FtsZ directs a second mode of peptidoglycan synthesis in Escherichia coli

A FtsZ Inhibitor That Can Utilize Siderophore-Ferric Iron Uptake Transporter Systems for Activity against Gram-Negative Bacterial Pathogens

The global threat of multidrug-resistant Gram-negative bacterial pathogens necessitates the development of new and effective antibiotics. FtsZ is an essential and highly conserved cytoskeletal protein that is an appealing antibacterial target for new antimicrobial therapeutics. However, the effectiveness of FtsZ inhibitors against Gram-negative species has been limited due in part to poor intracellular accumulation. To address this limitation, we have designed a FtsZ inhibitor (RUP4) that incorporates a chlorocatechol siderophore functionality that can chelate ferric iron (Fe3+) and utilizes endogenous siderophore uptake pathways to facilitate entry into Gram-negative pathogens. We show that RUP4 is active against both Klebsiella pneumoniae and Acinetobacter baumannii, with this activity being dependent on direct Fe3+ chelation and enhanced under Fe3+-limiting conditions. Genetic deletion studies in K. pneumoniae reveal that RUP4 gains entry through the FepA and CirA outer membrane transporters and the FhuBC inner membrane transporter. We also show that RUP4 exhibits bactericidal synergy against K. pneumoniae when combined with select antibiotics, with the strongest synergy observed with PBP2-targeting β-lactams or MreB inhibitors. In the aggregate, our studies indicate that incorporation of Fe3+-chelating moieties into FtsZ inhibitors is an appealing design strategy for enhancing activity against Gram-negative pathogens of global clinical significance.

Probing bacterial cell wall growth by tracing wall-anchored protein complexes

The dynamic assembly of the cell wall is key to the maintenance of cell shape during bacterial growth. Here, we present a method for the analysis of Escherichia coli cell wall growth at high spatial and temporal resolution, which is achieved by tracing the movement of fluorescently labeled cell wall-anchored flagellar motors. Using this method, we clearly identify the active and inert zones of cell wall growth during bacterial elongation. Within the active zone, the insertion of newly synthesized peptidoglycan occurs homogeneously in the axial direction without twisting of the cell body. Based on the measured parameters, we formulate a Bernoulli shift map model to predict the partitioning of cell wall-anchored proteins following cell division.

Insights into the Structure, Function, and Dynamics of the Bacterial Cytokinetic FtsZ-Ring

The FtsZ protein is a highly conserved bacterial tubulin homolog. In vivo, the functional form of FtsZ is the polymeric, ring-like structure (Z-ring) assembled at the future division site during cell division. While it is clear that the Z-ring plays an essential role in orchestrating cytokinesis, precisely what its functions are and how these functions are achieved remain elusive. In this article, we review what we have learned during the past decade about the Z-ring's structure, function, and dynamics, with a particular focus on insights generated by recent high-resolution imaging and single-molecule analyses. We suggest that the major function of the Z-ring is to govern nascent cell pole morphogenesis by directing the spatiotemporal distribution of septal cell wall remodeling enzymes through the Z-ring's GTP hydrolysis-dependent treadmilling dynamics. In this role, FtsZ functions in cell division as the counterpart of the cell shape-determining actin homolog MreB in cell elongation.

Cardiolipin Alters Rhodobacter sphaeroides Cell Shape by Affecting Peptidoglycan Precursor Biosynthesis

The phospholipid composition of the cell membrane influences the spatial and temporal biochemistry of cells. We studied molecular mechanisms connecting membrane composition to cell morphology in the model bacterium Rhodobacter sphaeroides. The peptidoglycan (PG) layer of the cell wall is a dominant component of cell mechanical properties; consequently, it has been an important antibiotic target. We found that the anionic phospholipid cardiolipin (CL) plays a role in determination of the shape of R. sphaeroides cells by affecting PG precursor biosynthesis. Removing CL in R. sphaeroides alters cell morphology and increases its sensitivity to antibiotics targeting proteins synthesizing PG. These studies provide a connection to spatial biochemical control in mitochondria, which contain an inner membrane with topological features in common with R. sphaeroides.

Cardiolipin (CL) is an anionic phospholipid that plays an important role in regulating protein biochemistry in bacteria and mitochondria. Deleting the CL synthase gene (Δcls) in Rhodobacter sphaeroides depletes CL and decreases cell length by 20%. Using a chemical biology approach, we found that a CL deficiency does not impair the function of the cell wall elongasome in R. sphaeroides; instead, biosynthesis of the peptidoglycan (PG) precursor lipid II is decreased. Treating R. sphaeroides cells with fosfomycin and d-cycloserine inhibits lipid II biosynthesis and creates phenotypes in cell shape, PG composition, and spatial PG assembly that are strikingly similar to those seen with R. sphaeroides Δcls cells, suggesting that CL deficiency alters the elongation of R. sphaeroides cells by reducing lipid II biosynthesis. We found that MurG—a glycosyltransferase that performs the last step of lipid II biosynthesis—interacts with anionic phospholipids in native (i.e., R. sphaeroides) and artificial membranes. Lipid II production decreases 25% in R. sphaeroides Δcls cells compared to wild-type cells, and overexpression of MurG in R. sphaeroides Δcls cells restores their rod shape, indicating that CL deficiency decreases MurG activity and alters cell shape. The R. sphaeroides Δcls mutant is more sensitive than the wild-type strain to antibiotics targeting PG synthesis, including fosfomycin, d-cycloserine, S-(3,4-dichlorobenzyl)isothiourea (A22), mecillinam, and ampicillin, suggesting that CL biosynthesis may be a potential target for combination chemotherapies that block the bacterial cell wall.

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.

Bacterial Cell Size: Multifactorial and Multifaceted

How cells establish, maintain, and modulate size has always been an area of great interest and fascination. Until recently, technical limitations curtailed our ability to understand the molecular basis of bacterial cell size control. In the past decade, advances in microfluidics, imaging, and high-throughput single-cell analysis, however, have led to a flurry of work revealing size to be a highly complex trait involving the integration of three core aspects of bacterial physiology: metabolism, growth, and cell cycle progression.

CbtA toxin of Escherichia coli inhibits cell division and cell elongation via direct and independent interactions with FtsZ and MreB

The toxin components of toxin-antitoxin modules, found in bacterial plasmids, phages, and chromosomes, typically target a single macromolecule to interfere with an essential cellular process. An apparent exception is the chromosomally encoded toxin component of the E. coli CbtA/CbeA toxin-antitoxin module, which can inhibit both cell division and cell elongation. A small protein of only 124 amino acids, CbtA, was previously proposed to interact with both FtsZ, a tubulin homolog that is essential for cell division, and MreB, an actin homolog that is essential for cell elongation. However, whether or not the toxic effects of CbtA are due to direct interactions with these predicted targets is not known. Here, we genetically separate the effects of CbtA on cell elongation and cell division, showing that CbtA interacts directly and independently with FtsZ and MreB. Using complementary genetic approaches, we identify the functionally relevant target surfaces on FtsZ and MreB, revealing that in both cases, CbtA binds to surfaces involved in essential cytoskeletal filament architecture. We show further that each interaction contributes independently to CbtA-mediated toxicity and that disruption of both interactions is required to alleviate the observed toxicity. Although several other protein modulators are known to target FtsZ, the CbtA-interacting surface we identify represents a novel inhibitory target. Our findings establish CbtA as a dual function toxin that inhibits both cell division and cell elongation via direct and independent interactions with FtsZ and MreB.

