Freeze-substitution of gram-negative eubacteria: general cell morphology and envelope profiles

Inactivation of swmA results in the loss of an outer cell layer in a swimming synechococcus strain

The mechanism of nonflagellar swimming of marine unicellular cyanobacteria remains poorly understood. SwmA is an abundant cell surface-associated 130-kDa glycoprotein that is required for the generation of thrust in Synechococcus sp. strain WH8102. Ultrastructural comparisons of wild-type cells to a mutant strain in which the gene encoding SwmA has been insertionally inactivated reveal that the mutant lacks a layer external to the outer membrane. Cryofixation and freeze-substitution are required for the preservation of this external layer. Freeze fracturing and etching reveal that this additional layer is an S-layer. How the S-layer might function in motility remains elusive; however, this work describes an ultrastructural component required for this unique type of swimming. In addition, the work presented here describes the envelope structure of a model swimming cyanobacterium.

Structure and function of TolC: the bacterial exit duct for proteins and drugs

The bacterial TolC protein plays a common role in the expulsion of diverse molecules, which include protein toxins and antibacterial drugs, from the cell. TolC is a trimeric 12-stranded alpha/beta barrel, comprising an alpha-helical trans-periplasmic tunnel embedded in the outer membrane by a contiguous beta-barrel channel. This structure establishes a 140 A long single pore fundamentally different to other membrane proteins and presents an exit duct to substrates, large and small, engaged at specific inner membrane translocases. TolC is open to the outside medium but is closed at its periplasmic entrance. When TolC is recruited by a substrate-laden translocase, the entrance is opened to allow substrate passage through a contiguous machinery spanning the entire cell envelope, from the cytosol to the external environment. Transition to the transient open state is achieved by an iris-like mechanism in which entrance alpha-helices undergo an untwisting realignment, thought to be stabilized by interaction with periplasmic helices of the translocase. TolC family proteins are ubiquitous among gram-negative bacteria, and the conserved entrance aperture presents a possible cheomotherapeutic target in multidrug-resistant pathogens.

Role of Cell Surface Lipopolysaccharides in Escherichia coli K12 adhesion and transport

The influence of bacterial surface lipopolysaccharides (LPS) on cell transport and adhesion has been examined by use of three mutants of Escherichia coli K12 with well-characterized LPS of different lengths and molecular composition. Two experimental techniques, a packed-bed column and a radial stagnation point flow system, were employed to investigate bacterial adhesion kinetics onto quartz surfaces over a wide range of solution ionic strengths. Although the two systems capture distinct deposition (adhesion) mechanisms because of their different hydrodynamics, similar deposition kinetics trends were observed for each bacterial strain. Bacterial deposition rates were directly related to the electrostatic double layer interaction between the bacteria and quartz surfaces, in qualitative agreement with classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. However, DLVO theory does not fully explain the deposition behavior for the bacterial strain with the lengthy, uncharged O-antigen portion of the LPS. Neither the length nor the charge characteristics of the LPS molecule directly correlated to deposition kinetics, suggesting a complex combination of cell surface charge heterogeneity and LPS composition controls the bacterial adhesive characteristics. It is further suggested that bacterial deposition behavior is determined by the combined influence of DLVO interactions, LPS-associated chemical interactions, and the hydrodynamics of the deposition system.

TolC--the bacterial exit duct for proteins and drugs

The TolC structure has unveiled a common mechanism for the movement of molecules, large and small, from the bacterial cell cytosol, across two membranes and the intervening periplasm, into the environment. Trimeric TolC is a remarkable cell exit duct that differs radically from other membrane proteins, comprising a 100-A long alpha-barrel that projects across the periplasmic space, anchored by a 40-A long beta-barrel spanning the outer membrane. The periplasmic entrance of TolC is closed until recruitment by substrate-specific translocases in the inner membrane triggers its transition to the open state, achieved by an iris-like 'untwisting' of the tunnel alpha-helices. TolC-dependent machineries present ubiquitous exit routes for virulence proteins and antibacterial drugs, and their conserved structure, specifically the electronegative TolC entrance constriction, may present a target for inhibitors of multidrug-resistant pathogens.

Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 A into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.

Degradation process of Mycobacterium leprae cells in infected tissue examined by the freeze-substitution method in electron microscopy

Mycobacterium leprae cells (strain Thai-53) harvested from infected mouse foot pads were examined by electron microscopy using the freeze-substitution technique. The population of M. leprae cells from the infected tissue consisted of a large number of degraded cells and a few normal cells. These thin sectioned cell profiles could be categorized into four groups depending on the alteration of the membrane structures, and the degradation process is considered to occur in stages, namely from stages 1 to 3. These are the normal cells with an asymmetrical membrane, a seemingly normal cell but with a symmetrical membrane (stage 1), a cell possessing contracted and highly concentrated cytoplasm with a membrane (stage 2), and a cell that has lost its membrane (stage 3). The peptidoglycan layer was found to remain intact in these cell groups.

