Compartmentalization of the periplasmic space at division sites in gram-negative bacteria

The Feline Calicivirus Leader of the Capsid Protein Has the Functional Characteristics of a Viroporin

The leader of the capsid (LC) protein is exclusive to the Vesivirus genus, and it is needed for successful feline calicivirus (FCV) replication, as well as an efficient apoptosis induction through the mitochondrial pathway. In this work, we aimed to determine if the LC protein from the FCV is a viroporin. Although lacking in a transmembrane domain or an amphipathic helix, the LC protein from the FCV is toxic when expressed in bacteria and it oligomerizes through disulfide bonds, which are both key characteristics of viroporins. An electron microscopy analysis of LC-expressing E. coli cells suggest that the protein induces osmotic stress. Moreover, we found that the previously studied C40A LC mutant, that fails to induce apoptosis and that hinders the replication cycle, also oligomerizes but it has a reduced toxicity and fails to induce osmotic stress in bacteria. We propose that the LC protein is a viroporin that acts as a disulfide bond-dependent antimicrobial peptide, similar to the Ebola virus delta peptide.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

Metabolic phospholipid labeling of intact bacteria enables a fluorescence assay that detects compromised outer membranes

Gram-negative bacteria possess an asymmetric outer membrane (OM) composed primarily of lipopolysaccharides (LPSs) on the outer leaflet and phospholipids (PLs) on the inner leaflet. The loss of this asymmetry due to mutations in the LPS biosynthesis or transport pathways causes the externalization of PLs to the outer leaflet of the OM and leads to OM permeability defects. Here, we used metabolic labeling to detect a compromised OM in intact bacteria. Phosphatidylcholine synthase expression in Escherichia coli allowed for the incorporation of exogenous propargylcholine into phosphatidyl(propargyl)choline and exogenous 1-azidoethyl-choline (AECho) into phosphatidyl(azidoethyl)choline (AEPC), as confirmed by LC/MS analyses. A fluorescent copper-free click reagent poorly labeled AEPC in intact wild-type cells but readily labeled AEPC from lysed cells. Fluorescence microscopy and flow cytometry analyses confirmed the absence of significant AEPC labeling from intact wild-type E. coli strains and revealed significant AEPC labeling in an E. coli LPS transport mutant (lptD4213) and an LPS biosynthesis mutant (E. coli lpxC101). Our results suggest that metabolic PL labeling with AECho is a promising tool for detecting a compromised bacterial OM, revealing aberrant PL externalization, and identifying or characterizing novel cell-active inhibitors of LPS biosynthesis or transport.

Molecular characterization and verification of azido-3,8-dideoxy-d-manno-oct-2-ulosonic acid incorporation into bacterial lipopolysaccharide

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an essential component of LPS in the outer leaflet of the Gram-negative bacterial outer membrane. Although labeling of Escherichia coli with the chemical reporter 8-azido-3,8-dideoxy-d-manno-oct-2-ulosonic acid (Kdo-N₃) has been reported, its incorporation into LPS has not been directly shown. We have now verified Kdo-N₃ incorporation into E. coli LPS at the molecular level. Using microscopy and PAGE analysis, we show that Kdo-N₃ is localized to the outer membrane and specifically incorporates into rough and deep-rough LPS. In an E. coli strain lacking endogenous Kdo biosynthesis, supplementation with exogenous Kdo restored full-length core-LPS, which suggests that the Kdo biosynthetic pathways might not be essential in vivo in the presence of sufficient exogenous Kdo. In contrast, exogenous Kdo-N₃ only restored a small fraction of core LPS with the majority incorporated into truncated LPS. The truncated LPS were identified as Kdo-N₃-lipid IV_A and (Kdo-N₃)₂-lipid IV_A by MS analysis. The low level of Kdo-N₃ incorporation could be partly explained by a 6-fold reduction in the specificity constant of the CMP-Kdo synthetase KdsB with Kdo-N₃ compared with Kdo. These results indicate that the azido moiety in Kdo-N₃ interferes with its utilization and may limit its utility as a tracer of LPS biosynthesis and transport in E. coli We propose that our findings will be helpful for researchers using Kdo and its chemical derivatives for investigating LPS biosynthesis, transport, and assembly in Gram-negative bacteria.

Determining the bacterial cell biology of Planctomycetes

Bacteria of the phylum Planctomycetes have been previously reported to possess several features that are typical of eukaryotes, such as cytosolic compartmentalization and endocytosis-like macromolecule uptake. However, recent evidence points towards a Gram-negative cell plan for Planctomycetes, although in-depth experimental analysis has been hampered by insufficient genetic tools. Here we develop methods for expression of fluorescent proteins and for gene deletion in a model planctomycete, Planctopirus limnophila, to analyse its cell organization in detail. Super-resolution light microscopy of mutants, cryo-electron tomography, bioinformatic predictions and proteomic analyses support an altered Gram-negative cell plan for Planctomycetes, including a defined outer membrane, a periplasmic space that can be greatly enlarged and convoluted, and an energized cytoplasmic membrane. These conclusions are further supported by experiments performed with two other Planctomycetes, Gemmata obscuriglobus and Rhodopirellula baltica. We also provide experimental evidence that is inconsistent with endocytosis-like macromolecule uptake; instead, extracellular macromolecules can be taken up and accumulate in the periplasmic space through unclear mechanisms.