Bacterially encoded toxin-antitoxin systems, which consist of a small toxin protein that is co-produced with a neutralizing antitoxin, are a potential avenue for the identification of novel antibiotic targets. These toxins typically target essential cellular processes, causing growth arrest or cell death when unchecked by the antitoxin. Our study is focused on the CbtA toxin of E. coli, which was known to inhibit both bacterial cell division and also bacterial cell elongation (the process by which rod-shaped bacteria grow prior to cell division). We report that the effects of CbtA on cell division and cell elongation are genetically separable, and that they are due to direct and independent interactions with its targets FtsZ and MreB, essential cytoskeletal proteins that direct cell division and cell elongation, respectively. Our genetic analysis defines the functionally relevant target surfaces on FtsZ and MreB; in the case of FtsZ this surface represents a novel inhibitory target. As a dual-function toxin that independently targets two essential cytoskeletal elements, CbtA could guide the design of dual-function antibiotics whose ability to independently target more than one essential cellular process might impede the development of drug resistance, which is a growing public health threat.

A novel gene, cilia flagella associated protein 44, encoding an enzyme cleaving FtsZ and tubulin contributes to the regulation of secretory pathway

We identified a novel gene, encoding the central region of the cilia and flagella associated protein 44 (Cfap44), that regulates trafficking of cellular components and morphology via the cleavage of cellular proteins (particularly β-tubulin). Although Cfap44 is registered in GenBank, the functions of both the central part and full-length protein are unknown except for a polymorphism associated with proprotein convertase 9 activity, the third gene of familiar hyper-cholesterolemia. In mice and humans, both unspliced and spliced RNAs were transcribed, and the spliced form was predominantly transcribed in the brain and embryonic tissues. In transfectants carrying this gene, various cellular processes such as cell division, transport of cellular components, and proteolytic processing of several proteins were found to be affected. The cleavage of β-tubulin was observed. A bacterial tubulin homolog, cell division protein FtsZ, was also cleaved in vivo and in vitro by the spliced form of Cfap44 product. Furthermore, the unspliced form showed proteolytic activity with low substrate specificity. Various biological activities of Cfap44 may be due to a direct effect of cleavage of β-tubulin inhibiting microtubule formation, or an indirect effect with the cross-recognition of the cleavage site between β-tubulin and other molecules.

Xanthomonas citri MinC Oscillates from Pole to Pole to Ensure Proper Cell Division and Shape

Xanthomonas citri (Xac) is the causal agent of citrus canker, a disease that affects citrus crops and causes economic impact worldwide. To further characterize cell division in this plant pathogen, we investigated the role of the protein MinC in cell division, chromosome segregation, and peptidoglycan incorporation by deleting the gene minC using allele exchange. Xac with minC deleted exhibited the classic Δmin phenotype observed in other bacteria deleted for min components: minicells and short filamentation. In addition we noticed the formation of branches, which is similar to what was previously described for Escherichia coli deleted for either min or for several low molecular weight penicillin-binding proteins (PBPs). The branching phenotype was medium dependent and probably linked to gluconeogenic growth. We complemented the minC gene by integrating gfp-minC into the amy locus. Xac complemented strains displayed a wild-type phenotype. In addition, GFP-MinC oscillated from pole to pole, similar to MinCD oscillations observed in E. coli and more recently in Synechococcus elongatus. Further investigation of the branching phenotype revealed that in branching cells nucleoid organization, divisome formation and peptidoglycan incorporation were disrupted.

Determinants of Bacterial Morphology: From Fundamentals to Possibilities for Antimicrobial Targeting

Bacterial morphology is extremely diverse. Specific shapes are the consequence of adaptive pressures optimizing bacterial fitness. Shape affects critical biological functions, including nutrient acquisition, motility, dispersion, stress resistance and interactions with other organisms. Although the characteristic shape of a bacterial species remains unchanged for vast numbers of generations, periodical variations occur throughout the cell (division) and life cycles, and these variations can be influenced by environmental conditions. Bacterial morphology is ultimately dictated by the net-like peptidoglycan (PG) sacculus. The species-specific shape of the PG sacculus at any time in the cell cycle is the product of multiple determinants. Some morphological determinants act as a cytoskeleton to guide biosynthetic complexes spatiotemporally, whereas others modify the PG sacculus after biosynthesis. Accumulating evidence supports critical roles of morphogenetic processes in bacteria-host interactions, including pathogenesis. Here, we review the molecular determinants underlying morphology, discuss the evidence linking bacterial morphology to niche adaptation and pathogenesis, and examine the potential of morphological determinants as antimicrobial targets.

Roles of the Essential Protein FtsA in Cell Growth and Division in Streptococcus pneumoniae

Streptococcus pneumoniae is an ovoid-shaped Gram-positive bacterium that grows by carrying out peripheral and septal peptidoglycan (PG) synthesis, analogous to model bacilli, such as Escherichia coli and Bacillus subtilis In the model bacilli, FtsZ and FtsA proteins assemble into a ring at midcell and are dedicated to septal PG synthesis but not peripheral PG synthesis; hence, inactivation of FtsZ or FtsA results in long filamentous cells unable to divide. Here, we demonstrate that FtsA and FtsZ colocalize at midcell in S. pneumoniae and that partial depletion of FtsA perturbs septum synthesis, resulting in elongated cells with multiple FtsZ rings that fail to complete septation. Unexpectedly, complete depletion of FtsA resulted in the delocalization of FtsZ rings and ultimately cell ballooning and lysis. In contrast, depletion or deletion of gpsB and sepF, which in B. subtilis are synthetically lethal with ftsA, resulted in enlarged and elongated cells with multiple FtsZ rings, with deletion of sepF mimicking partial depletion of FtsA. Notably, cell ballooning was not observed, consistent with later recruitment of these proteins to midcell after Z-ring assembly. The overproduction of FtsA stimulates septation and suppresses the cell division defects caused by the deletion of sepF and gpsB under some conditions, supporting the notion that FtsA shares overlapping functions with GpsB and SepF at later steps in the division process. Our results indicate that, in S. pneumoniae, both GpsB and SepF are involved in septal PG synthesis, whereas FtsA and FtsZ coordinate both peripheral and septal PG synthesis and are codependent for localization at midcell.IMPORTANCEStreptococcus pneumoniae (pneumococcus) is a clinically important human pathogen for which more therapies against unexploited essential targets, like cell growth and division proteins, are needed. Pneumococcus is an ovoid-shaped Gram-positive bacterium with cell growth and division properties that have important distinctions from those of rod-shaped bacteria. Gaining insights into these processes can thus provide valuable information to develop novel antimicrobials. Whereas rods use distinctly localized protein machines at different cellular locations to synthesize peripheral and septal peptidoglycans, we present evidence that S. pneumoniae organizes these two machines at a single location in the middle of dividing cells. Here, we focus on the properties of the actin-like protein FtsA as an essential orchestrator of peripheral and septal growth in this bacterium.