Cryo-transmission electron microscopy of frozen-hydrated sections of Escherichia coli and Pseudomonas aeruginosa

High-pressure freezing of Escherichia coli K-12 and Pseudomonas aeruginosa PAO1 in the presence of cryoprotectants provided consistent vitrification of cells so that frozen-hydrated sections could be cut, providing approximately 2-nm resolution of structure. The size and shape of the bacteria, as well as their surface and cytoplasmic constituents, were nicely preserved and compared well with other published high-resolution techniques. Cells possessed a rich cytoplasm containing a diffuse dispersion of ribosomes and genetic material. Close examination of cells revealed that the periplasmic space was compressed during cryosectioning, a finding which provided supporting evidence that this space is filled by a compressible gel. Since the outer membrane and peptidoglycan layer are bonded together via lipoproteins, the space between them (although still part of the periplasmic space) was not as compacted. Even when this cryosectioning compression was taken into account, there was still substantial variability in the width of the periplasmic space. It is possible that the protoplast has some capacity to float freely within the periplasm.

pH-induced conformational changes of AcrA, the membrane fusion protein of Escherichia coli multidrug efflux system

The multidrug efflux system AcrA-AcrB-TolC of Escherichia coli expels a wide range of drugs directly into the external medium from the bacterial cell. The mechanism of the efflux process is not fully understood. Of an elongated shape, AcrA is thought to span the periplasmic space coordinating the concerted operation of the inner and outer membrane proteins AcrB and TolC. In this study, we used site-directed spin labeling (SDSL) EPR (electron paramagnetic resonance) spectroscopy to investigate the molecular conformations of AcrA in solution. Ten AcrA mutants, each with an alanine to cysteine substitution, were engineered, purified, and labeled with a nitroxide spin label. EPR analysis of spin-labeled AcrA variants indicates that the side chain mobilities are consistent with the predicted secondary structure of AcrA. We further demonstrated that acidic pH induces oligomerization and conformational change of AcrA, and that the structural changes are reversible. These results suggest that the mechanism of action of AcrA in drug efflux is similar to the viral membrane fusion proteins, and that AcrA actively mediates the efflux of substrates.

Identification of oligomerization and drug-binding domains of the membrane fusion protein EmrA

Many pathogenic Gram-negative bacteria possess tripartite transporters that catalyze drug extrusion across the inner and outer membranes, thereby conferring resistance. These transporters consist of inner (IMP) and outer (OMP) membrane proteins, which are coupled by a periplasmic membrane fusion (MFP) protein. However, it is not know whether the MFP translocates the drug between the membranes, by acting as a channel, or whether it brings the IMP and OMP together, facilitating drug transfer. The MFP EmrA has an elongated periplasmic domain, which binds transported drugs, and is anchored to the inner membrane by a single alpha-helix, which contains a leucine zipper dimerization domain. Consistent with CD and hydrodynamic analyses, the periplasmic domain is predicted to be composed of a beta-sheet subdomain and an alpha-helical coiled-coil. We propose that EmrA forms a trimer in which the coiled-coils radiate across the periplasm, where they could sequester the OMP TolC. The "free" leucine zipper in the EmrA trimer might stabilize the interaction with the IMP EmrB, which also possesses leucine zipper motifs in the putative N- and C-terminal helices. The beta-sheet subdomain of EmrA would sit at the membrane surface adjacent to the EmrB, from which it receives the transported drug, inducing a conformational change that triggers the interaction with the OMP.

Characterization of the lipopolysaccharides and capsules of Shewanella spp

Electron microscopy, sodium dodecyl sulfate-polyacrylamide gel electrophoresis with silver staining and (1)H, (13)C, and (31)P-nuclear magnetic resonance (NMR) were used to detect and characterize the lipopolysaccharides (LPSs) of several Shewanella species. Many expressed only rough LPS; however, approximately one-half produced smooth LPS (and/or capsular polysaccharides). Some LPSs were affected by growth temperature with increased chain length observed below 25 degrees C. Maximum LPS heterogeneity was found at 15 to 20 degrees C. Thin sections of freeze-substituted cells revealed that Shewanella oneidensis, S. algae, S. frigidimarina, and Shewanella sp. strain MR-4 possessed either O-side chains or capsular fringes ranging from 20 to 130 nm in thickness depending on the species. NMR detected unusual sugars in S. putrefaciens CN32 and S. algae BrY(DL). It is possible that the ability of Shewanella to adhere to solid mineral phases (such as iron oxides) could be affected by the composition and length of surface polysaccharide polymers. These same polymers in S. algae may also contribute to this opportunistic pathogen's ability to promote infection.

