Exclusion of assembled MreB by anionic phospholipids at cell poles confers cell polarity for bidirectional growth

Relationship between the Rod complex and peptidoglycan structure in Escherichia coli

Peptidoglycan for elongation in Escherichia coli is synthesized by the Rod complex, which includes RodZ. Although various mutant strains of the Rod complex have been isolated, the relationship between the activity of the Rod complex and the overall physical and chemical structures of the peptidoglycan have not been reported. We constructed a RodZ mutant, termed RMR, and analyzed the growth rate, morphology, and other characteristics of cells producing the Rod complexes containing RMR. The growth and morphology of RMR cells were abnormal, and we isolated suppressor mutants from RMR cells. Most of the suppressor mutations were found in components of the Rod complex, suggesting that these suppressor mutations increase the integrity and/or the activity of the Rod complex. We purified peptidoglycan from wild-type, RMR, and suppressor mutant cells and observed their structures in detail. We found that the peptidoglycan purified from RMR cells had many large holes and different compositions of muropeptides from those of WT cells. The Rod complex may be a determinant not only for the whole shape of peptidoglycan but also for its highly dense structure to support the mechanical strength of the cell wall.

Coordinated peptidoglycan synthases and hydrolases stabilize the bacterial cell wall

Peptidoglycan (PG) defines cell shape and protects bacteria against osmotic stress. The growth and integrity of PG require coordinated actions between synthases that insert new PG strands and hydrolases that generate openings to allow the insertion. However, the mechanisms of their coordination remain elusive. Moenomycin that inhibits a family of PG synthases known as Class-A penicillin-binding proteins (aPBPs), collapses rod shape despite aPBPs being non-essential for rod-like morphology in the bacterium Myxococcus xanthus. Here, we demonstrate that inhibited PBP1a2, an aPBP, accelerates the degradation of cell poles by DacB, a hydrolytic PG peptidase. Moenomycin promotes the binding between DacB and PG and thus reduces the mobility of DacB through PBP1a2. Conversely, DacB also regulates the distribution and dynamics of aPBPs. Our findings clarify the action of moenomycin and suggest that disrupting the coordination between PG synthases and hydrolases could be more lethal than eliminating individual enzymes.

Plasma Membrane-Cell Wall Feedback in Bacteria

Most bacteria have cell wall peptidoglycan surrounding their plasma membranes. The essential cell wall provides a scaffold for the envelope, protection against turgor pressure and is a proven drug target. Synthesis of the cell wall involves reactions that span cytoplasmic and periplasmic compartments. Bacteria carry out the last steps of cell wall synthesis along their plasma membrane. The plasma membrane in bacteria is heterogeneous and contains membrane compartments. Here, I outline findings that highlight the emerging notion that plasma membrane compartments and the cell wall peptidoglycan are functionally intertwined. I start by providing models of cell wall synthesis compartmentalization within the plasma membrane in mycobacteria, Escherichia coli, and Bacillus subtilis. Then, I revisit literature that supports a role for the plasma membrane and its lipids in modulating enzymatic reactions that synthesize cell wall precursors. I also elaborate on what is known about bacterial lateral organization of the plasma membrane and the mechanisms by which organization is established and maintained. Finally, I discuss the implications of cell wall partitioning in bacteria and highlight how targeting plasma membrane compartmentalization serves as a way to disrupt cell wall synthesis in diverse species.

Cytoskeletal components can turn wall-less spherical bacteria into kinking helices

Bacterial cell shape is generally determined through an interplay between the peptidoglycan cell wall and cytoplasmic filaments made of polymerized MreB. Indeed, some bacteria (e.g., Mycoplasma) that lack both a cell wall and mreB genes consist of non-motile cells that are spherical or pleomorphic. However, other members of the same class Mollicutes (e.g., Spiroplasma, also lacking a cell wall) display a helical cell shape and kink-based motility, which is thought to rely on the presence of five MreB isoforms and a specific fibril protein. Here, we show that heterologous expression of Spiroplasma fibril and MreB proteins confers helical shape and kinking ability to Mycoplasma capricolum cells. Isoform MreB5 is sufficient to confer helicity and kink propagation to mycoplasma cells. Cryoelectron microscopy confirms the association of cytoplasmic MreB filaments with the plasma membrane, suggesting a direct effect on membrane curvature. However, in our experiments, the heterologous expression of MreBs and fibril did not result in efficient motility in culture broth, indicating that additional, unknown Spiroplasma components are required for swimming.

