Dynamics of the Bacterial Intermediate Filament Crescentin In Vitro and In Vivo

Background

Crescentin, the recently discovered bacterial intermediate filament protein, organizes into an extended filamentous structure that spans the length of the bacterium Caulobacter crescentus and plays a critical role in defining its curvature. The mechanism by which crescentin mediates cell curvature and whether crescentin filamentous structures are dynamic and/or polar are not fully understood.

Methodology/Principal Findings

Using light microscopy, electron microscopy and quantitative rheology, we investigated the mechanics and dynamics of crescentin structures. Live-cell microscopy reveals that crescentin forms structures in vivo that undergo slow remodeling. The exchange of subunits between these structures and a pool of unassembled subunits is slow during the life cycle of the cell however; in vitro assembly and gelation of C. crescentus crescentin structures are rapid. Moreover, crescentin forms filamentous structures that are elastic, solid-like, and, like other intermediate filaments, can recover a significant portion of their network elasticity after shear. The assembly efficiency of crescentin is largely unaffected by monovalent cations (K⁺, Na⁺), but is enhanced by divalent cations (Mg²⁺, Ca²⁺), suggesting that the assembly kinetics and micromechanics of crescentin depend on the valence of the ions present in solution.

Conclusions/Significance

These results indicate that crescentin forms filamentous structures that are elastic, labile, and stiff, and that their low dissociation rate from established structures controls the slow remodeling of crescentin in C. crescentus.     

Mutations in the Lipopolysaccharide Biosynthesis Pathway Interfere with Crescentin-Mediated Cell Curvature in Caulobacter crescentus

Bacterial cell morphogenesis requires coordination among multiple cellular systems, including the bacterial cytoskeleton and the cell wall. In the vibrioid bacterium Caulobacter crescentus, the intermediate filament-like protein crescentin forms a cell envelope-associated cytoskeletal structure that controls cell wall growth to generate cell curvature. We undertook a genetic screen to find other cellular components important for cell curvature. Here we report that deletion of a gene (wbqL) involved in the lipopolysaccharide (LPS) biosynthesis pathway abolishes cell curvature. Loss of WbqL function leads to the accumulation of an aberrant O-polysaccharide species and to the release of the S layer in the culture medium. Epistasis and microscopy experiments show that neither S-layer nor O-polysaccharide production is required for curved cell morphology per se but that production of the altered O-polysaccharide species abolishes cell curvature by apparently interfering with the ability of the crescentin structure to associate with the cell envelope. Our data suggest that perturbations in a cellular pathway that is itself fully dispensable for cell curvature can cause a disruption of cell morphogenesis, highlighting the delicate harmony among unrelated cellular systems. Using the wbqL mutant, we also show that the normal assembly and growth properties of the crescentin structure are independent of its association with the cell envelope. However, this envelope association is important for facilitating the local disruption of the stable crescentin structure at the division site during cytokinesis.Most bacterial species display a particular cellular morphology that is generally preserved across generations. The production and maintenance of shape require coordination among multiple cellular systems positioned at different places within the cell. The peptidoglycan cell wall, located external to the cytoplasmic membrane, is an important structural element that is required for shape maintenance. The processes governing the localization and timing of cell wall growth and turnover are likewise critical (8, 9, 23). The bacterial cytoskeleton is thought to play a central role in cell morphogenesis by exerting spatiotemporal control over peptidoglycan growth (8, 9, 23). In order for it to do so, there are numerous proteins that are required to connect cytoskeletal control mechanisms to the periplasmic enzymes that directly synthesize and modify the peptidoglycan cell wall. These proteins, such as MreC, MreD, RodA, and RodZ, are essential for maintenance of cell shape and are positioned in the cytoplasmic membrane to presumably link cytoskeletal elements in the cytoplasm to the activities of peptidoglycan-modifying enzymes in the periplasm (2, 5, 9, 23, 29). Some bacterial species also contain additional components that make important contributions to cell shape, such as cell wall teichoic acids in Gram-positive bacteria (9) and periplasmic flagella in spirochetes (41).In the vibrioid bacterium Caulobacter crescentus, an intermediate filament-like protein, crescentin, is required for cell curvature (3). Crescentin forms an intracellular filamentous structure that is associated with the cell wall and is thought to mechanically govern cell wall growth to produce cell curvature (7). The crescentin structure is localized along the inner curvature of the cell under the cytoplasmic membrane (3, 7) and is highly stable, with no detectable subunit exchange (10). The association between the crescentin structure and the cell envelope appears essential for its function, since an attachment-defective crescentin mutant is unable to support cell curvature (7). The function of the actin-like protein MreB is also critical for the envelope association of the crescentin structure (10), and MreB may provide one part of the connection between the crescentin structure and the peptidoglycan cell wall.Since bacterial morphogenesis requires multiple cellular components and systems, we used a genetic screen to find other factors important for cell curvature in C. crescentus. Surprisingly, we found that an alteration in the lipopolysaccharide (LPS) biosynthesis pathway can have a catastrophic effect on the ability of the crescentin structure to associate with the cell envelope and govern cell curvature.     

