Contraction of filaments of Escherichia coli after disruption of cell membrane by detergent

Molecular imaging of glycan chains couples cell-wall polysaccharide architecture to bacterial cell morphology

Biopolymer composite cell walls maintain cell shape and resist forces in plants, fungi and bacteria. Peptidoglycan, a crucial antibiotic target and immunomodulator, performs this role in bacteria. The textbook structural model of peptidoglycan is a highly ordered, crystalline material. Here we use atomic force microscopy (AFM) to image individual glycan chains in peptidoglycan from Escherichia coli in unprecedented detail. We quantify and map the extent to which chains are oriented in a similar direction (orientational order), showing it is much less ordered than previously depicted. Combining AFM with size exclusion chromatography, we reveal glycan chains up to 200 nm long. We show that altered cell shape is associated with substantial changes in peptidoglycan biophysical properties. Glycans from E. coli in its normal rod shape are long and circumferentially oriented, but when a spheroid shape is induced (chemically or genetically) glycans become short and disordered.

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.

Development of functionalised polyelectrolyte capsules using filamentous Escherichia coli cells

Background

Escherichia coli is one of the best studied microorganisms and finds multiple applications especially as tool in the heterologous production of interesting proteins of other organisms. The heterologous expression of special surface (S-) layer proteins caused the formation of extremely long E. coli cells which leave transparent tubes when they divide into single E. coli cells. Such natural structures are of high value as bio-templates for the development of bio-inorganic composites for many applications. In this study we used genetically modified filamentous Escherichia coli cells as template for the design of polyelectrolyte tubes that can be used as carrier for functional molecules or particles. Diversity of structures of biogenic materials has the potential to be used to construct inorganic or polymeric superior hybrid materials that reflect the form of the bio-template. Such bio-inspired materials are of great interest in diverse scientific fields like Biology, Chemistry and Material Science and can find application for the construction of functional materials or the bio-inspired synthesis of inorganic nanoparticles.

Results

Genetically modified filamentous E. coli cells were fixed in 2% glutaraldehyde and coated with alternating six layers of the polyanion polyelectrolyte poly(sodium-4styrenesulfonate) (PSS) and polycation polyelectrolyte poly(allylamine-hydrochloride) (PAH). Afterwards we dissolved the E. coli cells with 1.2% sodium hypochlorite, thus obtaining hollow polyelectrolyte tubes of 0.7 μm in diameter and 5–50 μm in length. For functionalisation the polyelectrolyte tubes were coated with S-layer protein polymers followed by metallisation with Pd(0) particles. These assemblies were analysed with light microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and transmission electron microscopy.

Conclusion

The thus constructed new material offers possibilities for diverse applications like novel catalysts or metal nanowires for electrical devices. The novelty of this work is the use of filamentous E. coli templates and the use of S-layer proteins in a new material construct.

Heterologous expression of the surface-layer-like protein SllB induces the formation of long filaments of Escherichia coli consisting of protein-stabilized outer membrane

Escherichia coliis one of the best studied micro-organisms and is the most widely used host in genetic engineering. The Gram-negative single cells are rod-shaped, and filaments are usually not found. Here, we describe the reproducible formation of elongatedE. colicells. During heterologous expression of the silent surface (S)-layer protein genesllBfromLysinibacillus sphaericusJG-A12 inE. coliBL21(DE3), the cells were arranged as long chains which were surrounded by highly stable sheaths. These filaments had a length of >100 μm. In the stationary growth phase, microscopic analyses demonstrated the formation of unusually long transparent tube-like structures which were enclosing separate single cells. The tube-like structures were isolated and analysed by SDS-PAGE, infrared-spectroscopy and different microscopic methods in order to identify their unusual composition and structure. The tube-like structures were found to be like outer membranes, containing high levels of proteins and to which the recombinant S-layer proteins were attached. Despite the entire structure being indicative of a disordered cell division, the bacterial cells were highly viable and stable. To our knowledge, this is the first time that the induction of drastic morphological changes inE. coliby the expression of a foreign protein has been reported.