Structure and carbohydrate analysis of the exopolysaccharide capsule of Pseudomonas putida G7

The aromatic hydrocarbon-degrading bacterium, Pseudomonas putida G7, produces exopolymers of potential interest in biotechnological applications. These exopolymers have been shown to have significant metal-binding ability. To initiate the study of the metal-polymer interactions, we explored the physical and chemical nature of the P. putida G7 exopolysaccharide, a major component of the exopolymer. A capsular structure was observed by light microscopy surrounding both planktonic and attached cells in biofilms after immunofluorescence staining with polyclonal antiserum raised against planktonic cells. Further work with planktonic cells showed that the immunostained capsule remained associated with young (log phase) cells, whereas older (stationary phase) cells lost their capsular material to the external milieu. Visualization of frozen, hydrated stationary phase cells by cryo-field emission scanning electron microscopy (cryoFESEM) revealed highly preserved extracellular material. In contrast, conventional scanning electron microscopy (SEM) of stationary phase cells showed rope-like material that most probably results from dehydrated and collapsed exopolymer. Both capsular and released exopolymers were separated from cells, and the released extracellular polysaccharide (EPS) was purified. Deoxycholate-polyacrylamide gel electrophoresis (PAGE) and silver/alcian blue staining of the partially purified material showed that it contained both EPS and lipopolysaccharide (LPS). Further purification of the EPS using a differential solubilization technique to remove LPS yielded highly purified EPS. Gas chromatography-mass spectrometry revealed that the purified EPS contained the monosaccharides, glucose, rhamnose, ribose, N-acetylgalactosamine and glucuronic acid. The structural and chemical properties of the P. putida EPS described here increase our understanding of the mechanisms of toxic metal binding by this well-known Proteobacterium.

The role of the TolC family in protein transport and multidrug efflux. From stereochemical certainty to mechanistic hypothesis

Gram-negative bacteria are enveloped by a system of two membranes, and they use specialized multicomponent, energy-driven pumps to transport molecules directly across this double-layered partition from the cell interior to the extra-cellular environment. One component of these pumps is embedded in the outer-membrane, and the paradigm for its structure and function is the TolC protein from Escherichia coli. A common component of a wide variety of efflux pumps, TolC and its homologues are involved in the export of chemically diverse molecules ranging from large protein toxins, such as alpha-hemolysin, to small toxic compounds, such as antibiotics. TolC family members thus play important roles in conferring pathogenic bacteria with both virulence and multidrug resistance. These pumps assemble reversibly in a transient process that brings together TolC or its homologue, an inner-membrane-associated periplasmic component, an integral inner-membrane translocase and the substrate itself. TolC can associate in this fashion with a variety of different partners to participate in the transport of diverse substrates. We review here the structure and function of TolC and the other components of the efflux/transport pump.

Characterization of in vitro interactions between a truncated TonB protein from Escherichia coli and the outer membrane receptors FhuA and FepA

High-affinity iron uptake in gram-negative bacteria depends upon TonB, a protein which couples the proton motive force in the cytoplasmic membrane to iron chelate receptors in the outer membrane. To advance studies on TonB structure and function, we expressed a recombinant form of Escherichia coli TonB lacking the N-terminal cytoplasmic membrane anchor. This protein (H(6)-'TonB; M(r), 24,880) was isolated in a soluble fraction of lysed cells and was purified by virtue of a hexahistidine tag located at its N terminus. Sedimentation experiments indicated that the H(6)-'TonB preparation was almost monodisperse and the protein was essentially monomeric. The value found for the Stokes radius (3.8 nm) is in good agreement with the value calculated by size exclusion chromatography. The frictional ratio (2.0) suggested that H(6)-'TonB adopts a highly asymmetrical form with an axial ratio of 15. H(6)-'TonB captured both the ferrichrome-iron receptor FhuA and the ferric enterobactin receptor FepA from detergent-solubilized outer membranes in vitro. Capture was enhanced by preincubation of the receptors with their cognate ligands. Cross-linking assays with the purified proteins in vitro demonstrated that there was preferential interaction between TonB and ligand-loaded FhuA. Purified H(6)-'TonB was found to be stable and thus shows promise for high-resolution structural studies.

Chunnel vision. Export and efflux through bacterial channel-tunnels

The Escherichia coli TolC protein is central to toxin export and drug efflux across the inner and outer cell membranes and the intervening periplasmic space. The crystal structure has revealed that TolC assembles into a remarkable alpha-helical trans-periplasmic cylinder (tunnel) embedded in the outer membrane by a contiguous beta-barrel (channel), so providing a large duct open to the outside environment. The channel-tunnel structure is conserved in TolC homologues throughout Gram-negative bacteria, and it is envisaged that they are recruited and opened, through a common mechanism, by substrate-specific inner-membrane complexes.