CozE is a member of the MreCD complex that directs cell elongation in Streptococcus pneumoniae

Most bacterial cells are surrounded by a peptidoglycan cell wall that is essential for their integrity. The major synthases of this exoskeleton are called penicillin-binding proteins (PBPs)^1,2. Surprisingly little is known about how cells control these enzymes, given their importance as drug targets. In the model Gram-negative bacterium Escherichia coli, outer membrane lipoproteins are critical activators of the class A PBPs (aPBPs)^3,4, bifunctional synthases capable of polymerizing and crosslinking peptidoglycan to build the exoskeletal matrix¹. Regulators of PBP activity in Gram-positive bacteria have yet to be discovered but are likely to be distinct due to the absence of an outer membrane. To uncover Gram-positive PBP regulatory factors, we used transposon-sequencing (Tn-Seq)⁵ to screen for mutations affecting the growth of Streptococcus pneumoniae cells when the aPBP synthase PBP1a was inactivated. Our analysis revealed a set of genes that were essential for growth in wild-type cells yet dispensable when pbp1a was deleted. The proteins encoded by these genes include the conserved cell wall elongation factors MreC and MreD^2,6,7, as well as a membrane protein of unknown function (SPD_0768) that we have named CozE (coordinator of zonal elongation). Our results indicate that CozE is a member of the MreCD complex of S. pneumoniae that directs the activity of PBP1a to the midcell plane where it promotes zonal cell elongation and normal morphology. CozE homologues are broadly distributed among bacteria, suggesting that they represent a widespread family of morphogenic proteins controlling cell wall biogenesis by the PBPs.

Beyond force generation: Why is a dynamic ring of FtsZ polymers essential for bacterial cytokinesis?

We propose that the essential function of the most highly conserved protein in bacterial cytokinesis, FtsZ, is not to generate a mechanical force to drive cell division. Rather, we suggest that FtsZ acts as a signal-processing hub to coordinate cell wall synthesis at the division septum with a diverse array of cellular processes, ensuring that the cell divides smoothly at the correct time and place, and with the correct septum morphology. Here, we explore how the polymerization properties of FtsZ, which have been widely attributed to force generation, can also be advantageous in this signal processing role. We suggest mechanisms by which FtsZ senses and integrates both mechanical and biochemical signals, and conclude by proposing experiments to investigate how FtsZ contributes to the remarkable spatial and temporal precision of bacterial cytokinesis.

Bacterial cell wall biogenesis is mediated by SEDS and PBP polymerase families functioning semi-autonomously

Multi-protein complexes organized by cytoskeletal proteins are essential for cell wall biogenesis in most bacteria. Current models of the wall assembly mechanism assume that class A penicillin-binding proteins (aPBPs), the targets of penicillin-like drugs, function as the primary cell wall polymerases within these machineries. Here, we use an in vivo cell wall polymerase assay in Escherichia coli combined with measurements of the localization dynamics of synthesis proteins to investigate this hypothesis. We find that aPBP activity is not necessary for glycan polymerization by the cell elongation machinery, as is commonly believed. Instead, our results indicate that cell wall synthesis is mediated by two distinct polymerase systems, shape, elongation, division, sporulation (SEDS)-family proteins working within the cytoskeletal machines and aPBP enzymes functioning outside these complexes. These findings thus necessitate a fundamental change in our conception of the cell wall assembly process in bacteria.

Lyme disease and relapsing fever Borrelia elongate through zones of peptidoglycan synthesis that mark division sites of daughter cells

Agents that cause Lyme disease, relapsing fever, leptospirosis, and syphilis belong to the phylum Spirochaetae-a unique lineage of bacteria most known for their long, spiral morphology. Despite the relevance to human health, little is known about the most fundamental aspects of spirochete growth. Here, using quantitative microscopy to track peptidoglycan cell-wall synthesis, we found that the Lyme disease spirochete Borrelia burgdorferi displays a complex pattern of growth. B. burgdorferi elongates from discrete zones that are both spatially and temporally regulated. In addition, some peptidoglycan incorporation occurs along the cell body, with the notable exception of a large region at the poles. Newborn cells inherit a highly active zone of peptidoglycan synthesis at midcell that contributes to elongation for most of the cell cycle. Concomitant with the initiation of nucleoid separation and cell constriction, second and third zones of elongation are established at the 1/4 and 3/4 cellular positions, marking future sites of division for the subsequent generation. Positioning of elongation zones along the cell is robust to cell length variations and is relatively precise over long distances (>30 µm), suggesting that cells ‟sense" relative, as opposed to absolute, cell length to establish zones of peptidoglycan synthesis. The transition from one to three zones of peptidoglycan growth during the cell cycle is also observed in relapsing fever Borrelia. However, this mode of growth does not extend to representative species from other spirochetal genera, suggesting that this distinctive growth mode represents an evolutionary divide in the spirochete phylum.

A Prophage-Encoded Small RNA Controls Metabolism and Cell Division in Escherichia coli

Hundreds of small RNAs (sRNAs) have been identified in diverse bacterial species, and while the functions of most remain unknown, some regulate key processes, particularly stress responses. The sRNA DicF was identified over 25 years ago as an inhibitor of cell division but since then has remained uncharacterized. DicF consists of 53 nucleotides and is encoded by a gene carried on a prophage (Qin) in the genomes of many Escherichia coli strains. We demonstrated that DicF inhibits cell division via direct base pairing with ftsZ mRNA to repress translation and prevent new synthesis of the bacterial tubulin homolog FtsZ. Systems analysis using computational and experimental methods identified additional mRNA targets of DicF: xylR and pykA mRNAs, encoding the xylose uptake and catabolism regulator and pyruvate kinase, respectively. Genetic analyses showed that DicF directly base pairs with and represses translation of these targets. Phenotypes of cells expressing DicF variants demonstrated that DicF-associated growth inhibition is not solely due to repression of ftsZ, indicating that the physiological consequences of DicF-mediated regulation extend beyond effects on cell division caused by reduced FtsZ synthesis. IMPORTANCE sRNAs are ubiquitous and versatile regulators of bacterial gene expression. A number of well-characterized examples in E. coli are highly conserved and present in the E. coli core genome. In contrast, the sRNA DicF (identified over 20 years ago but remaining poorly characterized) is encoded by a gene carried on a defective prophage element in many E. coli genomes. Here, we characterize DicF in order to better understand how horizontally acquired sRNA regulators impact bacterial gene expression and physiology. Our data confirm the long-hypothesized DicF-mediated regulation of ftsZ, encoding the bacterial tubulin homolog required for cell division. We further uncover DicF-mediated posttranscriptional control of metabolic gene expression. Ectopic production of DicF is highly toxic to E. coli cells, but the toxicity is not attributable to DicF regulation of ftsZ. Further work is needed to reveal the biological roles of and benefits for the host conferred by DicF and other products encoded by defective prophages.