MreB-Dependent Organization of the E. coli Cytoplasmic Membrane Controls Membrane Protein Diffusion

The functional organization of prokaryotic cell membranes, which is essential for many cellular processes, has been challenging to analyze due to the small size and nonflat geometry of bacterial cells. Here, we use single-molecule fluorescence microscopy and three-dimensional quantitative analyses in live Escherichia coli to demonstrate that its cytoplasmic membrane contains microdomains with distinct physical properties. We show that the stability of these microdomains depends on the integrity of the MreB cytoskeletal network underneath the membrane. We explore how the interplay between cytoskeleton and membrane affects trans-membrane protein (TMP) diffusion and reveal that the mobility of the TMPs tested is subdiffusive, most likely caused by confinement of TMP mobility by the submembranous MreB network. Our findings demonstrate that the dynamic architecture of prokaryotic cell membranes is controlled by the MreB cytoskeleton and regulates the mobility of TMPs.

Localization of the outer membrane protein OmpA2 in Caulobacter crescentus depends on the position of the gene in the chromosome

The outer membrane of Gram-negative bacteria is an essential structure involved in nutrient uptake, protection against harmful substances, and cell growth. Different proteins keep the outer membrane from blebbing out by simultaneously interacting with it and with the cell wall. These proteins have been mainly studied in enterobacteria, where OmpA and the Braun and Pal lipoproteins stabilize the outer membrane. Some degree of functional redundancy exists between these proteins, since none of them is essential but the absence of two of them results in a severe phenotype. Caulobacter crescentus has a different strategy to maintain its outer membrane, since it lacks the Braun lipoprotein and Pal is essential. In this work, we characterized OmpA2, an OmpA-like protein, in this bacterium. Our results showed that this protein is required for normal stalk growth and that it plays a minor role in the stability of the outer membrane. An OmpA2 fluorescent fusion protein showed that the concentration of this protein decreases from the stalk to the new pole. This localization pattern is important for its function, and it depends on the position of the gene locus in the chromosome and, as a consequence, in the cell. This result suggests that little diffusion occurs from the moment that the gene is transcribed until the mature protein attaches to the cell wall in the periplasm. This mechanism reveals the integration of different levels of information from protein function down to genome arrangement that allows the cell to self-organize.

Automated transmission-mode scanning electron microscopy (tSEM) for large volume analysis at nanoscale resolution

Transmission-mode scanning electron microscopy (tSEM) on a field emission SEM platform was developed for efficient and cost-effective imaging of circuit-scale volumes from brain at nanoscale resolution. Image area was maximized while optimizing the resolution and dynamic range necessary for discriminating key subcellular structures, such as small axonal, dendritic and glial processes, synapses, smooth endoplasmic reticulum, vesicles, microtubules, polyribosomes, and endosomes which are critical for neuronal function. Individual image fields from the tSEM system were up to 4,295 µm² (65.54 µm per side) at 2 nm pixel size, contrasting with image fields from a modern transmission electron microscope (TEM) system, which were only 66.59 µm² (8.160 µm per side) at the same pixel size. The tSEM produced outstanding images and had reduced distortion and drift relative to TEM. Automated stage and scan control in tSEM easily provided unattended serial section imaging and montaging. Lens and scan properties on both TEM and SEM platforms revealed no significant nonlinear distortions within a central field of ∼100 µm² and produced near-perfect image registration across serial sections using the computational elastic alignment tool in Fiji/TrakEM2 software, and reliable geometric measurements from RECONSTRUCT™ or Fiji/TrakEM2 software. Axial resolution limits the analysis of small structures contained within a section (∼45 nm). Since this new tSEM is non-destructive, objects within a section can be explored at finer axial resolution in TEM tomography with current methods. Future development of tSEM tomography promises thinner axial resolution producing nearly isotropic voxels and should provide within-section analyses of structures without changing platforms. Brain was the test system given our interest in synaptic connectivity and plasticity; however, the new tSEM system is readily applicable to other biological systems.