Bacterial cell curvature through mechanical control of cell growth

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

Molecular organization of Gram-negative peptidoglycan

The stress-bearing component of the bacterial cell wall--a multi-gigadalton bag-like molecule called the sacculus--is synthesized from peptidoglycan. Whereas the chemical composition and the 3-dimensional structure of the peptidoglycan subunit (in at least one conformation) are known, the architecture of the assembled sacculus is not. Four decades' worth of biochemical and electron microscopy experiments have resulted in two leading 3-D peptidoglycan models: "Layered" and "Scaffold", in which the glycan strands are parallel and perpendicular to the cell surface, respectively. Here we resolved the basic architecture of purified, frozen-hydrated sacculi through electron cryotomography. In the Gram-negative sacculus, a single layer of glycans lie parallel to the cell surface, roughly perpendicular to the long axis of the cell, encircling the cell in a disorganized hoop-like fashion.

Z-ring force and cell shape during division in rod-like bacteria

The life cycle of bacterial cells consists of repeated elongation, septum formation, and division. Before septum formation, a division ring called the Z-ring, which is made of a filamentous tubulin analog, FtsZ, is seen at the mid cell. Together with several other proteins, FtsZ is essential for cell division. Visualization of strains with GFP-labeled FtsZ shows that the Z-ring contracts before septum formation and pinches the cell into two equal halves. Thus, the Z-ring has been postulated to act as a force generator, although the magnitude of the contraction force is unknown. In this article, we develop a mathematical model to describe the process of growth and Z-ring contraction in rod-like bacteria. The elasticity and growth of the cell wall is incorporated in the model to predict the contraction speed, the cell shape, and the contraction force. With reasonable parameters, the model shows that a small force from the Z-ring (8 pN in Escherichia coli) is sufficient to accomplish division.

Murein (peptidoglycan) structure, architecture and biosynthesis in Escherichia coli

The periplasmic murein (peptidoglycan) sacculus is a giant macromolecule made of glycan strands cross-linked by short peptides completely surrounding the cytoplasmic membrane to protect the cell from lysis due to its internal osmotic pressure. More than 50 different muropeptides are released from the sacculus by treatment with a muramidase. Escherichia coli has six murein synthases which enlarge the sacculus by transglycosylation and transpeptidation of lipid II precursor. A set of twelve periplasmic murein hydrolases (autolysins) release murein fragments during cell growth and division. Recent data on the in vitro murein synthesis activities of the murein synthases and on the interactions between murein synthases, hydrolases and cell cycle related proteins are being summarized. There are different models for the architecture of murein and for the incorporation of new precursor into the sacculus. We present a model in which morphogenesis of the rod-shaped E. coli is driven by cytoskeleton elements competing for the control over the murein synthesis multi-enzyme complexes.

Cryo-electron microscopy of cell division in Staphylococcus aureus reveals a mid-zone between nascent cross walls

Cryo-electron microscopy of frozen-hydrated thin sections permits the observation of the real distribution of mass in biological specimens allowing the native structure of bacteria to be seen, including the natural orientation of their surface layers. Here, we use this approach to study the fine ultrastructure of the division site, or septum, of Staphylococcus aureus D(2)C. Frozen-hydrated sections revealed a differentiated cell wall at the septum, showing two high-density regions sandwiched between three low-density zones. The two zones adjacent to the membrane appeared as an extension of the periplasmic space seen in this organism's cell envelope and showed no distinguishing structures within them. Immediately next to these were higher-density zones that corresponded to nascent cross walls of the septum. Unexpectedly, a rather broad low-density zone was seen separating cross walls in the septum. This mid-zone of low density appeared inflated and without visible structures in isolated cell walls, which showed only the high-density zones of the septum. Here, we suggest that frozen-hydrated thin sections have captured a highly fragile septal region, the mid-zone, which results from the dynamic action of autolysis and actively separates daughter cells during division. The two zones next to the membranes are periplasmic spaces. Immediately next to these are the growing cross walls composed of peptidoglycan, teichoic acid and protein.