Conjugative junctions in RP4-mediated mating of Escherichia coli

The physical association of bacteria during conjugation mediated by the IncPalpha plasmid RP4 was investigated. Escherichia coli mating aggregates prepared on semisolid medium were ultrarapidly frozen using copper block freezing, followed by freeze substitution, thin sectioning, and transmission electron microscopy. In matings where the donor bacteria contained conjugative plasmids, distinctive junctions were observed between the outer membranes of the aggregates of mating cells. An electron-dense layer linked the stiffly parallel outer membranes in the junction zone, but there were no cytoplasmic bridges nor apparent breaks in the cell walls or membranes. In control experiments where the donors lacked conjugative plasmids, junctions were not observed. Previous studies have shown that plasmid RP4 carries operons for both plasmid DNA processing (Tra1) and mating pair formation (Tra2). In matings where donor strains carried Tra2 only or Tra2 plus the pilin-processing protease TraF, junctions were found but they were shorter and more interrupted than the wild type. If the donor strain had the pilin gene knocked out (trbC), junctions were still found. Thus, it appears that the electron-dense layer between the outer membranes of the conjugating cells is not composed of pilin.

Characterization of a major envelope protein from the rumen anaerobe Selenomonas ruminantium OB268

Cell envelopes from the Gram-negative staining but phylogenetically Gram-positive rumen anaerobe Selenomonas ruminantium OB268 contained a major 42 kDa heat modifiable protein. A similarly sized protein was present in the envelopes of Selenomonas ruminantium D1 and Selenomonas infelix. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of Triton X-100 extracted cell envelopes from S. ruminantium OB268 showed that they consisted primarily of the 42 kDa protein. Polyclonal antisera produced against these envelopes cross-reacted only with the 42 kDa major envelope proteins in both S. ruminantium D1 and S. infelix, indicating a conservation of antigenic structure among each of the major envelope proteins. The N-terminus of the 42 kDa S. ruminantium OB268 envelope protein shared significant homology with the S-layer (surface) protein from Thermus thermophilus, as well as additional envelope proteins containing the cell surface binding region known as a surface layer-like homologous (SLH) domain. Thin section analysis of Triton X-100 extracted envelopes demonstrated the presence of an outer bilayer over-laying the cell wall, and a regularly ordered array was visible following freeze-fracture etching through this bilayer. These findings suggest that the regularly ordered array may be composed of the 42 kDa major envelope protein. The 42 kDa protein has similarities with regularly ordered outer membrane proteins (rOMP) reported in certain Gram-negative and ancient eubacteria.

AcrA is a highly asymmetric protein capable of spanning the periplasm

AcrA protein is a component of the multi-drug efflux complex AcrAB-TolC of Escherichia coli. Judged by the hypersusceptibility phenotype of acrA mutants, the AcrAB-TolC system pumps out an extraordinarily wide variety of antibiotics, chemotherapeutic agents, detergents and dyes. This complex traverses both the inner and outer membranes of E. coli and catalyzes efflux of the drugs directly into the medium. The coordinated operation of the inner membrane transporter AcrB and outer membrane channel TolC is thought to be mediated by AcrA. The latter is a lipoprotein located in the periplasmic space. We show here that a lipid-deficient derivative of AcrA is functionally active as demonstrated by the complementation of the hypersusceptibility phenotype of the acrA mutant. Purified non-lipidated and intact forms of AcrA were able to restore, with similar efficiency, the activity of AcrA-dependent efflux of erythromycin in Ca2+-sucrose-treated E. coli cells. Using analytical ultracentrifugation and dynamic light scattering techniques we determined hydrodynamic properties of the non-lipidated AcrA and found that AcrA exists in solution as a highly asymmetric monomeric molecule with an axial ratio of 8. This elongated shape of AcrA is compatible with the hypothesis that this protein spans the periplasmic space coordinating the concerted operation of inner and outer membrane components of the complex.

Gram-negative bacteria produce membrane vesicles which are capable of killing other bacteria

Naturally produced membrane vesicles (MVs), isolated from 15 strains of gram-negative bacteria (Citrobacter, Enterobacter, Escherichia, Klebsiella, Morganella, Proteus, Salmonella, and Shigella strains), lysed many gram-positive (including Mycobacterium) and gram-negative cultures. Peptidoglycan zymograms suggested that MVs contained peptidoglycan hydrolases, and electron microscopy revealed that the murein sacculi were digested, confirming a previous modus operandi (J. L. Kadurugamuwa and T. J. Beveridge, J. Bacteriol. 174:2767-2774, 1996). MV-sensitive bacteria possessed A1alpha, A4alpha, A1gamma, A2alpha, and A4gamma peptidoglycan chemotypes, whereas A3alpha, A3beta, A3gamma, A4beta, B1alpha, and B1beta chemotypes were not affected. Pseudomonas aeruginosa PAO1 vesicles possessed the most lytic activity.