Lipopolysaccharide (LPS) inner-core phosphates are required for complete LPS synthesis and transport to the outer membrane in Pseudomonas aeruginosa PAO1

Gram-negative outer membrane (OM) integrity is maintained in part by Mg²⁺ cross-links between phosphates on lipid A and on core sugars of adjacent lipopolysaccharide (LPS) molecules. In contrast to other Gram-negative bacteria, waaP, encoding an inner-core kinase, could not be inactivated in Pseudomonas aeruginosa. To examine this further, expression of the kinases WaaP or WapP/WapQ/PA5006 was placed under the control of the arabinose-regulated pBAD promoter. Growth of these strains was arabinose dependent, confirming that core phosphorylation is essential in P. aeruginosa. Transmission electron micrographs of kinase-depleted cells revealed marked invaginations of the inner membrane. SDS-PAGE of total LPS from WaaP-depleted cells showed accumulation of a fast-migrating band. Mass spectrometry (MS) analysis revealed that LPS from these cells exhibits a unique truncated core consisting of two 3-deoxy-d-manno-octulosonic acids (Kdo), two l-glycero-d-manno-heptoses (Hep), and one hexose but completely devoid of phosphates, indicating that phosphorylation by WaaP is necessary for subsequent core phosphorylations. MS analysis of lipid A from WaaP-depleted cells revealed extensive 4-amino-4-deoxy-l-arabinose modification. OM prepared from these cells by Sarkosyl extraction of total membranes or by sucrose density gradient centrifugation lacked truncated LPS. Instead, truncated LPS was detected in the inner membrane fractions, consistent with impaired transport/assembly of this species into the OM.

Gram-negative bacteria have an outer membrane (OM) comprised of a phospholipid inner leaflet and a lipopolysaccharide (LPS) outer leaflet. The OM protects cells from toxic molecules and is important for survival during infection. The LPS core kinase gene waaP can be deleted in several Gram-negative bacteria but not in Pseudomonas aeruginosa. We used a controlled-expression system to deplete WaaP directly in P. aeruginosa cells, which halted growth. WaaP depletion also caused gross changes in cell morphology and led to the accumulation of an aberrant LPS lacking several core sugars and all core phosphates. The aberrant LPS failed to reach the OM, suggesting that WaaP is essential in P. aeruginosa because it is required to produce the full-length LPS that is recognized by the OM transport/assembly machinery in this organism. Therefore, WaaP may constitute a good target for the development of novel antipseudomonal agents.

Reduced lipopolysaccharide phosphorylation in Escherichia coli lowers the elevated ori/ter ratio in seqA mutants

The seqA defect in Escherichia coli increases the ori/ter ratio and causes chromosomal fragmentation, making seqA mutants dependent on recombinational repair (the seqA recA colethality). To understand the nature of this chromosomal fragmentation, we characterized DeltaseqA mutants and isolated suppressors of the DeltaseqA recA lethality. We demonstrate that our DeltaseqA alleles have normal function of the downstream pgm gene and normal ratios of the major phospholipids in the membranes, but increased surface lipopolysaccharide (LPS) phosphorylation. The predominant class of DeltaseqA recA suppressors disrupts the rfaQGP genes, reducing phosphorylation of the inner core region of LPS. The rfaQGP suppressors also reduce the elevated ori/ter ratio of the DeltaseqA mutants but, unexpectedly, the suppressed mutants still exhibit the high levels of chromosomal fragmentation and SOS induction, characteristic of the DeltaseqA mutants. We also found that colethality of rfaP with defects in the production of acidic phospholipids is suppressed by alternative initiation of chromosomal replication, suggesting that LPS phosphorylation stimulates replication initiation. The rfaQGP suppression of the seqA recA lethality provides genetic support for the surprising physical evidence that the oriC DNA forms complexes with the outer membrane.

The bacterial cytoskeleton

In recent years it has been shown that bacteria contain a number of cytoskeletal structures. The bacterial cytoplasmic elements include homologs of the three major types of eukaryotic cytoskeletal proteins (actin, tubulin, and intermediate filament proteins) and a fourth group, the MinD-ParA group, that appears to be unique to bacteria. The cytoskeletal structures play important roles in cell division, cell polarity, cell shape regulation, plasmid partition, and other functions. The proteins self-assemble into filamentous structures in vitro and form intracellular ordered structures in vivo. In addition, there are a number of filamentous bacterial elements that may turn out to be cytoskeletal in nature. This review attempts to summarize and integrate the in vivo and in vitro aspects of these systems and to evaluate the probable future directions of this active research field.

Direct visualization of red fluorescent lipoproteins indicates conservation of the membrane sorting rules in the family Enterobacteriaceae

Chimeras created by fusing the monomeric red fluorescent protein (RFP) to a bacterial lipoprotein signal peptide (lipoRFPs) were visualized in the cell envelope by epifluorescence microscopy. Plasmolysis of the bacteria separated the inner and outer membranes, allowing the specific subcellular localization of lipoRFPs to be determined in situ. When equipped with the canonical inner membrane lipoprotein retention signal CDSR, lipoRFP was located in the inner membrane in Escherichia coli, whereas the outer membrane sorting signal CSSR caused lipoRFP to localize to the outer membrane. CFSR-RFP was also routed to the outer membrane, but CFNSR-RFP was located in the inner membrane, consistent with previous data showing that this sequence functions as an inner membrane retention signal. These four lipoproteins exhibited identical localization patterns in a panel of members of the family Enterobacteriaceae, showing that the lipoprotein sorting rules are conserved in these bacteria and validating the use of E. coli as a model system. Although most predicted inner membrane lipoproteins in these bacteria have an aspartate residue after the fatty acylated N-terminal cysteine residue, alternative signals such as CFN can and probably do function in parallel, as indicated by the existence of putative inner membrane lipoproteins with this sequence at their N termini.