Osmosensing by bacteria: signals and membrane-based sensors

Bacteria can survive dramatic osmotic shifts. Osmoregulatory responses mitigate the passive adjustments in cell structure and the growth inhibition that may ensue. The levels of certain cytoplasmic solutes rise and fall in response to increases and decreases, respectively, in extracellular osmolality. Certain organic compounds are favored over ions as osmoregulatory solutes, although K+ fluxes are intrinsic to the osmoregulatory response for at least some organisms. Osmosensors must undergo transitions between "off" and "on" conformations in response to changes in extracellular water activity (direct osmosensing) or resulting changes in cell structure (indirect osmosensing). Those located in the cytoplasmic membranes and nucleoids of bacteria are positioned for indirect osmosensing. Cytoplasmic membrane-based osmosensors may detect changes in the periplasmic and/or cytoplasmic solvent by experiencing changes in preferential interactions with particular solvent constituents, cosolvent-induced hydration changes, and/or macromolecular crowding. Alternatively, the membrane may act as an antenna and osmosensors may detect changes in membrane structure. Cosolvents may modulate intrinsic biomembrane strain and/or topologically closed membrane systems may experience changes in mechanical strain in response to imposed osmotic shifts. The osmosensory mechanisms controlling membrane-based K+ transporters, transcriptional regulators, osmoprotectant transporters, and mechanosensitive channels intrinsic to the cytoplasmic membrane of Escherichia coli are under intensive investigation. The osmoprotectant transporter ProP and channel MscL act as osmosensors after purification and reconstitution in proteoliposomes. Evidence that sensor kinase KdpD receives multiple sensory inputs is consistent with the effects of K+ fluxes on nucleoid structure, cellular energetics, cytoplasmic ionic strength, and ion composition as well as on cytoplasmic osmolality. Thus, osmoregulatory responses accommodate and exploit the effects of individual cosolvents on cell structure and function as well as the collective contribution of cosolvents to intracellular osmolality.

The permeability of the wall fabric of Escherichia coli and Bacillus subtilis

To study the overall structure of the peptidoglycan fabric of the sacculi of gram-negative and gram-positive walls, actively growing cultures of Escherichia coli and Bacillus subtilis were treated with boiling sodium dodecyl sulfate solutions. The sacculi were then treated with enzymes to eliminate proteins and nucleic acids. These intact saccoli were probed with fluorescein-labeled dextrans with a range of known molecular weights. The penetration of the probes could be monitored by the negative-staining appearance in the fluorescence microscope. At several chosen times, the molecular weight fraction that allowed barely observable entry of the fluorescein-labeled probe and the molecular weight fraction that penetrated to achieve almost, but not quite, the concentration of probe in the solution external to the sacculi were determined. From three pairs of times and molecular weights that met one or the other of these two criteria, the effective pore size could be calculated. The minimum size of protein molecule that could diffuse through the pores was also calculated. Two mathematical models, which gave essentially the same results, were used to interpret the experimental data: one for the permeation of random coils through a surface containing holes and the other for rigid spheres diffusing through water-filled cylindrical pores. The mean estimate of the effective hole radius in walls from E. coli is 2.06 nm, and that of the effective hole size in walls from B. subtilis is 2.12 nm. These results are supported by experiments in which the loss of preloaded cells was monitored. Various fluorescein-labeled dextran samples were mixed with samples of intact cell walls, held for a long time, and then diluted. The efflux of the dextrans was monitored. Neither large nor small dextrans stained under these conditions. Only with dextran samples of a sufficiently small size were the sacculi filled during the preincubation period, and only with the largest of these could the probe not escape quickly. From the pore (or mesh) size, it can be concluded that the wall fabric of both organisms has few imperfections and that the major passageway is through the smallest possible pore, or "tessera," formed by the maximal cross-linking of the peptides from glycan chain to glycan chain compatible with the degree of rotational flexibility of the chains of repeating disaccharides of N-acetyl muramic acid and N-acetyl glucosamine. A tessera is composed of two chains of eight saccharides cross-linked by two octapeptides. The size of a globular hydrophilic molecule, if it did not bind to wall components, that could pass freely through the meshwork of an unstretched sacculus of either organism is roughly 25 kDa. We stress that this is only a rough estimate, and it may be possible for proteins of less than 50 kDa to pass through the native wall during normal growth conditions.