Defining the rate-limiting processes of bacterial cytokinesis

Bacterial cytokinesis is accomplished by the essential 'divisome' machinery. The most widely conserved divisome component, FtsZ, is a tubulin homolog that polymerizes into the 'FtsZ-ring' ('Z-ring'). Previous in vitro studies suggest that Z-ring contraction serves as a major constrictive force generator to limit the progression of cytokinesis. Here, we applied quantitative superresolution imaging to examine whether and how Z-ring contraction limits the rate of septum closure during cytokinesis in Escherichia coli cells. Surprisingly, septum closure rate was robust to substantial changes in all Z-ring properties proposed to be coupled to force generation: FtsZ's GTPase activity, Z-ring density, and the timing of Z-ring assembly and disassembly. Instead, the rate was limited by the activity of an essential cell wall synthesis enzyme and further modulated by a physical divisome-chromosome coupling. These results challenge a Z-ring-centric view of bacterial cytokinesis and identify cell wall synthesis and chromosome segregation as limiting processes of cytokinesis.

Dead-end intermediates in the enterobacterial common antigen pathway induce morphological defects in Escherichia coli by competing for undecaprenyl phosphate

Bacterial morphology is determined primarily by the architecture of the peptidoglycan (PG) cell wall, a mesh-like layer that encases the cell. To identify novel mechanisms that create or maintain cell shape in Escherichia coli, we used flow cytometry to screen a transposon insertion library and identified a wecE mutant that altered cell shape, causing cells to filament and swell. WecE is a sugar aminotransferase involved in the biosynthesis of enterobacterial common antigen (ECA), a non-essential outer membrane glycolipid of the Enterobacteriaceae. Loss of wecE interrupts biosynthesis of ECA and causes the accumulation of the undecaprenyl pyrophosphate-linked intermediate ECA-lipid II. The wecE shape defects were reversed by: (i) preventing initiation of ECA biosynthesis, (ii) increasing the synthesis of the lipid carrier undecaprenyl phosphate (Und-P), (iii) diverting Und-P to PG synthesis or (iv) promoting Und-P recycling. The results argue that the buildup of ECA-lipid II sequesters part of the pool of Und-P, which, in turn, adversely affects PG synthesis. The data strongly suggest there is competition for a common pool of Und-P, whose proper distribution to alternate metabolic pathways is required to maintain normal cell shape in E. coli.

Escherichia coli inner membrane protein YciB interacts with ZipA that is important for cell division

Escherichia coli propagates by undergoing cycles of lateral elongation, septum formation, and cell fission at the mid-cell. A large number of genes involved in these processes have been identified, but it is likely that others remain. A deletion mutant of yciB (ΔyciB) is shorter in the cell length compared to wild type and, in contrast, over-expression of yciB causes elongation of the cell. Furthermore, the septum localization of ZipA, an essential protein of cell division, is disturbed in a ΔyciB mutant. Purified YciB protein directly interacted with ZipA, which might indicate that YciB is involved in the cell envelope synthesis directed by ZipA in a PBP3-independent manner.

Cytoplasmic Domain of MscS Interacts with Cell Division Protein FtsZ: A Possible Non-Channel Function of the Mechanosensitive Channel in Escherichia Coli

Bacterial mechano-sensitive (MS) channels reside in the inner membrane and are considered to act as emergency valves whose role is to lower cell turgor when bacteria enter hypo-osmotic environments. However, there is emerging evidence that members of the Mechano-sensitive channel Small (MscS) family play additional roles in bacterial and plant cell physiology. MscS has a large cytoplasmic C-terminal region that changes its shape upon activation and inactivation of the channel. Our pull-down and co-sedimentation assays show that this domain interacts with FtsZ, a bacterial tubulin-like protein. We identify point mutations in the MscS C-terminal domain that reduce binding to FtsZ and show that bacteria expressing these mutants are compromised in growth on sublethal concentrations of β-lactam antibiotics. Our results suggest that interaction between MscS and FtsZ could occur upon inactivation and/or opening of the channel and could be important for the bacterial cell response against sustained stress upon stationary phase and in the presence of β-lactam antibiotics.

Structural constraints and dynamics of bacterial cell wall architecture

The peptidoglycan wall (PG) is a unique structure which confers physical strength and defined shape to bacteria. It consists of a net-like macromolecule of peptide interlinked glycan chains overlying the cell membrane. The structure and layout of the PG dictates that the wall has to be continuously modified as bacteria go through division, morphological differentiation, and adaptive responses. The PG is poorly known in structural terms. However, to understand morphogenesis a precise knowledge of glycan strand arrangement and of local effects of the different kinds of subunits is essential. The scarcity of data led to a conception of the PG as a regular, highly ordered structure which strongly influenced growth models. Here, we review the structure of the PG to define a more realistic conceptual framework. We discuss the consequences of the plasticity of murein architecture in morphogenesis and try to define a set of minimal structural constraints that must be fulfilled by any model to be compatible with present day information.

Mechanisms of bacterial morphogenesis: evolutionary cell biology approaches provide new insights

How Darwin's "endless forms most beautiful" have evolved remains one of the most exciting questions in biology. The significant variety of bacterial shapes is most likely due to the specific advantages they confer with respect to the diverse environments they occupy. While our understanding of the mechanisms generating relatively simple shapes has improved tremendously in the last few years, the molecular mechanisms underlying the generation of complex shapes and the evolution of shape diversity are largely unknown. The emerging field of bacterial evolutionary cell biology provides a novel strategy to answer this question in a comparative phylogenetic framework. This relatively novel approach provides hypotheses and insights into cell biological mechanisms, such as morphogenesis, and their evolution that would have been difficult to obtain by studying only model organisms. We discuss the necessary steps, challenges, and impact of integrating "evolutionary thinking" into bacterial cell biology in the genomic era.

Bacteria under the physical constraints of periodic micro-nanofluidic junctions reveal morphological plasticity and dynamic shifting of Min patterns

Morphological plasticity is an important survival strategy for bacteria adapting to stressful environments in response to new physical constraints. Here, we demonstrate Escherichia coli morphological plasticity can be induced by switching stress levels through the physical constraints of periodic micro-nanofluidic junctions. Moreover, the generation of diverse morphological aberrancies requires the intact functions of the divisome- and elongasome-directed pathways. It is also intriguing that the altered morphologies are developed in bacteria undergoing morphological reversion as stresses are removed. Cell filamentation underlies the most dominant morphological phenotypes, in which transitions between the novel pattern formations by the spatial regulators of the divisome, i.e., the Min system, are observed, suggesting their potential linkage during morphological reversion.