Localized cardiolipin synthesis is required for the assembly of MreB during the polarized cell division of Chlamydia trachomatis

Pathogenic Chlamydia species are coccoid bacteria that use the rod-shape determining protein MreB to direct septal peptidoglycan synthesis during their polarized cell division process. How the site of polarized budding is determined in this bacterium, where contextual features like membrane curvature are seemingly identical, is unclear. We hypothesized that the accumulation of the phospholipid, cardiolipin (CL), in specific regions of the cell membrane induces localized membrane changes that trigger the recruitment of MreB to the site where the bud will arise. To test this, we ectopically expressed cardiolipin synthase (Cls) and observed a polar distribution for this enzyme in Chlamydia trachomatis. In early division intermediates, Cls was restricted to the bud site where MreB is localized and peptidoglycan synthesis is initiated. The localization profile of 6xHis tagged Cls (Cls_6xH) throughout division mimicked the distribution of lipids that stain with NAO, a dye that labels CL. Treatment of Chlamydia with 3’,6-dinonylneamine (diNN), an antibiotic targeting CL-containing membrane domains, resulted in redistribution of Cls_6xH and NAO-staining phospholipids. In addition, 6xHis tagged MreB localization was altered by diNN treatment, suggesting an upstream regulatory role for CL-containing membranes in directing the assembly of MreB. This hypothesis is consistent with the observation that the clustered localization of Cls_6xH is not dependent upon MreB function or peptidoglycan synthesis. Furthermore, expression of a CL-binding protein at the inner membrane of C. trachomatis dramatically inhibited bacterial growth supporting the importance of CL in the division process. Our findings implicate a critical role for localized CL synthesis in driving MreB assembly at the bud site during the polarized cell division of Chlamydia.

Evolution of longitudinal division in multicellular bacteria of the Neisseriaceae family

Rod-shaped bacteria typically elongate and divide by transverse fission. However, several bacterial species can form rod-shaped cells that divide longitudinally. Here, we study the evolution of cell shape and division mode within the family Neisseriaceae, which includes Gram-negative coccoid and rod-shaped species. In particular, bacteria of the genera Alysiella, Simonsiella and Conchiformibius, which can be found in the oral cavity of mammals, are multicellular and divide longitudinally. We use comparative genomics and ultrastructural microscopy to infer that longitudinal division within Neisseriaceae evolved from a rod-shaped ancestor. In multicellular longitudinally-dividing species, neighbouring cells within multicellular filaments are attached by their lateral peptidoglycan. In these bacteria, peptidoglycan insertion does not appear concentric, i.e. from the cell periphery to its centre, but as a medial sheet guillotining each cell. Finally, we identify genes and alleles associated with multicellularity and longitudinal division, including the acquisition of amidase-encoding gene amiC2, and amino acid changes in proteins including MreB and FtsA. Introduction of amiC2 and allelic substitution of mreB in a rod-shaped species that divides by transverse fission results in shorter cells with longer septa. Our work sheds light on the evolution of multicellularity and longitudinal division in bacteria, and suggests that members of the Neisseriaceae family may be good models to study these processes due to their morphological plasticity and genetic tractability.

Rod-shaped bacteria typically elongate and divide by transverse fission, but a few species are known to divide longitudinally. Here, the authors use genomic, phylogenetic and microscopy techniques to shed light on the evolution of cell shape, multicellularity and division mode within the family Neisseriaceae.

The Role of Mre Factors and Cell Division in Peptidoglycan Growth in the Multicellular Cyanobacterium Anabaena

Bacteria in general serve two main tasks: cell growth and division. Both processes include peptidoglycan extension to allow cell expansion and to form the poles of the daughter cells, respectively. The cyanobacterium Anabaena forms filaments of communicated cells in which the outer membrane and the peptidoglycan sacculus, which is engrossed in the intercellular regions between contiguous cells, are continuous along the filament. During the growth of Anabaena, peptidoglycan incorporation was weak at the cell periphery. During cell division, midcell peptidoglycan incorporation matched the localization of the divisome, and incorporation persisted in the intercellular septa, even after the division was completed. MreB, MreC, and MreD were located throughout the cell periphery and, in contrast to other bacteria, also to the divisome all along midcell peptidoglycan growth. In Anabaena mutants bearing inactivated mreB, mreC, or mreD genes, which showed conspicuous alterations in the filament morphology, consecutive septal bands of peptidoglycan growth were frequently not parallel to each other and were irregularly spaced along the filament, reproducing the disposition of the Z-ring. Both lateral and septal growth was impaired in strains down-expressing Z-ring components, and MreB and MreD appeared to directly interact with some divisome components. We propose that, in Anabaena, association with the divisome is a way for localization of MreB, MreC, and MreD at the cell poles, where they regulate lateral, midcell, and septal peptidoglycan growth with the latter being involved in localization and maintenance of the intercellular septal-junction protein structures that mediate cell-cell communication along the filament.