Translocation of jellyfish green fluorescent protein via the Tat system of Escherichia coli and change of its periplasmic localization in response to osmotic up-shock

The bacterial twin arginine translocation (Tat) pathway is capable of exporting cofactor-containing enzymes into the periplasm. To assess the capacity of the Tat pathway to export heterologous proteins and to gain information about the property of the periplasm, we fused the twin arginine signal peptide of the trimethylamine N-oxide reductase to the jellyfish green fluorescent protein (GFP). Unlike the Sec pathway, the Tat system successfully exported correctly folded GFP into the periplasm of Escherichia coli. Interestingly, GFP appeared as a halo in most cells and occasionally showed a polar localization in wild type strains. When subjected to a mild osmotic up-shock, GFP relocalized very quickly at the two poles of the cells. The conversion from the halo structure to a periplasmic gathering at particular locations was also observed with spherical cells of the DeltarodA-pbpA mutant or of the wild type strain treated with lysozyme. Therefore, the periplasm is not a uniform compartment and the polarization of GFP is unlikely to be caused by simple invagination of the cytoplasmic membrane at the poles. Moreover, the polar gathering of GFP is reversible; the reversion was accelerated by glucose and inhibited by azide and carbonyl cyanide m-chlorophenylhydrazone, indicating an active adaptation of the bacteria to the osmolarity in the medium. These results strongly suggest a relocalization of periplasmic substances in response to environmental changes. The polar area might be the preferential zone where bacteria sense the change in the environment.

Bacterial cell division

Formation of the bacterial division septum is catalyzed by a number of essential proteins that assemble into a ring structure at the future division site. Assembly of proteins into the cytokinetic ring appears to occur in a hierarchial order that is initiated by the FtsZ protein, a structural and functional analog of eukaryotic tubulins. Placement of the division site at its correct location in Escherichia coli requires a division inhibitor (MinC), that is responsible for preventing septation at unwanted sites near the cell poles, and a topological specificity protein (MinE), that forms a ring at midcell and protects the midcell site from the division inhibitor. However, the mechanism responsible for identifying the position of the midcell site or the polar sites used for spore septum formation is still unclear. Regulation of the division process and its coordination with other cell cycle events, such as chromosome replication, are poorly understood. However, a protein has been identified in Caulobacter (CtrA) that regulates both the initiation of chromosome regulation and the transcription of ftsZ, and that may play an important role in the coordination process.

On the origin of branches in Escherichia coli

Some Escherichia coli strains with impaired cell division form branched cells at high frequencies during certain growth conditions. Here, we show that neither FtsI nor FtsZ activity is required for the development of branches. Buds did not form at specific positions along the cell surface during high-branching conditions. Antibiotics affecting cell wall synthesis had a positive effect on branch formation in the case of ampicillin, cephalexin, and penicillin G, whereas mecillinam and D-cycloserine had no substantial effect. Altering the cell morphology by nutritional shifts showed that changes in morphology preceded branching, indicating that the cell's physiological state rather than specific medium components induced branching. Finally, there was no increased probability for bud formation in the daughters of a cell with a bud or branch, showing that bud formation is a random event. We suggest that branch formation is caused by abnormalities in cell wall elongation rather than by aberrant cell division events.

Visualization of membrane domains in Escherichia coli

Bacterial membrane and nucleoids were stained concurrently by the lipophilic styryl dye FM 4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl) hexatrienyl)pyridinium dibromide] and 4',6-diamidino-2-phenylindole (DAPI), respectively, and studied using fluorescence microscopy imaging. Observation of plasmolysed cells indicated that FM 4-64 stained the inner membrane preferentially. In live Escherichia coli pbpB cells and filaments, prepared on wet agar slabs, an FM 4-64 staining pattern developed in the form of dark bands. In dividing cells, the bands occurred mainly at the constriction sites and, in filaments, between partitioning nucleoids. The FM 4-64 pattern of dark bands in filaments was abolished after inhibiting protein synthesis with chloramphenicol. It is proposed that the staining patterns reflect putative membrane domains formed by DNA-membrane interactions and have functional implications in cell division.

Targeting and modification of prokaryotic cell-cell junctions by tobacco mosaic virus cell-to-cell movement protein

The movement protein (MP) of tobacco mosaic virus (TMV) facilitates the cell-to-cell spread of infection by altering the structure and function of plasmodesmata, the intercellular communication channels in plants. Because the protein was shown to interfere with intercellular communication when expressed in the cyanobacterium Anabaena sp. strain PCC 7120, whether the ability of the protein to target and to modify intercellular communication channels in plants is conserved in this prokaryote was investigated. It was found that the MP localizes to the cell junctions and induces the formation of filamentous structures that traverse the septa. It is proposed that the protein interacts with host components that are similar between plants and Anabaena and that may be evolutionarily related. The observations in Anabaena suggest that the MP modifies plasmodesmata by forming a filamentous aggregate within the pore.

