chemical composition, as they contain N-acetylglucosamine and galactose monosaccharide residues and proteins, but lack DNA. Finally, we show that H. baltica holdfasts tolerate higher ionic strength than those of C. crescentus. We conclude that marine Caulobacterales holdfasts have physicochemical properties that maximize binding in high ionic strength environments.\n\nIMPORTANCEMost bacteria spend a large amount of their lifespan attached to surfaces, forming complex multicellular communities called biofilms. Bacteria can colonize virtually any surface, therefore they have adapted to bind efficiently in very different environments. In this study, we compare the adhesive holdfasts produced by the freshwater bacterium C. crescentus and a relative, the marine bacterium H. baltica. We show that H. baltica holdfasts have a different morphology and chemical composition, and tolerate high ionic strength. Our results show that H. baltica holdfast is an excellent model to study the effect of ionic strength on adhesion and providing insights on the physicochemical properties required for adhesion in the marine environment.

microbiology↗

bioRxiv2017

Layered structure and complex mechanochemistry of a strong bacterial adhesive

While designing adhesives that perform in aqueous environments has proven challenging for synthetic adhesives, microorganisms commonly produce bioadhesives that efficiently attach to a variety of substrates, including wet surfaces that remain a challenge for industrial adhesives. The aquatic bacterium Caulobacter crescentus uses a discrete polar polysaccharide complex, the holdfast, to strongly attach to surfaces and resist flow. The holdfast is extremely versatile and has an impressive adhesive strength. Here, we use atomic force microscopy (AFM) to unravel the complex structure of the holdfast and characterize its chemical constituents and their role in adhesion. We used purified holdfasts to dissect the intrinsic properties of this component as a biomaterial, without the effect of the bacterial cell body. Our data support a model where the holdfast is a heterogeneous material composed of two layers: a stiff nanoscopic core, covered by a sparse, flexible brush layer. These two layers contain not only N-acetyl-D-glucosamine (NAG), the only yet identified component present in the holdfast, but also peptides and DNA, which provide structure and adhesive character. Biochemical experiments suggest that, while polypeptides are the most important components for adhesive force, the presence of DNA mainly impacts the brush layer and initial adhesion, and NAG plays a primarily structural role within the core. Moreover, our results suggest that holdfast matures structurally, becoming more homogeneous over time. The unanticipated complexity of both the structure and composition of the holdfast likely underlies its distinctive strength as a wet adhesive and could inform the development of a versatile new family of adhesives.

biophysics↗