Microbial cell surface engineering for high-level synthesis of bio-products

Microbial cell surface layers, which mainly include the cell membrane, cell wall, periplasmic space, outer membrane, capsules, S-layers, pili, and flagella, control material exchange between the cell and the extracellular environment, and have great impact on production titers and yields of various bio-products synthesized by microbes. Recent research work has made exciting achievements in metabolic engineering using microbial cell surface components as novel regulation targets without direct modifications of the metabolic pathways of the desired products. This review article will summarize the accomplishments obtained in this emerging field, and will describe various engineering strategies that have been adopted in bacteria and yeasts for the enhancement of mass transfer across the cell surface, improvement of protein expression and folding, modulation of cell size and shape, and re-direction of cellular resources, all of which contribute to the construction of more efficient microbial cell factories toward the synthesis of a variety of bio-products. The existing problems and possible future directions will also be discussed.

Multiple conserved states characterize the twist landscape of the bacterial actin homolog MreB

Filament formation by cytoskeletal proteins is critical to their involvement in myriad cellular processes. The bacterial actin homolog MreB, which is essential for cell-shape determination in many rod-shaped bacteria, has served as a model system for studying the mechanics of cytoskeletal filaments. Previous molecular dynamics (MD) simulations revealed that the twist of MreB double protofilaments is dependent on the bound nucleotide, as well as binding to the membrane or the accessory protein RodZ, and MreB mutations that modulate twist also affect MreB spatial organization and cell shape. Here, we show that MreB double protofilaments can adopt multiple twist states during microsecond-scale MD simulations. A deep learning algorithm trained only on high- and low-twist states robustly identified all twist conformations across most perturbations of ATP-bound MreB, suggesting the existence of a conserved set of states whose occupancy is affected by each perturbation to MreB. Simulations replacing ATP with ADP indicated that twist states were generally stable after hydrolysis. These findings suggest a rich twist landscape that could provide the capacity to tune MreB activity and therefore its effects on cell shape.

Myxococcus xanthus as a Model Organism for Peptidoglycan Assembly and Bacterial Morphogenesis

A fundamental question in biology is how cell shapes are genetically encoded and enzymatically generated. Prevalent shapes among walled bacteria include spheres and rods. These shapes are chiefly determined by the peptidoglycan (PG) cell wall. Bacterial division results in two daughter cells, whose shapes are predetermined by the mother. This makes it difficult to explore the origin of cell shapes in healthy bacteria. In this review, we argue that the Gram-negative bacterium Myxococcus xanthus is an ideal model for understanding PG assembly and bacterial morphogenesis, because it forms rods and spheres at different life stages. Rod-shaped vegetative cells of M. xanthus can thoroughly degrade their PG and form spherical spores. As these spores germinate, cells rebuild their PG and reestablish rod shape without preexisting templates. Such a unique sphere-to-rod transition provides a rare opportunity to visualize de novo PG assembly and rod-like morphogenesis in a well-established model organism.

Direct Observation of Conversion From Walled Cells to Wall-Deficient L-Form and Vice Versa in Escherichia coli Indicates the Essentiality of the Outer Membrane for Proliferation of L-Form Cells

Gram-negative bacteria such as Escherichia coli are surrounded by an outer membrane, which encloses a peptidoglycan layer. Even if thinner than in many Gram-positive bacteria, the peptidoglycan in E. coli allows cells to withstand turgor pressure in hypotonic medium. In hypertonic medium, E. coli treated with a cell wall synthesis inhibitor such as penicillin G form wall-deficient cells. These so-called L-form cells grow well under anaerobic conditions (i.e., in the absence of oxidative stress), becoming deformed and dividing as L-form. Upon removal of the inhibitor, they return to the walled rod-shaped state. Recently, the outer membrane was reported to provide rigidity to Gram-negative bacteria and to strengthen wall-deficient cells. However, it remains unclear why L-form cells need the outer membrane for growth. Using a microfluidic system, we found that, upon treatment with the outer membrane-disrupting drugs polymyxin B and polymyxin B nonapeptide or with the outer membrane synthesis inhibitor CHIR-090, the cells lysed during cell deformation and division, indicating that the outer membrane was important even in hypertonic medium. L-form cells could return to rod-shaped when trapped in a narrow space, but not in a wide space, likely due to insufficient physical force. Outer membrane rigidity could be compromised by lack of outer membrane proteins; Lpp, OmpA, or Pal. Deletion of lpp caused cells to lyse during cell deformation and cell division. In contrast, ompA and pal mutants could be deformed and return to small oval cells even when less physical force was exerted. These results strongly suggest that wall-deficient E. coli cells require a rigid outer membrane to survive, but not too rigid to prevent them from changing cell shape.