Bacterial cell curvature through mechanical control of cell growth

The cytoskeleton is a key regulator of cell morphogenesis. Crescentin, a bacterial intermediate filament‐like protein, is required for the curved shape of Caulobacter crescentus and localizes to the inner cell curvature. Here, we show that crescentin forms a single filamentous structure that collapses into a helix when detached from the cell membrane, suggesting that it is normally maintained in a stretched configuration. Crescentin causes an elongation rate gradient around the circumference of the sidewall, creating a longitudinal cell length differential and hence curvature. Such curvature can be produced by physical force alone when cells are grown in circular microchambers. Production of crescentin in Escherichia coli is sufficient to generate cell curvature. Our data argue for a model in which physical strain borne by the crescentin structure anisotropically alters the kinetics of cell wall insertion to produce curved growth. Our study suggests that bacteria may use the cytoskeleton for mechanical control of growth to alter morphology.     

Growth Medium-Dependent Glycine Incorporation into the Peptidoglycan of Caulobacter crescentus

The peptidoglycan (PG) is a macromolecular component of the bacterial cell wall that maintains the shape and integrity of the cell. The PG of Caulobacter crescentus, unlike that of many other Gram-negative bacteria, has repeatedly been shown to contain significant amounts of glycine. This compositional peculiarity has been deemed an intrinsic characteristic of this species. By performing a comprehensive qualitative and quantitative analysis of the C. crescentus PG by high-performance liquid chromatography (HPLC) and mass spectrometry (MS), we show here that glycine incorporation into the C. crescentus PG depends on the presence of exogenous glycine in the growth medium. High levels of glycine were detected at the fifth position of the peptide side chains of PG isolated from C. crescentus cells grown in the complex laboratory medium PYE or in defined medium (M2G) supplemented with casamino acids or glycine alone. In contrast, glycine incorporation was undetectable when cells were grown in M2G medium lacking glycine. Remarkably, glycine incorporation into C. crescentus peptidoglycan occurred even in the presence of low millimolar to sub-millimolar concentrations of free glycine. High glycine content in the PG had no obvious effects on growth rates, mode of PG incorporation or cell morphology. Hence, the C. crescentus PG is able to retain its physiological functions in cell growth and morphogenesis despite significant alterations in its composition, in what we deem to be unprecedented plasticity.     

Physics of Bacterial Morphogenesis

Summary: Bacterial cells utilize three-dimensional (3D) protein assemblies to perform important cellular functions such as growth, division, chemoreception, and motility. These assemblies are composed of mechanoproteins that can mechanically deform and exert force. Sometimes, small-nucleotide hydrolysis is coupled to mechanical deformations. In this review, we describe the general principle for an understanding of the coupling of mechanics with chemistry in mechanochemical systems. We apply this principle to understand bacterial cell shape and morphogenesis and how mechanical forces can influence peptidoglycan cell wall growth. We review a model that can potentially reconcile the growth dynamics of the cell wall with the role of cytoskeletal proteins such as MreB and crescentin. We also review the application of mechanochemical principles to understand the assembly and constriction of the FtsZ ring. A number of potential mechanisms are proposed, and important questions are discussed.     