Structural and biochemical analysis of the sheath of Phormidium uncinatum

The sheath of the filamentous, gliding cyanobacterium Phormidium uncinatum was studied by using light and electron microscopy. In thin sections and freeze fractures the sheath was found to be composed of helically arranged carbohydrate fibrils, 4 to 7 nm in diameter, which showed a substantial degree of crystallinity. As in all other examined motile cyanobacteria, the arrangement of the sheath fibrils correlates with the motion of the filaments during gliding motility; i.e., the fibrils formed a right-handed helix in clockwise-rotating species and a left-handed helix in counterclockwise-rotating species and were radially arranged in nonrotating cyanobacteria. Since sheaths could only be found in old immotile cultures, the arrangement seems to depend on the process of formation and attachment of sheath fibrils to the cell surface rather than on shear forces created by the locomotion of the filaments. As the sheath in P. uncinatum directly contacts the cell surface via the previously identified surface fibril forming glycoprotein oscillin (E. Hoiczyk and W. Baumeister, Mol. Microbiol. 26:699-708, 1997), it seems reasonable that similar surface glycoproteins act as platforms for the assembly and attachment of the sheaths in cyanobacteria. In P. uncinatum the sheath makes up approximately 21% of the total dry weight of old cultures and consists only of neutral sugars. Staining reactions and X-ray diffraction analysis suggested that the fibrillar component is a homoglucan that is very similar but not identical to cellulose which is cross-linked by the other detected monosaccharides. Both the chemical composition and the rigid highly ordered structure clearly distinguish the sheaths from the slime secreted by the filaments during gliding motility.

Interactions between biofilms and the environment

The surfaces of bacteria are highly interactive with their environment. Whether the bacterium is Gram-negative or Gram-positive, most surfaces are charged at neutral pH because of the ionization of the reactive chemical groups which stud them. Since prokaryotes have a high surface area-to-volume ratio, this can have surprising ramifications. For example, many bacteria can concentrate dilute environmental metals on their surfaces and initiate the development of fine-grained minerals. In natural environments, it is not unusual to find such bacteria closely associated with the minerals which they have helped develop. Bacteria can be free-living (planktonic), but in most natural ecosystems they prefer to grow on interfaces as biofilms; supposedly to take advantage of the nutrient concentrative effect of the interface, although there must also be gained some protective value against predators and toxic agents. Using a Pseudomonas aeruginosa model system, we have determined that lipopolysaccharide is important in the initial attachment of this Gram-negative bacterium to interfaces and that this surface moiety subtly changes during biofilm formation. Using this same model system, we have also discovered that there is a natural tendency for Gram-negative bacteria to concentrate and package periplasmic components into membrane vesicles which bleb-off the surface. Since some of these components (e.g., peptidoglycan hydrolases) can degrade other surrounding cells, the vesicles could be predatory; i.e., a natural system by which neighboring bacteria are targeted and lysed, thereby liberating additional nutrients to the microbial community. This obviously would be of benefit to vesicle-producing bacteria living in biofilms containing mixed microbial populations.

Envelope structure of four gliding filamentous cyanobacteria

The cell walls of four gliding filamentous Oscillatoriaceae species comprising three different genera were studied by freeze substitution, freeze fracturing, and negative staining. In all species, the multilayered gram-negative cell wall is covered with a complex external double layer. The first layer is a tetragonal crystalline S-layer anchored on the outer membrane. The second array is formed by parallel, helically arranged surface fibrils with diameters of 8 to 12 nm. These fibrils have a serrated appearance in cross sections. In all cases, the orientation of the surface fibrils correlates with the sense of revolution of the filaments during gliding, i.e., clockwise in both Phormidium strains and counterclockwise in Oscillatoria princeps and Lyngbya aeruginosa. The lack of longitudinal corrugations or contractions of the surface fibrils and the identical appearances of motile and nonmotile filaments suggest that this structure plays a passive screw thread role in gliding. It is hypothesized that the necessary propulsive force is generated by shear forces between the surface fibrils and the continuing flow of secreted extracellular slime. Furthermore, the so-called junctional pores seem to be the extrusion sites of the slime. In motile cells, these pores exhibit a different staining behavior than that seen in nonmotile ones. In the former, the channels of the pores are filled with electron-dense material, whereas in the latter, the channels appear comparatively empty, highly contrasting the peptidoglycan. Finally, the presence of regular surface structures in other gliding prokaryotes is considered an indication that comparable structures are general features of the cell walls of gliding microbes.

A periplasmic insulin-cleaving proteinase (ICP) from Acinetobacter calcoaceticus sharing properties with protease III from Escherichia coli and IDE from eucaryotes