Alteration of Membrane Fluidity or Phospholipid Composition Perturbs Rotation of MreB Complexes in Escherichia coli

Gram-negative bacteria such as Escherichia coli are surrounded by inner and outer membranes and peptidoglycan in between, protecting the cells from turgor pressure and maintaining cell shape. The Rod complex, which synthesizes peptidoglycan, is composed of various proteins such as a cytoplasmic protein MreB, a transmembrane protein RodZ, and a transpeptidase PBP2. The Rod complex is a highly motile complex that rotates around the long axis of a cell. Previously, we had reported that anionic phospholipids (aPLs; phosphatidylglycerol and cardiolipin) play a role in the localization of MreB. In this study, we identified that cells lacking aPLs slow down Rod complex movement. We also found that at higher temperatures, the speed of movement increased in cells lacking aPLs, suggesting that membrane fluidity is important for movement. Consistent with this idea, Rod complex motion was reduced, and complex formation was disturbed in the cells depleted of FabA or FabB, which are essential for unsaturated fatty acid synthesis. These cells also showed abnormal morphology. Therefore, membrane fluidity is important for maintaining cell shape through the regulation of Rod complex formation and motility.

The role of the cytoskeletal proteins MreB and FtsZ in multicellular cyanobacteria

Multiseriate and true‐branching cyanobacteria are at the peak of prokaryotic morphological complexity. However, little is known about the mechanisms governing multiplanar cell division and morphogenesis. Here, we study the function of the prokaryotic cytoskeletal proteins, MreB and FtsZ in Fischerella muscicola PCC 7414 and Chlorogloeopsis fritschii PCC 6912. Vancomycin and HADA labeling revealed a mixed apical, septal, and lateral trichome growth mode in F. muscicola, whereas C. fritschii exhibits septal growth. In all morphotypes from both species, MreB forms either linear filaments or filamentous strings and can interact with FtsZ. Furthermore, multiplanar cell division in F. muscicola likely depends on FtsZ dosage. Our results lay the groundwork for future studies on cytoskeletal proteins in morphologically complex cyanobacteria.

Establishing rod shape from spherical, peptidoglycan-deficient bacterial spores

Chemical-induced spores of the Gram-negative bacterium Myxococcus xanthus are peptidoglycan (PG)-deficient. It is unclear how these spherical spores germinate into rod-shaped, walled cells without preexisting PG templates. We found that germinating spores first synthesize PG randomly on spherical surfaces. MglB, a GTPase-activating protein, forms a cluster that responds to the status of PG growth and stabilizes at one future cell pole. Following MglB, the Ras family GTPase MglA localizes to the second pole. MglA directs molecular motors to transport the bacterial actin homolog MreB and the Rod PG synthesis complexes away from poles. The Rod system establishes rod shape de novo by elongating PG at nonpolar regions. Thus, similar to eukaryotic cells, the interactions between GTPase, cytoskeletons, and molecular motors initiate spontaneous polarization in bacteria.

Critical Role for the Extended N Terminus of Chlamydial MreB in Directing Its Membrane Association and Potential Interaction with Divisome Proteins