The biophysics of the gram-negative periplasmic space

When subject to an osmotic 'up-shock', water flows outward from bacterial cytoplasm of the bacterium. Lipid bilayers can shrink very little in area and therefore must wrinkle to accommodate the smaller volume. The usual consequence is that all the layers of the cell envelope must become wrinkled together because they adhere to each other and must now cover a smaller surface. Plasmolysis spaces are formed if the cytoplasmic membrane (CM) separates from the other components of the wall. However, because the CM bilayer is essentially an incompressible two-dimensional liquid, this constraint restricts the location and shape of plasmolysis spaces. With mild up-shocks they form at the pole and around constricting regions in the cell. Elsewhere their creation requires the formation of endocytotic or exocytotic vesicles. The formation of endocytotic vesicles occurs in animal and plant cells as well as in bacterial cells. With stronger up-shocks tubular structures (Bayer adhesion sites), or other special geometric shapes (e.g., Scheie structures) allow the bilayer to surround an irregular shaped cytoplast. Periosmotic agents, that is, those that extract water from the periplasm as well as the cytoplasm, are molecules such as poly-vinyl-pyrrolidone and alpha-cyclodextrin that are too large to pass through the porins in the outer membrane. They were found to significantly inhibit the formation of plasmolysis spaces. Presumably, they inhibit the plasmolysis process, which requires that extracellular fluid enter between the CM and the outer membrane (OM). In the extreme case, with the dehydrating action of both osmotic agents and periosmotic agents, periplasmic space formation tends to be prevented and a new kind of space develops within the cytoplasm. We have designated these as 'cytoplasmic voids'. These novel structures are not bounded by lipid bilayers, in contrast to the endocytotic vesicles. These new spaces appear to result from the negative turgor pressure generated by the application of the combination of osmotic and periosmotic agents causing bubble formation. Several ideas in the literature about the wall biology (periseptal annuli, leading edge, osmotic pressure in the periplasm) are presented and critiqued. The basic criticism of these is that much of the phenomena can be explained because of the physics of the phospholipid bilayers and osmotic forces and thus does not imply the existence of a special control mechanism to regulate growth and division.

Proper placement of the Escherichia coli division site requires two functions that are associated with different domains of the MinE protein

The proper placement of the Escherichia coli division septum requires the MinE protein. MinE accomplishes this by imparting topological specificity to a division inhibitor coded by the minC and minD genes. As a result, the division inhibitor prevents septation at potential division sites that exist at the cell poles but permits septation at the normal division site at midcell. In this paper, we define two functions of MinE that are required for this effect and present evidence that different domains within the 88-amino acid MinE protein are responsible for each of these two functions. The first domain, responsible for the ability of MinE to counteract the activity of the MinCD division inhibitor, is located in a small region near the N terminus of the protein. The second domain, required for the topological specificity of MinE function, is located in the more distal region of the protein and affects the site specificity of placement of the division septum even when separated from the domain responsible for suppression of the activity of the division inhibitor.

Development of the cell-division site in FtsA- filaments

Early changes at cell-division sites were studied in non-septate filaments induced by growth of ftsATs mutant cells under non-permissive conditions. The positions of localized regions of plasmolysis were used as markers for the locations of partial and complete annular structures that are thought to be precursors of the periseptal annuli that flank the septum during cytokinesis. The results confirmed that these structures were localized at potential division sites and suggested a model in which older division sites play a role in the generation of new sites for future use, with each older site being used only once for this purpose. The results also suggest that the details of division-site development can profitably be studied in cells in which early events in the differentiation process are uncoupled from the septation event.

Early stages in development of the Escherichia coli cell-division site

Development of the Escherichia coli cell division site was studied in wild-type cells and in non-septate filaments of ftsZnull and ftsZTs mutant cells. Localized regions of plasmolysis were used as markers for the positions of annular structures that are thought to be related to the periseptal annuli that flank the ingrowing septum during cytokinesis. The results show that these structures are localized at potential division sites in non-septate filaments of FtsZ- cells, contrary to previous reports. The positions of the structures along the long axis of the cells in both wild-type cells and FtsZ- filaments were unaffected by the presence of plasmolysis bays at the cell poles. These results do not agree with a previous suggestion that the apparent association of plasmolysis bays with future division sites was artefactual. They support the view that division sites begin to differentiate before the initiation of septal ingrowth and that plasmolysis bays and the annular attachments that define them, mark the locations of these early events in the division process.

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.

Characterization of translucent segments observed in an smbA mutant of Escherichia coli

The smbA gene of Escherichia coli is essential for cell proliferation. The smbA2 mutant shows cold-sensitive colony formation at 22 degrees C. A novel morphological phenotype, formation of a translucent segment at midcell or at a cell pole, was observed by phase-contrast microscopy at a high frequency in the smbA2 mutant cells incubated in L medium lacking NaCl at 22 degrees C, but not observed in L medium containing 1% NaCl or 20% sucrose at the same temperature. No translucent segment was observed in the wild-type cells in any of the media used. Electron microscopic observation revealed that the translucent segments resulted from the enlargement of a periplasmic space by separation of the inner membrane from the peptidoglycan layer and the outer membrane.