Effects of (p)ppGpp on the Progression of the Cell Cycle of Caulobacter crescentus

Bacteria must control the progression of their cell cycle in response to nutrient availability. This regulation can be mediated by guanosine tetra- or pentaphosphate [(p)ppGpp], which are synthesized by enzymes of the RelA/SpoT homologue (Rsh) family, particularly under starvation conditions. Here, we study the effects of (p)ppGpp on the cell cycle of Caulobacter crescentus, an oligotrophic bacterium with a dimorphic life cycle. C. crescentus divides asymmetrically, producing a motile swarmer cell that cannot replicate its chromosome and a sessile stalked cell that is replication competent. The swarmer cell rapidly differentiates into a stalked cell in appropriate conditions. An artificial increase in the levels of (p)ppGpp in nonstarved C. crescentus cells was achieved by expressing a truncated relA gene from Escherichia coli, encoding a constitutively active (p)ppGpp synthetase. By combining single-cell microscopy, flow cytometry approaches, and swarming assays, we show that an increase in the intracellular concentration of (p)ppGpp is sufficient to slow down the swarmer-to-stalked cell differentiation process and to delay the initiation of chromosome replication. We also present evidence that the intracellular levels of two master regulators of the cell cycle of C. crescentus, DnaA and CtrA, are modulated in response to (p)ppGpp accumulation, even in the absence of actual starvation. CtrA proteolysis and DnaA synthesis seem indirectly inhibited by (p)ppGpp accumulation. By extending the life span of the motile nonreproductive swarmer cell and thus promoting dispersal and foraging functions over multiplication under starvation conditions, (p)ppGpp may play a central role in the ecological adaptation of C. crescentus to nutritional stresses.     

OmpW of Caulobacter crescentus Functions as an Outer Membrane Channel for Cations

Caulobacter crescentus is an oligotrophic bacterium that lives in dilute organic environments such as soil and freshwater. This bacterium represents an interesting model for cellular differentiation and regulation because daughter cells after division have different forms: one is motile while the other is non-motile and can adhere to surfaces. Interestingly, the known genome of C. crescentus does not contain genes predicted to code for outer membrane porins of the OmpF/C general diffusion type present in enteric bacteria or those coding for specific porins selective for classes of substrates. Instead, genes coding for 67 TonB-dependent outer membrane receptors have been identified, suggesting that active transport of specific nutrients may be the norm. Here, we report that high channel-forming activity was observed with crude outer membrane extracts of C. crescentus in lipid bilayer experiments, indicating that the outer membrane of C. crescentus contained an ion-permeable channel with a single-channel conductance of about 120 pS in 1M KCl. The channel-forming protein with an apparent molecular mass of about 20 kDa was purified to homogeneity. Partial protein sequencing of the protein indicated it was a member of the OmpW family of outer membrane proteins from Gram-negative bacteria. This channel was not observed in reconstitution experiments with crude outer membrane extracts of an OmpW deficient C. crescentus mutant. Biophysical analysis of the C. crescentus OmpW suggested that it has features that are special for general diffusion porins of Gram-negative outer membranes because it was not a wide aqueous channel. Furthermore, OmpW of C. crescentus seems to be different to known OmpW porins and has a preference for ions, in particular cations. A putative model for OmpW of C. crescentus was built on the basis of the known 3D-structures of OmpW of Escherichia coli and OprG of Pseudomonas aeruginosa using homology modeling. A comparison of the two known structures with the model of OmpW of C. crescentus suggested that it has a more hydrophilic interior and possibly a larger diameter.     

Differential modes of crosslinking establish spatially distinct regions of peptidoglycan in Caulobacter crescentus

The diversity of cell shapes across the bacterial kingdom reflects evolutionary pressures that have produced physiologically important morphologies. While efforts have been made to understand the regulation of some prototypical cell morphologies such as that of rod‐shaped Escherichia coli, little is known about most cell shapes. For Caulobacter crescentus, polar stalk synthesis is tied to its dimorphic life cycle, and stalk elongation is regulated by phosphate availability. Based on the previous observation that C. crescentus stalks are lysozyme‐resistant, we compared the composition of the peptidoglycan cell wall of stalks and cell bodies and identified key differences in peptidoglycan crosslinking. Cell body peptidoglycan contained primarily DD‐crosslinks between meso‐diaminopimelic acid and D‐alanine residues, whereas stalk peptidoglycan had more LD‐transpeptidation (meso‐diaminopimelic acid‐meso‐diaminopimelic acid), mediated by LdtD. We determined that ldtD is dispensable for stalk elongation; rather, stalk LD‐transpeptidation reflects an aging process associated with low peptidoglycan turnover in the stalk. We also found that lysozyme resistance is a structural consequence of LD‐crosslinking. Despite no obvious selection pressure for LD‐crosslinking or lysozyme resistance in C. crescentus, the correlation between these two properties was maintained in other organisms, suggesting that DAP‐DAP crosslinking may be a general mechanism for regulating bacterial sensitivity to lysozyme.     