Chlamydiae lack the conserved central coordinator protein of cell division FtsZ, a tubulin-like homolog. Current evidence indicates that Chlamydia uses the actin-like homolog, MreB, to substitute for the role of FtsZ in a polarized division mechanism. Interestingly, we observed MreB as a ring at the septum in dividing cells of Chlamydia We hypothesize that MreB, to substitute for FtsZ in Chlamydia, must possess unique properties compared to canonical MreB orthologs. Sequence differences between chlamydial MreB and orthologs in other bacteria revealed that chlamydial MreB possesses an extended N-terminal region, harboring predicted amphipathicity, as well as the conserved amphipathic helix found in other bacterial MreBs. The conserved amphipathic helix-directed green fluorescent protein (GFP) to label the membrane uniformly in Escherichia coli but the extended N-terminal region did not. However, the extended N-terminal region together with the conserved amphipathic region directed GFP to restrict the membrane label to the cell poles. In Chlamydia, the extended N-terminal region was sufficient to direct GFP to the membrane, and this localization was independent of an association with endogenous MreB. Importantly, mutating the extended N-terminal region to reduce its amphipathicity resulted in the accumulation of GFP in the cytosol of the chlamydiae and not in the membrane. The N-terminal domain of MreB was not required for homotypic interactions but was necessary for interactions with cell division components RodZ and FtsK. Our data provide mechanistic support for chlamydial MreB to serve as a substitute for FtsZ by forming a ringlike structure at the site of polarized division.IMPORTANCE Chlamydia trachomatis is an obligate intracellular pathogen, causing sexually transmitted diseases and trachoma. The study of chlamydial physiology is important for developing novel therapeutic strategies for these diseases. Chlamydiae divide by a unique MreB-dependent polarized cell division process. In this study, we investigated unique properties of chlamydial MreB and observed that chlamydial species harbor an extended N-terminal region possessing amphipathicity. MreB formed a ring at the septum, like FtsZ in Escherichia coli, and its localization was dependent upon the amphipathic nature of its extended N terminus. Furthermore, this region is crucial for the interaction of MreB with cell division proteins. Given these results, chlamydial MreB likely functions at the septum as a scaffold for divisome proteins to regulate cell division in this organism.

Chiral twisting in a bacterial cytoskeletal polymer affects filament size and orientation

In many rod-shaped bacteria, the actin homolog MreB directs cell-wall insertion and maintains cell shape, but it remains unclear how structural changes to MreB affect its organization in vivo. Here, we perform molecular dynamics simulations for Caulobacter crescentus MreB to extract mechanical parameters for inputs into a coarse-grained biophysical polymer model that successfully predicts MreB filament properties in vivo. Our analyses indicate that MreB double protofilaments can exhibit left-handed twisting that is dependent on the bound nucleotide and membrane binding; the degree of twisting correlates with the length and orientation of MreB filaments observed in vitro and in vivo. Our molecular dynamics simulations also suggest that membrane binding of MreB double protofilaments induces a stable membrane curvature of similar magnitude to that observed in vivo. Thus, our multiscale modeling correlates cytoskeletal filament size with conformational changes inferred from molecular dynamics simulations, providing a paradigm for connecting protein filament structure and mechanics to cellular organization and function.

The actin homolog MreB directs cell-wall insertion and maintains cell shape in many rod-shaped bacteria. Here, Shi et al. perform molecular dynamics simulations for MreB to extract mechanical parameters for inputs into a coarse-grained biophysical polymer model that predicts MreB filament properties.

The transpeptidase PBP2 governs initial localization and activity of the major cell-wall synthesis machinery in E. coli

Bacterial shape is physically determined by the peptidoglycan cell wall. The cell-wall-synthesis machinery responsible for rod shape in Escherichia coli is the processive 'Rod complex'. Previously, cytoplasmic MreB filaments were thought to govern formation and localization of Rod complexes based on local cell-envelope curvature. Using single-particle tracking of the transpeptidase and Rod-complex component PBP2, we found that PBP2 binds to a substrate different from MreB. Depletion and localization experiments of other putative Rod-complex components provide evidence that none of those provide the sole rate-limiting substrate for PBP2 binding. Consistently, we found only weak correlations between MreB and envelope curvature in the cylindrical part of cells. Residual correlations do not require curvature-based Rod-complex initiation but can be attributed to persistent rotational motion. We therefore speculate that the local cell-wall architecture provides the cue for Rod-complex initiation, either through direct binding by PBP2 or through an unknown intermediate.

RodZ: a key-player in cell elongation and cell division in Escherichia coli

RodZ is required for determination of cell shape in rod-shaped bacterium, such as Escherichia coli. RodZ is a transmembrane protein and forms a supramolecular complex called the Rod complex with other proteins, such as MreB-actin and peptidoglycan synthesis enzymes (for e.g., PBP2). Deletion of the rodZ gene changes the cell shape from rod to round or ovoid. Another supramolecular complex called divisome that controls cell division mainly consists of FtsZ-tubulin. MreB directly interacts with FtsZ and this interaction is critical to trigger a transition from cell elongation to cell division. Recently, we found that RodZ also directly interacts with FtsZ, and RodZ recruits MreB to the divisome. Formation of the division ring, called Z ring, is delayed if RodZ does not interact with FtsZ, indicating that RodZ might facilitate the formation of the Z ring during the cell division process. In this mini-review, we have summarized the roles of RodZ in cell elongation and cell division, especially based on our recent study.