Peptidoglycan plasticity in bacteria: emerging variability of the murein sacculus and their associated biological functions

The peptidoglycan (PG) sacculus once thought to be just a reinforcing, static and uniform structure, is fast becoming recognized as a dynamic cell constituent involved in every aspect of bacterial physiology. Recent advances showed that in addition to 'classical' tasks-as an essential element to define bacterial shape, size, division and resistance to osmotic stress-the sacculus plays very important roles in many other fields. The very few chemical and structural changes that were once considered as bizarre, or maybe exotic exceptions, are now universally accepted as fundamental pieces in bacterial cell wall adaptation to different kinds of environmental stresses; immune response; intra-specific and inter-specific signalling and antibiotics, just to mention a few. Most, if not all, of these implications are a consequence of the enormous adaptability of PG metabolism to cope with changing conditions, a characteristic for which the term plasticity is proposed. Here we overview and comment on a number of recent contributions on the cell wall adaptive responses to environmental challenges that has greatly impacted the already high complexity of the PG biology field. These new evidences have revived the interest in PG plasticity as an exciting and trendy topic in current microbiology which considers this variability as the trustworthy picture of bacterial PG in nature.

Different walls for rods and balls: the diversity of peptidoglycan

Peptidoglycan performs the essential role of resisting turgor in the cell walls of most bacteria. It determines cell shape, and its biosynthesis is the target for many important antibiotics. The fundamental chemical building blocks of peptidoglycan are conserved: repeating disaccharides cross-linked by peptides. However, these blocks come in many varieties and can be assembled in different ways. So beyond the fundamental similarity, prodigious chemical, organizational and architectural diversity is revealed. Here, we track the evolution of our current understanding of peptidoglycan and underpinning technical and methodological developments. The origin and function of chemical diversity is discussed with respect to some well-studied example species. We then explore how this chemistry is manifested in elegant and complex peptidoglycan organization and how this is interpreted in different and sometimes controversial architectural models. We contend that emerging technology brings about the possibility of achieving a complete understanding of peptidoglycan chemistry, through architecture, to the way in which diverse species and populations of cells meet the challenges of maintaining viability and growth within their environmental niches, by exploiting the bioengineering versatility of peptidoglycan.

Modes of cell wall growth differentiation in rod-shaped bacteria

A bacterial cell takes on the challenge to preserve and reproduce its shape at every generation against a substantial internal pressure by surrounding itself with a mechanical support, a peptidoglycan cell wall. The enlargement of the cell wall via net incorporation of precursors into the pre-existing wall conditions bacterial growth and morphology. However, generation, reproduction and/or modification of a specific shape requires that the incorporation takes place at precise locations for a defined time period. Much has been learnt in the past few years about the biochemistry of the peptidoglycan synthesis process, but topological approaches to the understanding of shape generation have been hindered by a lack of appropriate techniques. Recent technological advances are paving the way for substantial progress in understanding the mechanisms of bacterial morphogenesis. Here we review the latest developments, focusing on the impact of new techniques on the precise mapping of cell wall growth sites.

A mutation in the promoter region of zipA, a component of the divisome, suppresses the shape defect of RodZ-deficient cells

RodZ is important for maintaining the rod shape of Escherichia coli. Loss of RodZ causes conversion of the rod shape to a round shape and a growth rate slower than that of wild-type cells. Suppressor mutations that simultaneously restore both the growth rates and the rod shape were isolated. Most of the suppressor mutations are found in mreB, mrdA, or mrdB. One of the mutations was in the promoter region of zipA, which encodes a crucial component of the cell division machinery. In this study, we investigated the mechanism of the suppression by this mutation. ZipA was slightly but significantly increased in the suppressor cells and led to a delay in cell division. While round-shaped mreB and mrdA mutants lose cell bipolarity, we found that round-shaped rodZ mutants retained cell bipolarity. Therefore, we concluded that a delay in the completion of septation provides extra time to elongate the cell laterally so that the zipA suppressor mutant is able to recover its ovoid or rod shape. The suppression by zipA demonstrates that the regulation of timing of septation potentially contributes to the conversion of morphology in bacterial cells.

From models to pathogens: how much have we learned about Streptococcus pneumoniae cell division?

Streptococcus pneumoniae is an oval-shaped Gram-positive coccus that lives in intimate association with its human host, both as a commensal and pathogen. The seriousness of pneumococcal infections and the spread of multi-drug resistant strains call for new lines of intervention. Bacterial cell division is an attractive target to develop antimicrobial drugs. This review discusses the recent advances in understanding S. pneumoniae growth and division, in comparison with the best studied rod-shaped models, Escherichia coli and Bacillus subtilis. To maintain their shape, these bacteria propagate by peripheral and septal peptidoglycan synthesis, involving proteins that assemble into distinct complexes called the elongasome and the divisome, respectively. Many of these proteins are conserved in S. pneumoniae, supporting the notion that the ovococcal shape is also achieved by rounds of elongation and division. Importantly, S. pneumoniae and close relatives with similar morphology differ in several aspects from the model rods. Overall, the data support a model in which a single large machinery, containing both the peripheral and septal peptidoglycan synthesis complexes, assembles at midcell and governs growth and division. The mechanisms generating the ovococcal or coccal shape in lactic-acid bacteria have likely evolved by gene reduction from a rod-shaped ancestor of the same group.

Cell wall elongation mode in Gram-negative bacteria is determined by peptidoglycan architecture

Cellular integrity and morphology of most bacteria is maintained by cell wall peptidoglycan, the target of antibiotics essential in modern healthcare. It consists of glycan strands, cross-linked by peptides, whose arrangement determines cell shape, prevents lysis due to turgor pressure and yet remains dynamic to allow insertion of new material, and hence growth. The cellular architecture and insertion pattern of peptidoglycan have remained elusive. Here we determine the peptidoglycan architecture and dynamics during growth in rod-shaped Gram-negative bacteria. Peptidoglycan is made up of circumferentially oriented bands of material interspersed with a more porous network. Super-resolution fluorescence microscopy reveals an unexpected discontinuous, patchy synthesis pattern. We present a consolidated model of growth via architecture-regulated insertion, where we propose only the more porous regions of the peptidoglycan network that are permissive for synthesis.

Bacterial cell wall peptidoglycan is essential for viability and shape determination. Using high-resolution microscopy, Foster and colleagues elucidate the peptidoglycan architecture and insertion pattern in Escherichia coli and other Gram-negative bacteria, and propose a new model for cell wall elongation.