The membrane binding C-terminus of protein A from Staphylococcus aureus affects its cellular localization and causes structural deformation when expressed in Escherichia coli

Protein A from Staphylococcus aureus is a powerful diagnostic reagent and has several uses in human disease therapy. Expression in non-pathogenic Escherichia coli containing recombinant plasmids coding for this protein has increased its availability, but can reduce the stability of the plasmid-bearing host. By employing immune electron microscopy, we have determined that E. coli containing stable plasmids coding for a truncated version of protein A, without the membrane binding site, secrete this protein through the cytoplasmic membrane and into the periplasmic space, where it accumulates. E. coli containing unstable plasmids, however, which code for the complete protein including the membrane-binding site, target the protein into the cytoplasmic membrane. This accumulation of protein A in the E. coli cytoplasmic membrane inhibits the formation of septa between dividing cells and results in aberrant elongated, multi-chromosomal forms.

Plasmolysis bays in Escherichia coli: are they related to development and positioning of division sites?

Plasmolysis bays, induced in Escherichia coli by hypertonic treatment, are flanked by zones of adhesion between the plasma membrane and the cell wall. To test the proposition of Cook et al. (W. R. Cook, F. Joseleau-Petit, T. J. MacAlister, and L. I. Rothfield, Proc. Natl. Acad. Sci. USA 84:7144-7148, 1987) that these zones, called periseptal annuli, play a role in determining the division site, we analyzed the positions of these zones by phase-contrast and electron microscopy. In situ treatment of cells grown in agar showed that the youngest cell pole was the most susceptible to plasmolysis, whereas the constriction site was resistant. Lateral bays occurred only at some distance from a polar bay or a resistant constriction site. Orienting cells with their most prominently plasmolyzed polar bay in one direction showed that the lateral bays were always displaced away from the polar bay at about half the distance to the other cell pole. If no poles were plasmolyzed, lateral bays occurred either in the centers of nonconstricting cells or at the 1/4 or 3/4 position of cell length in constricting cells. The asymmetric positions of lateral plasmolysis bays, caused by their abrupt displacement in the presence of polar bays or constriction sites, does not confirm the periseptal annulus model (Cook et al.), which predicts a gradual and symmetric change in the position of lateral bays with increasing cell length. Our analysis indicates that plasmolysis bays have no relation to the development and positioning of the future division site.

Interactions of Escherichia coli membrane lipoproteins with the murein sacculus

Bifunctional cross-linking reagents were used to identify cell envelope proteins that interacted with the murein sacculus. This revealed that a number of [3H]leucine-labeled proteins and [3H]palmitate-labeled lipoproteins were reproducibly cross-linked to the sacculus in plasmolyzed cells. The results suggested that most of the cell envelope lipoproteins, and not only the murein lipoprotein, mediate interactions between the murein sacculus and the inner and/or outer membrane of the cell.

Polar localization of a bacterial chemoreceptor

The bacterial chemotaxis signal transducer MCP is an integral membrane receptor protein. The chemoreceptor is localized at the flagellum-bearing pole of Caulobacter crescentus swarmer cells. Amino-terminal sequences of the MCP target the protein to the membrane while the carboxy-terminal portion of the protein is responsible for polar localization. The C. crescentus and Escherichia coli MCPs have highly conserved carboxy-terminal domains, and when an E. coli MCP is expressed in C. crescentus, it is targeted to the swarmer cell progeny. These results suggest that subcellular localization of a prokaryotic protein involves interaction of specific regions of the protein with unique cell sites that contain either localized binding proteins or a specific secretory apparatus.

Dynamics of PhiX174 protein E-mediated lysis of Escherichia coli

Expression of cloned gene E of bacteriophage PhiX174 induces lysis by formation of a transmembrane tunnel structure in the cell envelope of Escherichia coli. Ultrastructural studies of the location of the lysis tunnel indicate that it is preferentially located at the septum or at polar regions of the cell. Furthermore, the diameter and shape of individual tunnel structures vary greatly indicating that its structure is not rigid. Apparently, the contours of individual lysis tunnels are determined by enlarged meshes in the peptidoglycan net and the force produced at its orifice, by the outflow of cytoplasmic content. Once the tunnel is formed the driving force for the lysis process is the osmotic pressure difference between cytoplasm and medium. During the lysis process areas of the cytoplasmic membrane which are not tightly attached to the envelope are extended inward by the negative pressure produced during lysis. After cell lysis external medium can diffuse through the lysis tunnel filling the inner cell space of the still rigid bacterial ghosts.