Phosphate Starvation Triggers Production and Secretion of an Extracellular Lipoprotein in Caulobacter crescentus

Life in oligotrophic environments necessitates quick adaptive responses to a sudden lack of nutrients. Secretion of specific degradative enzymes into the extracellular medium is a means to mobilize the required nutrient from nearby sources. The aquatic bacterium Caulobacter crescentus must often face changes in its environment such as phosphate limitation. Evidence reported in this paper indicates that under phosphate starvation, C. crescentus produces a membrane surface-anchored lipoprotein named ElpS subsequently released into the extracellular medium. A complete set of 12 genes encoding a type II secretion system (T2SS) is located adjacent to the elpS locus in the C. crescentus genome. Deletion of this T2SS impairs release of ElpS in the environment, which surprisingly remains present at the cell surface, indicating that the T2SS is not involved in the translocation of ElpS to the outer membrane but rather in its release. Accordingly, treatment with protease inhibitors prevents release of ElpS in the extracellular medium suggesting that ElpS secretion relies on a T2SS-secreted protease. Finally, secretion of ElpS is associated with an increase in alkaline phosphatase activity in culture supernatants, suggesting a role of the secreted protein in inorganic phosphate mobilization. In conlusion, we have shown that upon phosphate starvation, C. crescentus produces an outer membrane bound lipoprotein, ElpS, which is further cleaved and released in the extracellular medium in a T2SS-dependent manner. Our data suggest that ElpS is associated with an alkaline phosphatase activity, thereby allowing the bacterium to gather inorganic phosphates from a poor environment.     

Function and Localization Dynamics of Bifunctional Penicillin-Binding Proteins in Caulobacter crescentus

The peptidoglycan cell wall of bacteria is a complex macromolecule composed of glycan strands that are cross-linked by short peptide bridges. Its biosynthesis involves a conserved group of enzymes, the bifunctional penicillin-binding proteins (bPBPs), which contain both a transglycosylase and a transpeptidase domain, thus being able to elongate the glycan strands and, at the same time, generate the peptide cross-links. The stalked model bacterium Caulobacter crescentus possesses five bPBP paralogs, named Pbp1A, PbpC, PbpX, PbpY, and PbpZ, whose function is still incompletely understood. In this study, we show that any of these proteins except for PbpZ is sufficient for growth and normal morphogenesis when expressed at native or elevated levels, whereas inactivation of all five paralogs is lethal. Growth analyses indicate a central role of PbpX in the resistance of C. crescentus against the noncanonical amino acid d-alanine. Moreover, we show that PbpX and PbpY localize to the cell division site. Their recruitment to the divisome is dependent on the essential cell division protein FtsN and likely involves interactions with FtsL and the putative peptidoglycan hydrolase DipM. The same interaction pattern is observed for Pbp1A and PbpC, although these proteins do not accumulate at midcell. Our findings demonstrate that the bPBPs of C. crescentus are, to a large extent, redundant and have retained the ability to interact with the peptidoglycan biosynthetic machineries responsible for cell elongation, cytokinesis, and stalk growth. Nevertheless, they may preferentially act in specific peptidoglycan biosynthetic complexes, thereby facilitating the independent regulation of distinct growth processes.     

Tightly regulated and heritable division control in single bacterial cells

The robust surface adherence property of the aquatic bacterium Caulobacter crescentus permits visualization of single cells in a linear microfluidic culture chamber over an extended number of generations. The division rate of Caulobacter in this continuous-flow culture environment is substantially faster than in other culture apparati and is independent of flow velocity. Analysis of the growth and division of single isogenic cells reveals that the cell cycle control network of this bacterium generates an oscillatory output with a coefficient of variation lower than that of all other bacterial species measured to date. DivJ, a regulator of polar cell development, is necessary for maintaining low variance in interdivision timing, as transposon disruption of divJ significantly increases the coefficient of variation of both interdivision time and the rate of cell elongation. Moreover, interdivision time and cell division arrest are significantly correlated between mother and daughter cells, providing evidence for epigenetic inheritance of cell division behavior in Caulobacter. The single-cell growth/division results reported here suggest that future predictive models of Caulobacter cell cycle regulation should include parameters describing the variance and inheritance properties of this system.     