Significance of plasmolysis spaces as markers for periseptal annuli and adhesion sites

During hyperosmotic shock, the protoplast and stretched-out peptidoglycan layer first shrink together until the turgor pressure in the cell is relieved. Being non-compressible, the outer and inner membranes must fold their superfluous surfaces. While the protoplast contracts further, the inner membrane rearranges into plasmolysis spaces visible by phase-contrast microscopy. Two opposing theories predict a similar positioning of spaces in dividing cells and filaments: the 'periseptal annulus model', based on adhesion zones, involved in the predetermination of the division site; and a 'restricted, random model', based on physical properties of the protoplast. Strong osmotic shock causes retraction of the inner membrane over almost the entire surface forming the so-called 'Bayer bridges'. These tubular adhesion sites are preserved by chemical fixation, and can be destroyed by cryofixation and freeze-substitution of unfixed cells. Both the regular positioning of the plasmolysis spaces and the occurrence of tubular adhesion sites can be explained on the basis of physical properties of the membrane which necessitate rearrangements by membrane flow during shrinkage of the protoplast.

Elastic, flexible peptidoglycan and bacterial cell wall properties

The peptidoglycan sacculus serves as a mechanical framework for the cell walls of most eubacteria and largely determines cell shape. The notion that the structure is a rigid shell is contradicted by findings that peptidoglycan can expand and contract. Thus, the sacculus functions as an elastic, flexible, polyionic, amphoteric, restraining network.

Elasticity of the sacculus of Escherichia coli

Preparations of purified peptidoglycan of Escherichia coli (i.e., sacculi) were studied by low-angle laser light scattering. Control experiments and theoretical calculations based on the Rayleigh-Gans theory showed that the mean sacculus surface area could be accurately inferred from measurements with our apparatus by using computer routines developed previously. Large changes in the mean saccular surface area resulted from alterations in the stress caused by varying the net charge on the sacculi. The net charge was affected by altering the suspending medium pH, causing carboxyl and amino groups in the peptidoglycan to gain or lose protons, or by acetylation or succinylation of the amino groups. A preponderance of either plus or minus charges caused an expansion of the mean sacculus surface area. The largest increase in area probably represents the elastic limit of the peptidoglycan and was 300% above the area of isoionic sacculi. This degree of expansion is consistent with possible conformations of the intact peptidoglycan structure without necessitating rupture of the wall fabric. Our findings concerning saccular elasticity provide support for the surface stress theory. It provides a mechanism so that bacteria can grow and divide while maintaining turgor pressure, without the necessity of having and using proteins to do the mechanical work.

Mechanical properties of Bacillus subtilis cell walls: effects of removing residual culture medium

Experiments are described in which the tensile strength, the initial (Youngs') modulus, and other mechanical properties of the bacterial cell wall were obtained as functions of relative humidity (RH) in the range of 20 to 95%. These properties were deduced from tensile tests on bacterial thread, a fiber consisting of many highly aligned cells of Bacillus subtilis, from which residual culture medium had been removed by immersion in water. Reasons are given to support the idea that the mechanical properties of bacterial thread relate directly to those of the cylinder wall and that they are not influenced by septa, cytoplasm, or the thread assembly. The data show that the cell wall, like many other heteropolymers, is visco-elastic. When dry, it behaves like a glassy polymer with a tensile strength of about 300 MPa and a modulus of about 13 GPa. When wet, its behavior is more like a rubbery polymer with a tensile strength of about 13 MPa and a modulus of about 30 MPa. Thus, the cell wall is stronger than previously reported. Walls of this strength would be able to bear a turgor pressure of 2.6 MPa (about 26 atm). The dynamic behavior suggests a wide range of relaxation times. The way in which mechanical behavior depends strongly on humidity is discussed in terms of possible hydrogen bond density and the ordering of water molecules. Cell walls in threads containing residual culture medium TB are, except at low RH, 10 times more flexible and about 4 times less strong. All of their mechanical properties appear to vary with change in RH in a manner similar to those of walls from which the culture medium has been washed, but with a downshift of about 18% RH.