Introduction of proteins into living bacterial cells: distribution of labeled HU protein in Escherichia coli

Growing bacterial cells forming division septa have sites near the septa that are sensitive to EDTA shock. Cells treated with EDTA incorporate proteins and other molecules from the surrounding medium, probably via vesiclelike lesions at the septa that are induced by EDTA. The amount of protein taken up is proportional to the protein concentration in the permeabilization medium. Incorporated molecules equilibrate throughout the cytoplasm, and those with affinity for DNA bind to the nucleoid. Conditions that promote the viability of permeabilized cells and help to avoid otherwise irreversible effects of EDTA are defined. Procedures for selecting cells that have incorporated protein and for studying the distribution of the protein and its effects in growing-dividing cells are described. The procedure may have several applications to molecular and cellular biology; however, we describe here the localization in living cells of the histonelike protein HU. Fluorescence microscopy of cells containing different amounts of fluorescein-labeled HU (varied from approximately 10(3) to 10(5) molecules per cell) showed that the HU concentrates in the nucleoid and is uniformly distributed throughout this structure. Control experiments demonstrated that unlabeled interior parts of the nucleoid can be resolved when labeled proteins that do not bind DNA or enter the nucleoid are introduced into living cells. It was concluded that in vivo added HU binds primarily DNA and that there are no intrinsic restrictions on major regions of the nucleoid to which the added HU protein may bind.

Biogenesis of cell division sites in ftsA and ftsZ filaments

The development of nascent cell division sites was studied in Escherichia coli strains containing ftsAts and ftsZts mutations in which septal development is arrested after shift to the restrictive temperature. Division sites were studied by measuring positions of plasmolysis bays, a visible marker for periseptal annuli. Annuli are circumferential zones of membrane adhesion which represent the earliest known structural differentiation at developing septal sites. Two patterns of annulus development were observed in the mutant strains. Normal numbers of new annuli were generated in filaments of both mutants, but in ftsZ filaments annuli failed to mature and to become properly localized, suggesting that ftsZ gene product is first required for maturation of annuli. In contrast, mature annuli accumulated at division sites in ftsA filaments, suggesting that the ftsA gene product is required at a stage after maturation and localization of nascent annuli.

Expression of positional information during cell differentiation of Caulobacter

The asymmetric targeting of proteins to the Caulobacter predivisional cell poles yields dissimilar progeny. We show that the products of transcriptional reporter gene fusions to a flagellin gene and to the flagellar hook operon are segregated to the progeny swarmer cell. This segregation does not depend on sequences within the mRNA, but on the upstream regulatory region. The subset of developmentally regulated flagellar genes that exhibit mRNA segregation has the same upstream cis-acting elements: an activator-binding site known as the ftr sequence and an IHF-binding site. We propose that these genes are preferentially transcribed from the chromosome in the incipient swarmer cell pole of the predivisional cell.

To shape a cell: an inquiry into the causes of morphogenesis of microorganisms

We recognize organisms first and foremost by their forms, but how they grow and shape themselves still largely passes understanding. The objective of this article is to survey what has been learned of morphogenesis of walled eucaryotic microorganisms as a set of problems in cellular heredity, biochemistry, physiology, and organization. Despite the diversity of microbial forms and habits, some common principles can be discerned. (i) That the form of each organism represents the expression of a genetic program is almost universally taken for granted. However, reflection on the findings with morphologically aberrant mutants suggests that the metaphor of a genetic program is misleading. Cellular form is generated by a web of interacting chemical and physical processes, whose every strand is woven of multiple gene products. The relationship between genes and form is indirect and cumulative; therefore, morphogenesis must be addressed as a problem not of molecular genetics but of cellular physiology. (ii) The shape of walled cells is determined by the manner in which the wall is laid down during growth and development. Turgor pressure commonly, perhaps always, supplies the driving force for surface enlargement. Cells yield to this scalar force by localized, controlled wall synthesis; their forms represent variations on the theme of local compliance with global force. (iii) Growth and division in bacteria display most immediately the interplay of hydrostatic pressure, localized wall synthesis, and structural constraints. Koch's surface stress theory provides a comprehensive and quantitative framework for understanding bacterial shapes. (iv) In the larger and more versatile eucaryotic cells, expansion is mediated by the secretion of vesicles. Secretion and ancillary processes, such as cytoplasmic transport, are spatially organized on the micrometer scale. The diversity of vectorial physiology and of the forms it generates is illustrated by examples: apical growth of fungal hyphae, bud formation in yeasts, germination of fucoid zygotes, and development of cells of Nitella, Closterium, and other unicellular algae. (v) Unicellular organisms, no less than embryos, have a remarkable capacity to impose spatial order upon themselves with or without the help of directional cues. Self-organization is reviewed here from two perspectives: the theoretical exploration of morphogens, gradients, and fields, and experimental study of polarization in Fucus cells, extension of hyphal tips, and pattern formation in ciliates. Here is the heart of the matter, yet self-organization remains nearly as mysterious as it was a century ago, a subject in search of a paradigm.