Division-site localization of RodZ is required for efficient Z ring formation in Escherichia coli

Bacteria such as Escherichia coli must coordinate cell elongation and cell division. Elongation is regulated by an elongasome complex containing MreB actin and the transmembrane protein RodZ, which regulates assembly of MreB, whereas division is regulated by a divisome complex containing FtsZ tubulin. These complexes were previously thought to function separately. However, MreB has been shown to directly interact with FtsZ to switch to cell division from cell elongation, indicating that these complexes collaborate to regulate both processes. Here, we investigated the role of RodZ in the regulation of cell division. RodZ localized to the division site in an FtsZ-dependent manner. We also found that division-site localization of MreB was dependent on RodZ. Formation of a Z ring was delayed by deletion of rodZ, suggesting that division-site localization of RodZ facilitated the formation or stabilization of the Z ring during early cell division. Thus, RodZ functions to regulate MreB assembly during cell elongation and facilitates the formation of the Z ring during cell division in E. coli.

Mechanics and dynamics of translocating MreB filaments on curved membranes

MreB is an actin homolog that is essential for coordinating the cell wall synthesis required for the rod shape of many bacteria. Previously we have shown that filaments of MreB bind to the curved membranes of bacteria and translocate in directions determined by principal membrane curvatures to create and reinforce the rod shape (Hussain et al., 2018). Here, in order to understand how MreB filament dynamics affects their cellular distribution, we model how MreB filaments bind and translocate on membranes with different geometries. We find that it is both energetically favorable and robust for filaments to bind and orient along directions of largest membrane curvature. Furthermore, significant localization to different membrane regions results from processive MreB motion in various geometries. These results demonstrate that the in vivo localization of MreB observed in many different experiments, including those examining negative Gaussian curvature, can arise from translocation dynamics alone.

How to Build a Bacterial Cell: MreB as the Foreman of E. coli Construction

Cell shape matters across the kingdoms of life, and cells have the remarkable capacity to define and maintain specific shapes and sizes. But how are the shapes of micron-sized cells determined from the coordinated activities of nanometer-sized proteins? Here, we review general principles that have surfaced through the study of rod-shaped bacterial growth. Imaging approaches have revealed that polymers of the actin homolog MreB play a central role. MreB both senses and changes cell shape, thereby generating a self-organizing feedback system for shape maintenance. At the molecular level, structural and computational studies indicate that MreB filaments exhibit tunable mechanical properties that explain their preference for certain geometries and orientations along the cylindrical cell body. We illustrate the regulatory landscape of rod-shape formation and the connectivity between cell shape, cell growth, and other aspects of cell physiology. These discoveries provide a framework for future investigations into the architecture and construction of microbes.

Subcellular Organization: A Critical Feature of Bacterial Cell Replication

Spatial organization is a hallmark of all living systems. Even bacteria, the smallest forms of cellular life, display defined shapes and complex internal organization, showcasing a highly structured genome, cytoskeletal filaments, localized scaffolding structures, dynamic spatial patterns, active transport, and occasionally, intracellular organelles. Spatial order is required for faithful and efficient cellular replication and offers a powerful means for the development of unique biological properties. Here, we discuss organizational features of bacterial cells and highlight how bacteria have evolved diverse spatial mechanisms to overcome challenges cells face as self-replicating entities.

Relation between rotation of MreB actin and cell width of Escherichia coli

Bacterial cells, including Escherichia coli and Bacillus subtilis, continuously elongate and divide. Although the cell width is maintained during cell cycle, the molecular mechanisms involved in its regulation remain unknown. MreB has been implicated to play a role in maintaining cell width. Several point mutations in mreB that affect cell width have been identified. The MreB protein forms clusters or polymers in the cell and moves along annular tracks perpendicular to the long axis. This rotation is coupled with peptidoglycan synthesis. Here, we focused on two MreB mutants, MreB^A125V and MreB^A174T . Cells producing MreB^A125V and MreB^A174T were thinner and thicker than WT cells, and MreB^A125V and MreB^A174T rotated faster and slower than WT MreB, respectively. We observed that the rotation rate correlated with the cell wall synthesis rate. Thus, we conclude that the velocity of MreB rotation also affects cell width, that is, the faster the MreB rotates, the thinner the cell width is.

Stress-induced adaptive morphogenesis in bacteria

Bacteria thrive in virtually all environments. Like all other living organisms, bacteria may encounter various types of stresses, to which cells need to adapt. In this chapter, we describe how cells cope with stressful conditions and how this may lead to dramatic morphological changes. These changes may not only allow harmless cells to withstand environmental insults but can also benefit pathogenic bacteria by enabling them to escape from the immune system and the activity of antibiotics. A better understanding of stress-induced morphogenesis will help us to develop new approaches to combat such harmful pathogens.