A periplasmic insulin-cleaving proteinase (ICP), purified to its electrophoretic homogeneity in the SDS-PAGE from the Gram-negative bacterium Acinetobacter calcoaceticus, was examined and compared in its properties with the protease III (protease Pi, pitrilysin, EC 3.4.99.44) of Escherichia coli and the insulin-destroying proteinase (IDE, insulinase, EC 3.4.99.45) from eucaryotes. The enzyme was proven to be a metalloprotease like protease III and IDE, as was shown by the inhibitory effects exerted by EDTA and o-phenanthroline. Furthermore, dialysis against EDTA and o-phenanthroline led to a complete loss of activity, which could be restored by addition of Co2+, and, to a lesser extent, but at a lower metal ion concentration by Zn2+. Similar to protease III and IDE, ICP prefers the cleavage of small polypeptides (insulin, insulin B-chain, glucagon) to the cleavage of proteins (casein, human serum albumin, globin) and was inactive against synthetic amino acid derivates (esters, p-nitranilides, and furoylacroleyl substrates) of subtilisin, thermolysin, trypsin, and chymotrypsin. The peptide-bond-specificity of the ICP in the cleavage of the oxidized insulin B-chain was investigated and the results were compared to the specificity of protease III of E. coli, IDE, protease-24,11, and thermolysin. Cleavage sites in the oxidized insulin B-chain generated by ICP are Asn3-Gln4, His10-Leu11, Ala14-Leu15, Leu17-Val18, Gly23-Phe24, Phe24-Phe25, and Phe25-Tyr26. Principally, ICP cleaves between hydrophobic amino acids and amides. The ICP shares one of the only two cleavage sites with the protease III and four sites with the IDE.

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.

Structural differentiation of the Bacillus subtilis 168 cell wall

Exponential-growth-phase cultures of Bacillus subtilis 168 were probed with polycationized ferritin (PCF) or concanavalin A (localized by the addition of horseradish peroxidase conjugated to colloidal gold) to distinguish surface anionic sites and teichoic acid polymers, respectively. Isolated cell walls, lysozyme-digested cell walls, and cell walls treated with mild alkali to remove teichoic acid were also treated with PCF. After labelling, whole cells and walls were processed for electron microscopy by freeze-substitution. Thin sections of untreated cells showed a triphasic, fibrous wall extending more than 30 nm beyond the cytoplasmic membrane. Measurements of wall thickness indicated that the wall was thicker at locations adjacent to septa and at pole-cylinder junctions (P < 0.001). Labelling studies showed that at saturating concentrations the PCF probe labelled the outermost limit of the cell wall, completely surrounding individual cells. However, at limiting PCF concentrations, labelling was observed at only discrete cell surface locations adjacent to or overlying septa and at the junction between pole and cylinder. Labelling was rarely observed along the cell cylinder or directly over the poles. Cells did not label along the cylindrical wall until there was visible evidence of a developing septum. Identical labelling patterns were observed by using concanavalin A-horseradish peroxidase-colloidal gold. Neither probe appeared to penetrate between the fibers of the wall. We suggest that the fibrous appearance of the wall seen in freeze-substituted cells reflects turnover of the wall matrix, that the specificity of labelling to discrete sites on the cell surface is indicative of regions of extreme hydrolytic activity in which alpha-glucose residues of the wall teichoic acids and electronegative sites (contributed by phosphate and carboxyl groups of the teichoic acids and carboxyl groups of the peptidoglycan polymers) are more readily accessible to our probes, and that the wall of exponentially growing B. subtilis cells contains regions of structural differentiation.

Ultrastructure and chemical composition of the sheath of Leptothrix discophora SP-6

Light microscopy and transmission electron microscopy of thin sections and metal-shadowed specimens showed that the sheath of Leptothrix discophora SP-6 (ATCC 51168) is a tube-like extracellular polymeric structure consisting of a condensed fabric of 6.5-nm-diameter fibrils underlying a more diffuse outer capsular layer. In thin sections, outer membrane bridges seen to contact the inner sheath layer suggested that the sheath fabric was attached to the outer layer of the gram-negative cell wall. The capsular polymers showed an affinity for cationic colloidal iron and polycationic ferritin, indicating that they carry a negative charge. Cell-free sheaths were isolated by treatment with a mixture of lysozyme, EDTA, and N-lauroylsarcosine (Sarkosyl) or sodium dodecyl sulfate (SDS). Both Sarkosyl- and SDS-isolated sheaths were indistinguishable in microscopic appearance. However, the Mn-oxidizing activity of Sarkosyl-isolated sheaths was more stable than that of SDS-isolated sheaths. The Sarkosyl-isolated sheaths also contained more 2-keto-3-deoxyoctanoic acid and more outer membrane protein than SDS-isolated sheaths. The oven-dried mass of detergent-isolated sheaths represented approximately 9% of the total oven-dried biomass of SP-6 cultures; the oven-dried sheaths contained 38% C, 6.9% N, 6% H, and 2.1% S and approximately 34 to 35% carbohydrate (polysaccharide), 23 to 25% protein, 8% lipid, and 4% inorganic ash. Gas-liquid chromatography showed that the polysaccharide was an approximately 1:1 mixture of uronic acids (glucuronic, galacturonic, and mannuronic acids and at least one other unidentified uronic acid) and an amino sugar (galactosamine). Neutral sugars were not detected. Amino acid analysis showed that sheath proteins were enriched in cysteine (6 mol%). The cysteine residues in the sheath proteins probably provide sulfhydryls for disulfide bonds that play an important role in maintaining the structural integrity of the sheath (D. Emerson and W.C. Ghiorse, J. Bacteriol. 175:7819-7827, 1993).