Dynamical Modeling of the Cell Cycle and Cell Fate Emergence in Caulobacter crescentus

The division of Caulobacter crescentus, a model organism for studying cell cycle and differentiation in bacteria, generates two cell types: swarmer and stalked. To complete its cycle, C. crescentus must first differentiate from the swarmer to the stalked phenotype. An important regulator involved in this process is CtrA, which operates in a gene regulatory network and coordinates many of the interactions associated to the generation of cellular asymmetry. Gaining insight into how such a differentiation phenomenon arises and how network components interact to bring about cellular behavior and function demands mathematical models and simulations. In this work, we present a dynamical model based on a generalization of the Boolean abstraction of gene expression for a minimal network controlling the cell cycle and asymmetric cell division in C. crescentus. This network was constructed from data obtained from an exhaustive search in the literature. The results of the simulations based on our model show a cyclic attractor whose configurations can be made to correspond with the current knowledge of the activity of the regulators participating in the gene network during the cell cycle. Additionally, we found two point attractors that can be interpreted in terms of the network configurations directing the two cell types. The entire network is shown to be operating close to the critical regime, which means that it is robust enough to perturbations on dynamics of the network, but adaptable to environmental changes.     

Programmed Flagellar Ejection in Caulobacter crescentus Leaves PL-subcomplexes

The bacterial flagellum consists of a long extracellular filament that is rotated by a motor embedded in the cell envelope. While flagellar assembly has been extensively studied,¹ the disassembly process remains less well understood. In addition to the programmed flagellar ejection that occurs during the life cycle of Caulobacter crescentus, we and others have recently shown that many bacterial species lose their flagella under starvation conditions, leaving relic structures in the outer membrane.^2–7 However, it remains unknown whether the programmed flagellar ejection of C. crescentus leaves similar relics or not. Here, we imaged the various stages of the C. crescentus life cycle using electron cryo-tomography (cryo-ET) and found that flagellar relic subcomplexes, akin to those produced in the starvation-induced process, remain as a result of flagellar ejection during cell development. This similarity suggests that the programmed flagellar ejection of C. crescentus might share a common evolutionary path with the more general, and likely more ancient,³ starvation-related flagellar loss.     

Repurposing Hsp90 inhibitors as antibiotics targeting histidine kinases

To address the growing need for new antimicrobial agents, we explored whether inhibition of bacterial signaling machinery could inhibit bacterial growth. Because bacteria rely on two-component signaling systems to respond to environmental changes, and because these systems are both highly conserved and mediated by histidine kinases, inhibiting histidine kinases may provide broad spectrum antimicrobial activity. The histidine kinase ATP binding domain is conserved with the ATPase domain of eukaryotic Hsp90 molecular chaperones. To find a chemical scaffold for compounds that target histidine kinases, we leveraged this conservation. We screened ATP competitive Hsp90 inhibitors against CckA, an essential histidine kinase in Caulobacter crescentus that controls cell growth, and showed that the diaryl pyrazole is a promising scaffold for histidine kinase inhibition. We synthesized a panel of derivatives and found that they inhibit the histidine kinases C. crescentus CckA and Salmonella PhoQ but not C. crescentus DivJ; and they inhibit bacterial growth in both Gram-negative and Gram-positive bacterial strains.     

Intermediate filament-like cytoskeleton of Caulobacter crescentus

Eukaryotic cytoskeleton consists of three main types of filaments: actin microfilaments, microtubules and intermediate filaments (IFs). Actin and tubulin-like proteins are also found in bacteria where they perform diverse cytoskeletal functions. IFs, however, are considered to be a characteristic constituent of metazoan cells only, where they (among other functions) are involved in determination and maintenance of cell shape and cellular integrity. Surprisingly, a coiled coil-rich protein called crescentin was recently shown to play a key role in determining the complex curved and helical cell shapes of the bacterium Caulobacter crescentus, and to exhibit several characteristic properties of animal IF proteins. First, the arrangement of the coiled coil domains of crescentin closely resembles the tripartite molecular architecture of IF proteins. Second, crescentin also possesses the defining biochemical property of IF proteins to assemble into 10-nm-wide filaments in vitro without cofactors. Furthermore, crescentin forms a higher-order helical structure in vivo, which is localized asymmetrically along the concave side of the cell. In close association with the cell membrane, the crescentin structure promotes the helical growth of the cell and thereby determines a curved or a helical shape, depending on the length of the cell. The unexpected finding of an IF-like element in a bacterium raises several interesting questions concerning, for example, the molecular mechanisms whereby complex and asymmetric cell shapes are generated by different bacteria, or the functional and evolutionary relatedness of crescentin to animal IF proteins.     