Peptidoglycan

The peptidoglycan sacculus is a net-like polymer that surrounds the cytoplasmic membrane in most bacteria. It is essential to maintain the bacterial cell shape and protect from turgor. The peptidoglycan has a basic composition, common to all bacteria, with species-specific variations that can modify its biophysical properties or the pathogenicity of the bacteria. The synthesis of peptidoglycan starts in the cytoplasm and the precursor lipid II is flipped across the cytoplasmic membrane. The new peptidoglycan strands are synthesised and incorporated into the pre-existing sacculus by the coordinated activities of peptidoglycan synthases and hydrolases. In the model organism Escherichia coli there are two complexes required for the elongation and division. Each of them is regulated by different proteins from both the cytoplasmic and periplasmic sides that ensure the well-coordinated synthesis of new peptidoglycan.

Genomewide phenotypic analysis of growth, cell morphogenesis, and cell cycle events in Escherichia coli

Cell size, cell growth, and cell cycle events are necessarily intertwined to achieve robust bacterial replication. Yet, a comprehensive and integrated view of these fundamental processes is lacking. Here, we describe an image‐based quantitative screen of the single‐gene knockout collection of Escherichia coli and identify many new genes involved in cell morphogenesis, population growth, nucleoid (bulk chromosome) dynamics, and cell division. Functional analyses, together with high‐dimensional classification, unveil new associations of morphological and cell cycle phenotypes with specific functions and pathways. Additionally, correlation analysis across ~4,000 genetic perturbations shows that growth rate is surprisingly not predictive of cell size. Growth rate was also uncorrelated with the relative timings of nucleoid separation and cell constriction. Rather, our analysis identifies scaling relationships between cell size and nucleoid size and between nucleoid size and the relative timings of nucleoid separation and cell division. These connections suggest that the nucleoid links cell morphogenesis to the cell cycle.

Is Longitudinal Division in Rod-Shaped Bacteria a Matter of Swapping Axis?

The morphology of bacterial species shows a wealth of variation from star-shaped to spherical and rod- to spiral-shaped, to mention a few. Their mode of growth and division is also very diverse and flexible ranging from polar growth and lateral surface increase to midcell expansion and from perpendicular to longitudinal asymmetric division. Gammaproteobacterial rod-shaped species such as Escherchia coli divide perpendicularly and grow in length, whereas the genetically very similar rod-shaped symbiotic Thiosymbion divide longitudinally, and some species even divide asynchronously while growing in width. The ovococcal Streptococcus pneumoniae also lengthens and divides perpendicularly, yet it is genetically very different from E. coli. Are these differences as dramatic as is suggested by visual inspection, or can they all be achieved by subtle variation in the regulation of the same protein complexes that synthesize the cell envelope? Most bacteria rely on the cytoskeletal polymer FtsZ to organize cell division, but only a subset of species use the actin homolog MreB for length growth, although some of them are morphologically not that different. Poles are usually negative determinant for cell division. Curved cell poles can be inert or active with respect to peptidoglycan synthesis, can localize chemotaxis and other sensing proteins or other bacterial equipment, such as pili, depending on the species. But what is actually the definition of a pole? This review discusses the possible common denominators for growth and division of distinct and similar bacterial species.

Host-Polarized Cell Growth in Animal Symbionts

To determine the fundamentals of cell growth, we must extend cell biological studies to non-model organisms. Here, we investigated the growth modes of the only two rods known to widen instead of elongating, Candidatus Thiosymbion oneisti and Thiosymbion hypermnestrae. These bacteria are attached by one pole to the surface of their respective nematode hosts. By incubating live Ca. T. oneisti and T. hypermnestrae with a peptidoglycan metabolic probe, we observed that the insertion of new cell wall starts at the poles and proceeds inward, concomitantly with FtsZ-based membrane constriction. Remarkably, in Ca. T. hypermnestrae, the proximal, animal-attached pole grows before the distal, free pole, indicating that the peptidoglycan synthesis machinery is host oriented. Immunostaining of the symbionts with an antibody against the actin homolog MreB revealed that it was arranged medially—that is, parallel to the cell long axis—throughout the symbiont life cycle. Given that depolymerization of MreB abolished newly synthesized peptidoglycan insertion and impaired divisome assembly, we conclude that MreB function is required for symbiont widening and division. In conclusion, our data invoke a reassessment of the localization and function of the bacterial actin homolog.