A periplasmic intermediate in the extracellular secretion pathway of Pseudomonas aeruginosa exotoxin A

Pseudomonas aeruginosa exotoxin A is synthesized with a secretion signal peptide typical of proteins whose final destination is the periplasm. However, exotoxin A is released from the cell without a detectable periplasmic pool, suggesting that additional determinants in this protein are important for recognition by a specialized machinery of extracellular secretion. The role of the N terminus of the mature exotoxin A in this recognition was investigated. A series of exotoxin A proteins with amino acid substitutions for the glutamic acid pair at the +2 and +3 positions were constructed by mutagenesis of the exotoxin A gene. These N-terminal acidic residues of the mature exotoxin A protein were found to be important not only for efficient processing of the precursor protein but also for extracellular localization of the toxin. The mutated exotoxin A proteins, in which a glutamic acid at the +2 position was replaced by a lysine or a double substitution of lysine and glutamine for the pair of adjacent glutamic acids, accumulated in precursor forms in the mixed cytoplasmic and membrane fractions, which was not seen with the wild-type exotoxin A. The processing of the precursor form of one exotoxin A mutant, in which the glutamic acid at the +2 position was replaced with a glutamine, was not affected. Moreover, a substantial fraction of the mature forms of all three mutants of exotoxin A accumulated in the periplasm, while wild-type exotoxin A could be detected only extracellularly. The periplasmic pools of these variants of exotoxin A could therefore represent the intermediate state during extracellular secretion. The signal for extracellular localization may be located in a small region near the amino terminus of the mature protein or could consist of several regions that are brought together after the polypeptide has folded. Alternatively, the acidic residues may be important for ensuring a conformation essential for exotoxin A to traverse the outer membrane.

Ultrastructural examination of the lipopolysaccharides of Pseudomonas aeruginosa strains and their isogenic rough mutants by freeze-substitution

The majority of Pseudomonas aeruginosa strains synthesize two antigenically distinct types of lipopolysaccharide (LPS), namely, a serotype-specific B-band LPS and a common antigen A-band LPS. A-band LPS consists of uncharged poly-D-rhamnan, which does not bind uranyl ions and is difficult to stain for electron microscopy; the highly charged B-band LPS is more easily visualized. We selected two wild-type strains, PAO1 (serotype O5) and IATS O6 (serotype O6), generated isogenic mutants from them, and examined the distribution of LPS on the surface of these organisms by freeze-substitution and electron microscopy. On PAO1 cells, which express both A-band and B-band LPSs, a 31- to 36-nm-wide fringe extending perpendicularly from the outer membrane was observed. A fine fibrous material was also observed on the surface of serotype O6 (A+ B+) cells, although this material did not form a uniform layer. When the LPS-deficient mutants, strains AK1401 (A+ B-), AK 1012 (A- B-), rd7513 (A- B-), and R5 (an IATS O6-derived rough mutant; A- B-), were examined, no extraneous material was apparent above the bilayer. However, an asymmetrical staining pattern was observed on the outer leaflet of the outer membrane of each of these mutants, presumably conforming to the anionic charge distribution of the core region of the rough LPS. In all cases, expression of the LPS types was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining. When optical densitometry on electron microscopy negatives was used to analyze the outer membrane staining profiles, subtle differences in the degrees of core deficiency among rough mutants were detectable. This is the first time an electron microscopy technique has preserved the infrastructure produced in the outer membrane by its constituent macromolecules. We conclude that freeze-substitution electron microscopy is effective in the visualization of LPS morphotypes.

Reevaluation of envelope profiles and cytoplasmic ultrastructure of mycobacteria processed by conventional embedding and freeze-substitution protocols

The cell envelope architectures and cytoplasmic structures of Mycobacterium aurum CIPT 1210005, M. fortuitum, M. phlei 425, and M. thermoresistible ATCC 19527 were compared by conventional embedding and freeze-substitution methods. To ascertain the integrity of cells during each stage of the processing regimens, [1-14C]acetate was incorporated into the mycolic acids of mycobacterial walls, and the extraction of labeled mycolic acids was monitored by liquid scintillation counting. Radiolabeled mycolic acids were extracted by both processing methods; however, freeze-substitution resulted in the extraction of markedly less radiolabel. During conventional processing of cells, most of the radiolabel was extracted during the dehydration stage, whereas postsubstitution washes in acetone yielded the greatest loss of radiolabel during freeze-substitution. Conventional embedding frequently produced cells with condensed fibrous nucleoids and occasional mesosomes. Their cell walls were relatively thick (approximately 25 nm) but lacked substance. Freeze-substituted cells appeared more robust, with well-dispersed nucleoids and ribosomes. The walls of all species were much thinner than those of their conventionally processed counterparts, but these stained well, which was an indication of more wall substance; the fabric of these walls, in particular the plasma membrane, appeared highly condensed and tightly apposed to the peptidoglycan. Some species possessed a thick, irregular outer layer that was readily visualized in the absence of exogenous stabilizing agents by freeze-substitution. Since freeze-substituted mycobacteria retained a greater percentage of mycolic acids in their walls, and probably other labile wall and cytoplasmic constituents, we believe that freeze-substitution provides a more accurate image of structural organization in mycobacteria than that achieved by conventional procedures.