The bacterial cytoskeleton: an intermediate filament-like function in cell shape

Various cell shapes are encountered in the prokaryotic world, but how they are achieved is poorly understood. Intermediate filaments (IFs) of the eukaryotic cytoskeleton play an important role in cell shape in higher organisms. No such filaments have been found in prokaryotes. Here, we describe a bacterial equivalent to IF proteins, named crescentin, whose cytoskeletal function is required for the vibrioid and helical shapes of Caulobacter crescentus. Without crescentin, the cells adopt a straight-rod morphology. Crescentin has characteristic features of IF proteins including the ability to assemble into filaments in vitro without energy or cofactor requirements. In vivo, crescentin forms a helical structure that colocalizes with the inner cell curvatures beneath the cytoplasmic membrane. We propose that IF-like filaments of crescentin assemble into a helical structure, which by applying its geometry to the cell, generates a vibrioid or helical cell shape depending on the length of the cell.     

Modeling the temporal dynamics of master regulators and CtrA proteolysis in Caulobacter crescentus cell cycle

The cell cycle of Caulobacter crescentus involves the polar morphogenesis and an asymmetric cell division driven by precise interactions and regulations of proteins, which makes Caulobacter an ideal model organism for investigating bacterial cell development and differentiation. The abundance of molecular data accumulated on Caulobacter motivates system biologists to analyze the complex regulatory network of cell cycle via quantitative modeling. In this paper, We propose a comprehensive model to accurately characterize the underlying mechanisms of cell cycle regulation based on the study of: a) chromosome replication and methylation; b) interactive pathways of five master regulatory proteins including DnaA, GcrA, CcrM, CtrA, and SciP, as well as novel consideration of their corresponding mRNAs; c) cell cycle-dependent proteolysis of CtrA through hierarchical protease complexes. The temporal dynamics of our simulation results are able to closely replicate an extensive set of experimental observations and capture the main phenotype of seven mutant strains of Caulobacter crescentus. Collectively, the proposed model can be used to predict phenotypes of other mutant cases, especially for nonviable strains which are hard to cultivate and observe. Moreover, the module of cyclic proteolysis is an efficient tool to study the metabolism of proteins with similar mechanisms.     

Mycoplasma pneumoniae,an Underutilized Model for Bacterial Cell Biology

In recent decades, bacterial cell biology has seen great advances, and numerous model systems have been developed to study a wide variety of cellular processes, including cell division, motility, assembly of macromolecular structures, and biogenesis of cell polarity. Considerable attention has been given to these model organisms, which include Escherichia coli, Bacillus subtilis, Caulobacter crescentus, and Myxococcus xanthus. Studies of these processes in the pathogenic bacterium Mycoplasma pneumoniae and its close relatives have also been carried out on a smaller scale, but this work is often overlooked, in part due to this organism''s reputation as minimalistic and simple. In this minireview, I discuss recent work on the role of the M. pneumoniae attachment organelle (AO), a structure required for adherence to host cells, in these processes. The AO is constructed from proteins that generally lack homology to those found in other organisms, and this construction occurs in coordination with cell cycle events. The proteins of the M. pneumoniae AO share compositional features with proteins with related roles in model organisms. Once constructed, the AO becomes activated for its role in a form of gliding motility whose underlying mechanism appears to be distinct from that of other gliding bacteria, including Mycoplasma mobile. Together with the FtsZ cytoskeletal protein, motility participates in the cell division process. My intention is to bring this deceptively complex organism into alignment with the better-known model systems.     

A sweet twist gets Bacillus into shape

A protective organelle that is essential for viability under most conditions, the cell wall is a dynamic structure that is continuously remodelled with the growth of the bacterial cell. Because the cell wall also moulds the bacterium, the mechanisms of cell wall homeostasis can be deciphered using cell shape as a convenient proxy. In this issue of Molecular Microbiology, Foulquier et al. illuminate a connection between cell shape regulation and metabolism in Bacillus subtilis. They find that the putative NAD(P)‐binding enzyme YvcK organizes into helical subcellular structures that exert shape control by directing the cell wall biosynthetic enzyme PBP1 along the cell cylinder and to the septum, a function shared with the MreB actin cytoskeleton. Unlike MreB, however, the role of YvcK in cell shape control is manifested only on certain carbon sources, presumably by way of a previously unknown metabolic feed that taps into cell morphogenesis.