Pende et al. show that cell growth is host oriented in two marine nematode-attached bacteria. In contrast to what is observed in model rods, the actin homolog MreB of the symbionts is arranged parallel to the cell long axis throughout the cell cycle. This medial MreB ring is essential for symbiont growth and division.

Recent advances in understanding how rod-like bacteria stably maintain their cell shapes

Cell shape and cell volume are important for many bacterial functions. In recent years, we have seen a range of experimental and theoretical work that led to a better understanding of the determinants of cell shape and size. The roles of different molecular machineries for cell-wall expansion have been detailed and partially redefined, mechanical forces have been shown to influence cell shape, and new connections between metabolism and cell shape have been proposed. Yet the fundamental determinants of the different cellular dimensions remain to be identified. Here, we highlight some of the recent developments and focus on the determinants of rod-like cell shape and size in the well-studied model organisms Escherichia coli and Bacillus subtilis.

The periplasmic disordered domain of RodZ promotes its self-interaction in Escherichia coli

Rod shape of bacterial cells such as Escherichia coli is mainly regulated by a supramolecular complex called elongasome including MreB actin. Deletion of the mreB gene in rod-shaped bacterium E. coli results in round-shaped cells. RodZ was isolated as a determinant of rod shape in E. coli, Caulobacter crescentus and Bacillus subtilis and it has been shown to be an interaction partner and a regulator of assembly of MreB through its cytoplasmic domain. As opposed to functions of the N-terminal cytoplasmic domain of RodZ, functions of the C-terminal periplasmic domain including a disordered region are still unclear. To understand it, we adopted an in vivo photo-cross-linking assay to analyze interaction partners to identify proteins which interact with RodZ via its periplasmic domain, finding that the RodZ self-interacts in the periplasmic disordered domain. Self-interaction of RodZ was affected by MreB actin. Deletion of this region resulted in aberrant cell shape. Our results suggest that MreB binding to the cytoplasmic domain of RodZ causes structural changes in the disordered periplasmic domain of RodZ. We also found that the disordered domain of RodZ contributes to fine-tune rod shape in E. coli.

The bacterial Sec system is required for the organization and function of the MreB cytoskeleton

The Sec system is responsible for protein insertion, translocation and secretion across membranes in all cells. The bacterial actin homolog MreB controls various processes, including cell wall synthesis, membrane organization and polarity establishment. Here we show that the two systems genetically interact and that components of the Sec system, especially the SecA motor protein, are essential for spatiotemporal organization of MreB in E. coli, as evidenced by the accumulation of MreB at irregular sites in Sec-impaired cells. MreB mislocalization in SecA-defective cells significantly affects MreB-coordinated processes, such as cell wall synthesis, and induce formation of membrane invaginations enriched in high fluidity domains. Additionally, MreB is not recruited to the FtsZ ring in secA mutant cells, contributing to division arrest and cell filamentation. Our results show that all these faults are due to improper targeting of MreB to the membrane in the absence of SecA. Thus, when we reroute RodZ, MreB membrane-anchor, by fusing it to a SecA-independent integral membrane protein and overproducing it, MreB localization is restored and the defect in cell division is corrected. Notably, the RodZ moiety is not properly inserted into the membrane, strongly suggesting that it only serves as a bait for placing MreB around the cell circumference. Finally, we show that MreB localization depends on SecA also in C. crescentus, suggesting that regulation of MreB by the Sec system is conserved in bacteria. Taken together, our data reveal that the secretion system plays an important role in determining the organization and functioning of the cytoskeletal system in bacteria.

The notion that bacterial cells have intricate spatial organization, which affects many vital processes, is relatively new and, hence, the underlying mechanisms are largely unknown. The general secretion system and the cytoskeleton are central systems, each known to organize functions associated with certain cellular domains, in both eukaryotes and prokaryotes. While the role of the Sec system in membrane protein translocation and secretion has been largely explored, not much in known about its role in inner cell organization. We show that the Sec system is important for the localization pattern and functionality of the bacterial cytoskeletal system, which controls cell shape, cell division and polarity. Our findings highlight the Sec system as a central coordinator that controls cellular functions on both sides of the membrane.

Polar localization of MreB actin is inhibited by anionic phospholipids in the rod-shaped bacterium Escherichia coli

Bacterial actin MreB is required for the maintenance of cell polarity. MreB is located underneath the cell membrane and mainly localizes at a central cylindrical part of the cell. In addition, it has recently been found that anionic phospholipids (aPLs: phosphatidylglycerol and cardiolipin) play a crucial role in excluding MreB from the cell poles. Subcellular localization of MreB is positively and negatively regulated by membrane curvature and aPLs, respectively.