Direct interaction of FtsZ and MreB is required for septum synthesis and cell division in Escherichia coli

How bacteria coordinate cell growth with division is not well understood. Bacterial cell elongation is controlled by actin-MreB while cell division is governed by tubulin-FtsZ. A ring-like structure containing FtsZ (the Z ring) at mid-cell attracts other cell division proteins to form the divisome, an essential protein assembly required for septum synthesis and cell separation. The Z ring exists at mid-cell during a major part of the cell cycle without contracting. Here, we show that MreB and FtsZ of Escherichia coli interact directly and that this interaction is required for Z ring contraction. We further show that the MreB-FtsZ interaction is required for transfer of cell-wall biosynthetic enzymes from the lateral to the mature divisome, allowing cells to synthesise the septum. Our observations show that bacterial cell division is coupled to cell elongation via a direct and essential interaction between FtsZ and MreB.

Cell sorting enriches Escherichia coli mutants that rely on peptidoglycan endopeptidases to suppress highly aberrant morphologies

Bacterial morphology imparts physiological advantages to cells in different environments and, judging by the fidelity with which shape is passed to daughter cells, is a tightly regulated characteristic. Surprisingly, only in the past 10 to 15 years has significant headway been made in identifying the mechanisms by which cells create and maintain particular shapes. One reason for this is that the relevant discoveries have relied heavily on the arduous, somewhat subjective process of manual microscopy. Here, we show that flow cytometry, coupled with the sorting capability of fluorescence-activated cell sorting (FACS), can detect, quantify, and enrich bacteria with morphological alterations. The light scattering properties of several highly aberrant morphological mutants of Escherichia coli were characterized by flow cytometry. Cells from a region that overlapped the distribution of normal rod-shaped cells were collected by FACS and reincubated. After 4 to 15 iterations of this enrichment process, suppressor mutants were isolated that returned almost all the population to a near-normal shape. Suppressors were successfully isolated from strains lacking three or four penicillin binding proteins (PBPs) but not from a mutant lacking a total of seven PBPs. The peptidoglycan endopeptidase, AmpH, was identified as being important for the suppression process, as was a related endopeptidase, MepA. The results validate the use of cell sorting as a means for studying bacterial morphology and identify at least one new class of enzymes required for the suppression of cell shape defects.

ZipA is required for FtsZ-dependent preseptal peptidoglycan synthesis prior to invagination during cell division

Rod-shaped bacteria grow by a repetitive cycle of elongation followed by division, and the mechanisms responsible for these two processes have been studied for decades. However, little is known about what happens during the transition between the two activities. At least one event occurs after elongation ends and before division commences, that being the insertion of new cell wall peptidoglycan into a narrowly circumscribed ribbon around midcell where septation is destined to take place. This insertion does not depend on the presence of the septation-specific protein PBP3 and is therefore known as PBP3-independent peptidoglycan synthesis (PIPS). Here we report that only FtsZ and ZipA are required to generate PIPS in wild-type Escherichia coli. PIPS does not require the participation of other members of the divisome, the MreB-directed cell wall elongation complex, alternate peptidoglycan synthases, the major peptidoglycan amidases, or any of the low-molecular-weight penicillin binding proteins. ZipA-directed PIPS may represent an intermediate stage that connects cell wall elongation to septal invagination and may be the reason ZipA is essential in the gammaproteobacteria.

Osmolality-dependent relocation of penicillin-binding protein PBP2 to the division site in Caulobacter crescentus

The synthesis of the peptidoglycan cell wall is carefully regulated in time and space. In nature, this essential process occurs in cells that live in fluctuating environments. Here we show that the spatial distributions of specific cell wall proteins in Caulobacter crescentus are sensitive to small external osmotic upshifts. The penicillin-binding protein PBP2, which is commonly branded as an essential cell elongation-specific transpeptidase, switches its localization from a dispersed, patchy pattern to an accumulation at the FtsZ ring location in response to osmotic upshifts as low as 40 mosmol/kg. This osmolality-dependent relocation to the division apparatus is initiated within less than a minute, while restoration to the patchy localization pattern is dependent on cell growth and takes 1 to 2 generations. Cell wall morphogenetic protein RodA and penicillin-binding protein PBP1a also change their spatial distribution by accumulating at the division site in response to external osmotic upshifts. Consistent with its ecological distribution, C. crescentus displays a narrow range of osmotolerance, with an upper limit of 225 mosmol/kg in minimal medium. Collectively, our findings reveal an unsuspected level of environmental regulation of cell wall protein behavior that is likely linked to an ecological adaptation.

Escherichia coli low-molecular-weight penicillin-binding proteins help orient septal FtsZ, and their absence leads to asymmetric cell division and branching

Escherichia coli cells lacking low-molecular-weight penicillin-binding proteins (LMW PBPs) exhibit morphological alterations that also appear when the septal protein FtsZ is mislocalized, suggesting that peptidoglycan modification and division may work together to produce cell shape. We found that in strains lacking PBP5 and other LMW PBPs, higher FtsZ concentrations increased the frequency of branched cells and incorrectly oriented Z rings by 10- to 15-fold. Invagination of these rings produced improperly oriented septa, which in turn gave rise to asymmetric cell poles that eventually elongated into branches. Branches always originated from the remnants of abnormal septation events, cementing the relationship between aberrant cell division and branch formation. In the absence of PBP5, PBP6 and DacD localized to nascent septa, suggesting that these PBPs can partially substitute for the loss of PBP5. We propose that branching begins when mislocalized FtsZ triggers the insertion of inert peptidoglycan at unusual positions during cell division. Only later, after normal cell wall elongation separates the patches, do branches become visible. Thus, a relationship between the LMW PBPs and cytoplasmic FtsZ ultimately affects cell division and overall shape.

Helical insertion of peptidoglycan produces chiral ordering of the bacterial cell wall

The regulation of cell shape is a common challenge faced by organisms across all biological kingdoms. In nearly all bacteria, cell shape is determined by the architecture of the peptidoglycan cell wall, a macromolecule consisting of glycan strands crosslinked by peptides. In addition to shape, cell growth must also maintain the wall structural integrity to prevent lysis due to large turgor pressures. Robustness can be accomplished by establishing a globally ordered cell-wall network, although how a bacterium generates and maintains peptidoglycan order on the micron scale using nanometer-sized proteins remains a mystery. Here, we demonstrate that left-handed chirality of the MreB cytoskeleton in the rod-shaped bacterium Escherichia coli gives rise to a global, right-handed chiral ordering of the cell wall. Local, MreB-guided insertion of material into the peptidoglycan network naturally orders the glycan strands and causes cells to twist left-handedly during elongational growth. Through comparison with the right-handed twisting of Bacillus subtilis cells, our work supports a common mechanism linking helical insertion and chiral cell-wall ordering in rod-shaped bacteria. These physical principles of cell growth link the molecular structure of the bacterial cytoskeleton, mechanisms of wall synthesis, and the coordination of cell-wall architecture.