Capsule structure of Proteus mirabilis (ATCC 49565)

Proteus mirabilis 2573 (ATCC 49565) produces an acidic capsular polysaccharide which was shown from glycose analysis, carboxyl reduction, methylation, periodate oxidation, and the application of one dimensional and two-dimensional high-resolution nuclear magnetic resonance techniques to be a high-molecular-weight polymer of branched trisaccharide units composed of 2-acetamido-2-deoxy-D-glucose (N-acetyl-D-glucosamine), 2-acetamido-2,6-dideoxy-L-galactose (N-acetyl-L-fucosamine), and D-glucuronic acid, having the structure: [formula: see text] P. mirabilis 2573 also produces an O:6 serotype lipopolysaccharide in which the O-chain component has the same structure as the homologous capsular polysaccharide. This is the first report of a defined capsular polysaccharide in this bacterial genus.

Freeze-substitution studies of bacteria

Typically, models of bacterial structure combine biochemical data obtained from bulk analyses of cell populations with electron microscopic observation of individual cells. Recent development of a battery of cryotechniques specific for biological electron microscopy have begun to supercede routine procedures such as conventional thin sectioning. One of these cryotechniques, freeze-substitution, combines the advantages of ultrarapid freezing with standard microtomy methods. This technique is particularly well suited to the examination of bacterial structure and has yielded additional ultrastructural information consistent with biochemical data but often challenging models of cell structure obtained from conventional microscopical methods. In addition to retaining more accurately the spatial distribution of cell components, freeze-substitution has been successfully combined with immunochemical labelling techniques and has enabled identification and localization of specific molecules both within the cell and on the cell surface. In this review, I describe current ideas on bacterial ultrastructure, modified in accordance with new data obtained from recent freeze-substitution studies.

Surface layers of bacteria

Since bacteria are so small, microscopy has traditionally been used to study them as individual cells. To this end, electron microscopy has been a most powerful tool for studying bacterial surfaces; the viewing of macromolecular arrangements of some surfaces is now possible. This review compares older conventional electron-microscopic methods with new cryotechniques currently available and the results each has produced. Emphasis is not placed on the methodology but, rather, on the importance of the results in terms of our perception of the makeup and function of bacterial surfaces and their interaction with the surrounding environment.

Amount of peptidoglycan in cell walls of gram-negative bacteria

The amount of diaminopimelic acid (Dap) in the cell wall of Escherichia coli was measured in two ways. A radiochemical method first described by us in 1985 (F. B. Wientjes, E. Pas, P. E. M. Taschner, and C. L. Woldringh, J. Bacteriol. 164:331-337, 1985) is based on the steady-state incorporation of [3H]Dap during several generations. Knowing the cell concentration and the specific activity of the [3H]Dap, one can calculate the number of Dap molecules per sacculus. The second method measures the Dap content chemically in sacculi isolated from a known number of cells. With both methods, a value of 3.5 x 10(6) Dap molecules per sacculus was obtained. Combined with electron microscopic measurements of the surface area of the cells, the data indicate an average surface area per disaccharide unit of ca. 2.5 nm2. This finding suggests that the peptidoglycan is basically a monolayered structure.

Identification, isolation, and structural studies of extracellular polysaccharides produced by Caulobacter crescentus

Caulobacters are adherent prosthecate bacteria that are members of bacterial biofouling communities in many environments. Investigation of the cell surface carbohydrates produced by two strains of the freshwater Caulobacter crescentus, CB2A and CB15A, revealed a hitherto undetected extracellular polysaccharide (EPS) or capsule. Isolation and characterization of the EPS fractions showed that each strain produced a unique neutral EPS which could not be readily removed from the cell surface by washing. Monosaccharide analysis showed that the main CB2A EPS contained D-glucose, D-gulose, and D-fucose in a ratio of 3:1:1, whereas the CB15A EPS fraction contained D-galactose, D-glucose, D-mannose, and D-fucose in approximately equal amounts. Methylation analysis of the main CB2A EPS showed the presence of terminal glucose and gulose groups, 3-linked fucosyl, and two 3,4-linked glucosyl units, thus confirming the pentasaccharide repeating unit indicated by 1H nuclear magnetic resonance analysis. Similar studies of the CB15A EPS revealed a tetrasaccharide repeating unit consisting of terminal galactose, 4-linked fucosyl, 3-linked glucosyl, and 3,4-linked mannosyl residues. EPS was not detectable by thin-section electron microscopy techniques, including some methods designed to preserve or enhance capsules, nor was the EPS readily detected on the cell surface by scanning electron microscopy when conventional fixation techniques were used; however, a structure consistent with EPS was revealed when samples were prepared by cryofixation and freeze-substitution methods.