Cell wall mechanical properties as measured with bacterial thread made from Bacillus subtilis

Engineering approaches used in the study of textile fibers have been applied to the measurement of mechanical properties of bacterial cell walls by using the Bacillus subtilis bacterial thread system. Improved methods have been developed for the production of thread and for measuring its mechanical properties. The best specimens of thread produced from cultures of strain FJ7 grown in TB medium at 20 degrees C varied in diameter by a factor of 1.09 over a 30-mm thread length. The stress-strain behavior of cell walls was determined over the range of relative humidities between 11 and 98%. Measurements of over 125 specimens indicated that cell wall behaved like other viscoelastic polymers, both natural and man-made, exhibiting relaxation under constant elongation and recovery upon load removal. This kinetic behavior and also the cell wall strength depended greatly on humidity. The recovery from extension observed after loading even up to a substantial fraction of the breaking load indicated that the properties measured were those of cell wall material rather than of behavior of the thread assemblage. Control experiments showed that neither drying of thread nor the length of time it remained dry before testing influenced the mechanical properties of the cell walls. Specimens drawn from TB medium and then washed in water and redrawn were found to be stiffer and stronger than controls not washed. However, tensile properties were not changed by exposure of cells to lysozyme before thread production. This suggests that glycan backbones are not arranged along the length of the cell cylinder. The strength of the cell wall in vivo was estimated by extrapolation to 100% relative humidity to be about 3 N/mm2. Walls of this strength would be able to bear a turgor pressure of 6 atm (ca. 607.8 kPa), but if the increase in strength of water-washed threads was appropriate, the figure could be 24 atm (ca. 2,431.2 kPa).

Elasticity of the human red cell membrane skeleton. Effects of temperature and denaturants

The molecular basis for the elasticity of the human erythrocyte membrane was explored. Skeletons were released from ghosts in Triton X-100 and their dimensions followed by dark-field microscopy and packed volume. The rest size of skeletons was assumed to reflect the balance point between expansion (deformation) driven by electrostatic repulsions among the excess of fixed negative charges on the proteins and contraction (recovery) driven by their elasticity. The size of skeletons decreased with increasing temperature. This finding suggests that entropy drives elasticity. The requisite entropy change could be associated with either the configurational freedom of flexible protein chains or with the solvation of side chains exposed during protein dissociation (hydrophobic effects). To distinguish between these two alternatives, we tested the impact of two weak denaturants, 10% ethanol and 20 nM lithium 3,5-diiodosalicylate. Both agents reversibly promoted the expansion of skeletons, presumably by reducing their elasticity. Since the conformation of random coils and globular proteins should not be significantly altered by these mild treatments, this finding strongly suggests a role for weak interdomain and/or interprotein associations. We conclude that the elasticity of the red cell membrane skeleton may not derive from the configurational entropy of flexible coils. Rather, the elastic energy may arise from reversible dissociations of weak but specific intramolecular and/or intermolecular contacts, presumably within deformed spectrin filaments.

Nephelometric determination of turgor pressure in growing gram-negative bacteria

Gas vesicles were used as probes to measure turgor pressure in Ancylobacter aquaticus. The externally applied pressure required to collapse the vesicles in turgid cells was compared with that in cells whose turgor had been partially or totally removed by adding an impermeable solute to the external medium. Since gram-negative bacteria do not have rigid cell walls, plasmolysis is not expected to occur in the same way as it does in the cells of higher plants. Bacterial cells shrink considerably before plasmolysis occurs in hyperosmotic media. The increase in pressure required to collapse 50% of the vesicles as external osmotic pressure increases is less than predicted from the degree of osmotically inducible shrinkage seen with this organism or with another gram-negative bacterium. This feature complicates the calculation of the turgor pressure as the difference between the collapse pressure of vesicles with and without sucrose present in the medium. We propose a new model of the relationship between turgor pressure and the cell wall stress in gram-negative bacteria based on the behavior of an ideal elastic container when the pressure differential across its surface is decreased. We developed a new curve-fitting technique for evaluating bacterial turgor pressure measurements.