Recent advances in pneumococcal peptidoglycan biosynthesis suggest new vaccine and antimicrobial targets

Streptococcus pneumoniae is a serious human respiratory pathogen that has the capacity to evade capsule-based vaccines and to develop multidrug antibiotic resistance. This review summarizes recent advances in understanding the mechanisms and regulation of peptidoglycan (PG) biosynthesis that result in ellipsoid-shaped, ovococcus Streptococcus cells. New results support a two-state model for septal and peripheral PG synthesis at mid-cell, involvement of essential cell division proteins in PG remodeling, and mid-cell localization of proteins that organize PG biosynthesis and that form the protein translocation apparatus. PG biosynthesis proteins have already turned up as promising vaccine candidates and targets of antibiotics. Properties of several recently characterized proteins that mediate or regulate PG biosynthesis suggest a source of additional targets for therapies against pneumococcus.

Spore formation in Myxococcus xanthus is tied to cytoskeleton functions and polysaccharide spore coat deposition

Myxococcus xanthus is a Gram-negative bacterium that differentiates into environmentally resistant spores. Spore differentiation involves septation-independent remodelling of the rod-shaped vegetative cell into a spherical spore and deposition of a thick and compact spore coat outside of the outer membrane. Our analyses suggest that spore coat polysaccharides are exported to the cell surface by the Exo outer membrane polysaccharide export/polysaccharide co-polymerase 2a (OPX/PCP-2a) machinery. Conversion of the capsule-like polysaccharide layer into a compact spore coat layer requires the Nfs proteins which likely form a complex in the cell envelope. Mutants in either nfs, exo or two other genetic loci encoding homologues of polysaccharide synthesis enzymes fail to complete morphogenesis from rods to spherical spores and instead produce a transient state of deformed cell morphology before reversion into typical rods. We additionally provide evidence that the cell cytoskeletal protein, MreB, plays an important role in rod to spore morphogenesis and for spore outgrowth. These studies provide evidence that this novel Gram-negative differentiation process is tied to cytoskeleton functions and polysaccharide spore coat deposition.

Chemical-biological studies of subcellular organization in bacteria

The subcellular organization of biological molecules is a critical determinant of many bacterial processes, including growth, replication of the genome, and division, yet the details of many mechanisms that control intracellular organization remain unknown. Decoding this information will impact the field of bacterial physiology and can provide insight into eukaryotic biology, including related processes in mitochondria and chloroplasts. Small molecule probes provide unique advantages in studying these mechanisms and manipulating the organization of biomolecules in live bacterial cells. In this review, we describe small molecules that are available for investigating subcellular organization in bacteria, specifically targeting FtsZ, MreB, peptidoglycan, and lipid bilayers. We discuss how these probes have been used to study microbiological questions and conclude by providing suggestions about important areas in which chemical-biological approaches will have a revolutionary impact on the study of bacterial physiology.

Salmonella enterica serovar typhimurium colonizing the lumen of the chicken intestine grows slowly and upregulates a unique set of virulence and metabolism genes

The pattern of global gene expression in Salmonella enterica serovar Typhimurium bacteria harvested from the chicken intestinal lumen (cecum) was compared with that of a late-log-phase LB broth culture using a whole-genome microarray. Levels of transcription, translation, and cell division in vivo were lower than those in vitro. S. Typhimurium appeared to be using carbon sources, such as propionate, 1,2-propanediol, and ethanolamine, in addition to melibiose and ascorbate, the latter possibly transformed to d-xylulose. Amino acid starvation appeared to be a factor during colonization. Bacteria in the lumen were non- or weakly motile and nonchemotactic but showed upregulation of a number of fimbrial and Salmonella pathogenicity island 3 (SPI-3) and 5 genes, suggesting a close physical association with the host during colonization. S. Typhimurium bacteria harvested from the cecal mucosa showed an expression profile similar to that of bacteria from the intestinal lumen, except that levels of transcription, translation, and cell division were higher and glucose may also have been used as a carbon source.

The requirement for pneumococcal MreC and MreD is relieved by inactivation of the gene encoding PBP1a

MreC and MreD, along with the actin homologue MreB, are required to maintain the shape of rod-shaped bacteria. The depletion of MreCD in rod-shaped bacteria leads to the formation of spherical cells and the accumulation of suppressor mutations. Ovococcus bacteria, such as Streptococcus pneumoniae, lack MreB homologues, and the functions of the S. pneumoniae MreCD (MreCD(Spn)) proteins are unknown. mreCD are located upstream from the pcsB cell division gene in most Streptococcus species, but we found that mreCD and pcsB are transcribed independently. Similarly to rod-shaped bacteria, we show that mreCD are essential in the virulent serotype 2 D39 strain of S. pneumoniae, and the depletion of MreCD results in cell rounding and lysis. In contrast, laboratory strain R6 contains suppressors that allow the growth of ΔmreCD mutants, and bypass suppressors accumulate in D39 ΔmreCD mutants. One class of suppressors eliminates the function of class A penicillin binding protein 1a (PBP1a). Unencapsulated Δpbp1a D39 mutants have smaller diameters than their pbp1a(+) parent or Δpbp2a and Δpbp1b mutants, which lack other class A PBPs and do not show the suppression of ΔmreCD mutations. Suppressed ΔmreCD Δpbp1a double mutants form aberrantly shaped cells, some with misplaced peptidoglycan (PG) biosynthesis compared to that of single Δpbp1a mutants. Quantitative Western blotting showed that MreC(Spn) is abundant (≈8,500 dimers per cell), and immunofluorescent microscopy (IFM) located MreCD(Spn) to the equators and septa of dividing cells, similarly to the PBPs and PG pentapeptides indicative of PG synthesis. These combined results are consistent with a model in which MreCD(Spn) direct peripheral PG synthesis and control PBP1a localization or activity.

Mutations in the nucleotide binding pocket of MreB can alter cell curvature and polar morphology in Caulobacter

The maintenance of cell shape in Caulobacter crescentus requires the essential gene mreB, which encodes a member of the actin superfamily and the target of the antibiotic, A22. We isolated 35 unique A22-resistant Caulobacter strains with single amino acid substitutions near the nucleotide binding site of MreB. Mutations that alter cell curvature and mislocalize the intermediate filament crescentin cluster on the back surface of MreB's structure. Another subset have variable cell widths, with wide cell bodies and actively growing thin extensions of the cell poles that concentrate fluorescent MreB. We found that the extent to which MreB localization is perturbed is linearly correlated with the development of pointed cell poles and variable cell widths. Further, we find that a mutation to glycine of two conserved aspartic acid residues that are important for nucleotide hydrolysis in other members of the actin superfamily abolishes robust midcell recruitment of MreB but supports a normal rate of growth. These mutant strains provide novel insight into how MreB's protein structure, subcellular localization, and activity contribute to its function in bacterial cell shape.