Biosynthesis of a membrane adhesion zone fraction throughout the cell cycle of Escherichia coli

Synchronized cells of Escherichia coli were pulse-labeled with [3H]leucine and subjected to membrane fractionation to determine whether a fraction that is enriched for membrane-murein adhesion zones (fraction OML) was preferentially generated at specific times during the cell cycle, as previously suggested from studies of lkyD and cha mutants. Contrary to this prediction, the experiments showed that OML was formed continuously during the division cycle.

Transfer of preformed terminal C5b-9 complement complexes into the outer membrane of viable gram-negative bacteria: effect on viability and integrity

An efficient fusion system between Gram-negative bacteria and liposomes incorporating detergent-extracted C5b-9 complexes has been developed that allows delivery of preformed terminal complexes to the cell envelope (Tomlinson et al., 1989b). Fusion of Salmonella minnesota Re595 and Escherichia coli 17 with C5b-9-incorporated liposomes resulted in the transfer of 1900 C5b-9 complexes to each target bacterial cell. No loss in viability of bacteria was observed following fusion, even though the deposotion of 900 complexes onto the envelope following exposure to lysozyme-free serum effected a greater than 99% loss of viability. Increased sensitivity to antibiotics normally excluded from the cell by an integral outer membrane (OM), as well as the ability of the chromogenic substrate PADAC to gain access to periplasmically located beta-lactamase, indicated that transferred C5b-9 complexes functioned as water-filled channels through the OM. A similar conclusion was drawn from measurements demonstrating the uptake by cells of the lipophilic cation tetraphenylphosphonium (bromide), a result further indicating that the membrane potential across the cytoplasmic membrane was maintained following C5b-9 transfer to the OM. Examination of S. minnesota Re595 by electron microscopy revealed no obvious difference between cells exposed to lethal concentrations of lysozyme-free serum and cells following fusion with C5b-9-incorporated liposomes. These data suggest either that there are critical sites in the OM to which liposome-delivered C5b-9 complexes are unable to gain access or that bacterial cell death is related to events occurring during polymerization of C9 on the cell surface.

A division inhibitor and a topological specificity factor coded for by the minicell locus determine proper placement of the division septum in E. coli

The E. coli minicell locus (minB) was shown to code for three gene products (MinC, MinD, and MinE) whose coordinate action is required for proper placement of the division spetum. Studies of the phenotypic effects of expression of the three genes, alone and in all possible combinations, indicated the following: cell poles contain potential division sites that will support additional septation events unless specifically inactivated; the minC and minD gene products act in concert to form a nonspecific inhibitor of septation that is capable of blocking cell division at all potential division sites; and the minE gene codes for a topological specificity factor that, in wild-type cells, prevents the division inhibitor from acting at internal division sites while permitting it to block septation at polar sites.

Isolation and properties of minB, a complex genetic locus involved in correct placement of the division site in Escherichia coli

Mutation of Escherichia coli minicell locus (minB) results in aberrant placement of the division septum. In this paper we report the isolation and characterization of the minB locus. Replacement of the chromosomal minB+ allele by cloned minB sequences containing transposon insertions resulted in the minicell phenotype, indicating that minB+ function is required to maintain the normal division pattern. Paradoxically, overexpression of the locus also resulted in the minicell phenotype. The locus codes for several peptides whose expression is coordinately affected by transposon mutations that also eliminate minB+ function. A subset of the minB peptides is sufficient to prevent minicell formation in minB1 mutants or to induce minicell formation when overproduced in wild-type strains, implicating these peptides in the normal process of localization of the division site. The results indicate that minB is a complex locus whose expression must be maintained within certain limits to maintain the normal pattern of localization of the division septum.

Proposed mechanism for generation and localization of new cell division sites during the division cycle of Escherichia coli

The earliest detectable event at future sites of cell division in Escherichia coli is the appearance of paired periseptal annuli that flank the site of formation of the division septum. The development and localization of these structures were followed as the cell progressed through the division cycle. The data suggest that (i) new periseptal annuli are generated from annuli already in position at the midpoint of the newborn cell; (ii) the nascent annuli are then displaced laterally during cell elongation to positions at 1/4 and 3/4 cell length; and (iii) the annuli at 1/4 and 3/4 cell length are retained during division, becoming the midpoint annuli of the newborn cells at the sites of the forthcoming division septum. The results indicate that the sites of future divisions can be identified and committed to the division process prior to the division cycle in which these sites are utilized for septum formation, and they suggest a model in which preexisting sites of cell division generate future division sites by a replication/displacement mechanism.

Membrane-murein attachment at the leading edge of the division septum: a second membrane-murein structure associated with morphogenesis of the gram-negative bacterial division septum

Electron microscopy of plasmolyzed cells of Salmonella typhimurium revealed a continuous zone of membrane-murein attachment at the leading edge of the division septum at all stages of septal invagination. The membrane-murein attachment site had a characteristic ultrastructural appearance and remained as a bacterial birth scar at the new pole of each of the two daughter cells after cell separation. The continuous zone of membrane-murein attachment at the leading septal edge represents the second organelle based on a topologically ordered domain of membrane-murein adhesion to be described at the site of cell division.