Characterization of CrgA, a new partner of the Mycobacterium tuberculosis peptidoglycan polymerization complexes

The role(s) in cell division of the Mycobacterium tuberculosis Rv0011c gene product, a homolog of the Streptomyces CrgA protein that is responsible for coordinating growth and cytokinesis in sporogenic aerial hyphae, is largely unknown. We show that an enhanced cyan fluorescent protein-M. tuberculosis CrgA (ECFP-CrgA(MT)) fusion protein is localized to the cell membrane, midcell, and cell pole regions in Mycobacterium smegmatis. Furthermore, the ECFP-CrgA(MT) fusion protein colocalized with FtsZ-enhanced yellow fluorescent protein (EYFP) in M. smegmatis. Bacterial two-hybrid assays indicated strong interactions of M. tuberculosis CrgA with FtsZ, FtsQ, and the class B penicillin-binding proteins, FtsI (PBPB) and PBPA. The midcell localization of CrgA(MT) was severely compromised under conditions of FtsZ depletion, which indicated that CrgA localizes to the midcell region after assembly of the FtsZ ring. M. tuberculosis cells with reduced CrgA levels were elongated and grew more slowly than wild-type cells, which indicated defects in cell division, whereas CrgA overproduction did not show growth defects. A M. smegmatis ΔcrgA strain exhibited a bulged cell morphology, elongated cells with a chain-like phenotype, cells with polar bulbous structures, and a modest growth defect. FtsZ and FtsI levels were not affected in cells producing altered levels of CrgA. Septal and membrane localization of GFP-FtsI was enhanced by CrgA overproduction and was diminished in a ΔcrgA strain, which indicates that one role of CrgA is to promote and/or stabilize FtsI localization. Overall, these data indicate that CrgA is a novel member of the cell division complex in mycobacteria and possibly facilitates septum formation.

Mechanisms for maintaining cell shape in rod-shaped Gram-negative bacteria

For the rod-shaped Gram-negative bacterium Escherichia coli, changes in cell shape have critical consequences for motility, immune system evasion, proliferation and adhesion. For most bacteria, the peptidoglycan cell wall is both necessary and sufficient to determine cell shape. However, how the synthesis machinery assembles a peptidoglycan network with a robustly maintained micron-scale shape has remained elusive. To explore shape maintenance, we have quantified the robustness of cell shape in three Gram-negative bacteria in different genetic backgrounds and in the presence of an antibiotic that inhibits division. Building on previous modelling suggesting a prominent role for mechanical forces in shape regulation, we introduce a biophysical model for the growth dynamics of rod-shaped cells to investigate the roles of spatial regulation of peptidoglycan synthesis, glycan-strand biochemistry and mechanical stretching during insertion. Our studies reveal that rod-shape maintenance requires insertion to be insensitive to fluctuations in cell-wall density and stress, and even a simple helical pattern of insertion is sufficient for over sixfold elongation without significant loss in shape. In addition, we demonstrate that both the length and pre-stretching of newly inserted strands regulate cell width. In sum, we show that simple physical rules can allow bacteria to achieve robust, shape-preserving cell-wall growth.

A novel in vivo cell-wall labeling approach sheds new light on peptidoglycan synthesis in Escherichia coli

Peptidoglycan synthesis and turnover in relation to cell growth and division has been studied by using a new labeling method. This method involves the incorporation of fluorescently labeled peptidoglycan precursors into the cell wall by means of the cell-wall recycling pathway. We show that Escherichia coli is able to import exogenous added murein tripeptide labeled with N-7-nitro-2,1,3-benzoxadiazol-4-yl (AeK-NBD) into the cytoplasm where it enters the peptidoglycan biosynthesis route, resulting in fluorescent labels specifically located in the cell wall. When wild-type cells were grown in the presence of the fluorescent peptide, peptidoglycan was uniformly labeled in cells undergoing elongation. Cells in the process of division displayed a lack of labeled peptidoglycan at mid-cell. Analysis of labeling patterns in cell division mutants showed that the occurrence of unlabeled peptidoglycan is dependent on the presence of FtsZ, but independent of FtsQ and FtsI. Accumulation of fluorescence at the division sites of a triple amidase mutant (ΔamiABC) revealed that AeK-NBD is incorporated into septal peptidoglycan. AmiC was shown to be involved in the rapid removal of labeled peptidoglycan side chains at division sites in wild-type cells. Because septal localization of AmiC is dependent on FtsQ and FtsI, this points to the presence of another peptidoglycan hydrolase activity directly dependent on FtsZ.

The bacterial cytoskeleton

Bacteria, like eukaryotes, employ cytoskeletal elements to perform many functions, including cell morphogenesis, cell division, DNA partitioning, and cell motility. They not only possess counterparts of eukaryotic actin, tubulin, and intermediate filament proteins, but they also have cytoskeletal elements of their own. Unlike the rigid sequence and structural conservation often observed for eukaryotic cytoskeletal proteins, the bacterial counterparts can display considerable diversity in sequence and function across species. Their wide range of function highlights the flexibility of core cytoskeletal protein motifs, such that one type of cytoskeletal element can perform various functions, and one function can be performed by different types of cytoskeletal elements.

A new morphogenesis pathway in bacteria: unbalanced activity of cell wall synthesis machineries leads to coccus-to-rod transition and filamentation in ovococci

Bacteria display a variety of shapes, which have biological relevance. In most eubacteria, cell shape is maintained by the tough peptidoglycan (PG) layer of the cell wall, the sacculus. The organization of PG synthesis machineries, orchestrated by different cytoskeletal elements, determines the specific shapes of sacculi. In rod-shaped bacteria, the actin-like (MreB) and the tubuline-like (FtsZ) cytoskeletons control synthesis of the sidewall (elongation) and the crosswall (septation) respectively. Much less is known concerning cell morphogenesis in cocci, which lack MreB proteins. While spherical cocci exclusively display septal growth, ovococci additionally display peripheral growth, which is responsible of the slight longitudinal expansion that generates their ovoid shape. Here, we report that the ovococcus Lactococcus lactis has the ability to become rod-shaped. L. lactis IL1403 wild-type cells form long aseptate filaments during both biofilm and planktonic growth in a synthetic medium. Nascent PG insertion and the division protein FtsK localize in multiple peripheral rings regularly spaced along the filaments. We show that filamentation results from septation inhibition, and that penicillin-binding proteins PBP2x and PBP2b play a direct role in this process. We propose a model for filament formation in L. lactis, and discuss the possible biological role of such morphological differentiation.

Bacterial shape: two-dimensional questions and possibilities

Events in the past decade have made it both possible and interesting to ask how bacteria create cells of defined length, diameter, and morphology. The current consensus is that bacterial shape is determined by the coordinated activities of cytoskeleton complexes that drive cell elongation and division. Cell length is most easily explained by the timing of cell division, principally by regulating the activity of the FtsZ protein. However, the question of how cells establish and maintain a specific and uniform diameter is, by far, much more difficult to answer. Mutations associated with the elongation complex often alter cell width, though it is not clear how. Some evidence suggests that diameter is strongly influenced by events during cell division. In addition, surprising new observations show that the bacterial cell wall is more highly malleable than previously believed and that cells can alter and restore their shapes by relying only on internal mechanisms.