Virulence factors are released from Pseudomonas aeruginosa in association with membrane vesicles during normal growth and exposure to gentamicin: a novel mechanism of enzyme secretion

Pseudomonas aeruginosa blebs-off membrane vesicles (MVs) into culture medium during normal growth. Release of these vesicles increased approximately threefold after exposure of the organism to four times the MIC of gentamicin. Natural and gentamicin-induced membrane vesicles (n-MVs and g-MVs and g-MVs, respectively) were isolated by filtration and differential centrifugation, and several of their biological activities were characterized. Electron microscopy of both n-MVs and g-MVs revealed that they were spherical bilayer MVs with a diameter of 50 to 150 nm. Immunoelectron microscopy and Western blot (immunoblot) analysis of the vesicles demonstrated the presence of B-band lipopolysaccharide (LPS), with a slightly higher proportion of B-band LPS in g-MVs than in n-MVs. A-band LPS was occasionally detected in g-MVs but not in n-MVs. In addition to LPS, several enzymes, such as phospholipase C, protease, hemolysin, and alkaline phosphatase, which are known to contribute to the pathogenicity of Pseudomonas infections were found to be present in both vesicle types. Both types of vesicles contained DNA, with a significantly higher content in g-MVs. These vesicles could thus play an important role in genetic transformation and disease by serving as a transport vehicle for DNA and virulence factors and are presumably involved in septic shock.

Localization of penicillin-binding proteins to the splitting system of Staphylococcus aureus septa by using a mercury-penicillin V derivative

Precise localization of penicillin-binding protein (PBP)-antibiotic complexes in a methicillin-sensitive Staphylococcus aureus strain (BB255), its isogenic heterogeneous methicillin-resistant transductant (BB270), and a homogeneous methicillin-resistant strain (Col) was investigated by high-resolution electron microscopy. A mercury-penicillin V (Hg-pen V) derivative was used as a heavy metal-labeled, electron-dense probe for accurately localizing PBPs in situ in single bacterial cells during growth. The most striking feature of thin sections was the presence of an abnormally large (17 to 24 nm in width) splitting system within the thick cross walls or septa of Hg-pen V-treated bacteria of all strains. Untreated control cells possessed a thin, condensed splitting system, 7 to 9 nm in width. A thick splitting system was also distinguishable in unstained thin sections, thereby confirming that the electron contrast of this structure was not attributed to binding of bulky heavy metal stains usually used for electron microscopy. Biochemical analyses demonstrated that Hg-pen V bound to isolated plasma membranes as well as sodium dodecyl sulfate-treated cell walls and that two or more PBPs in each strain bound to this antibiotic. In contrast, the splitting system in penicillin V-treated bacteria was rarely visible after 30 min in the presence of antibiotic. These findings suggest that while most PBPs were associated with the plasma membrane, a proportion of PBPs were located within the fabric of the cell wall, in particular, in the splitting system. Inhibition of one or more high-M(r) PBPs by beta-lactam antibiotics modified the splitting system and cross-wall structure, therefore supporting a role for these PBPs in the synthesis and architectural design of these structures in S. aureus.

Scanning probe microscopy in microbiology

Scanning probe microscopy (SPM) is emerging as an important alternative to electron microscopy as a technique for analyzing submicron details on biological surfaces. Microbiological specimens such as viruses, bacteriophages, and ordered bacterial surface layers and membranes have played an important role in the development of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) in cellular and molecular biology. Early STM studies involving metal-coated bacteriophage T4 polyheads, Methanospirillum hungatei, and Deinococcus radiodurans HPI layer clearly demonstrated that resolution was comparable to TEM on similarly prepared specimens and only limited by metal graininess. However, except for thin films or layers, novel biological information has been difficult to obtain since imaging native surfaces of such biomaterials as proteins or nucleic acids by STM proved to be unreliable. With the development of atomic force microscopes, which allow imaging of similar native structures, SPM applications have widened to include straightforward surface structure analysis, analysis of surface elastic and inelastic properties, bonding force measurements between molecules, and micro-manipulations of such individual molecules as DNA. AFM images have progressed from relatively crude representations of specimen topography to nanometer scale representations of native hydrated surfaces. It appears from the study of microbiological specimens that direct visualization of dynamic molecular events or processes may soon become a reality.

Unusually Stable Spinae from a Freshwater Chlorobium sp

A green Chlorobium sp. with spinae, strain JSB1, was isolated from an enrichment culture previously obtained from Fayetteville Green Lake, N.Y. (J. S. Brooke, J. B. Thompson, T. J. Beveridge, and S. F. Koval, Arch. Microbiol. 157:319-322, 1992). Cells were gram-negative, nonmotile rods which contained bacteriochlorophyll c and chlorosomes. Spinae were best seen by transmission electron microscopy in thin sections of cells fixed in the presence of tannic acid. High-resolution scanning electron microscopy showed the spinae randomly distributed at the cell surface and at the junctions between cells. Spinae were physically sheared from cells and isolated from the culture supernatant by ultrafiltration. As observed by electron microscopy, spinae demonstrated unusual structural stability when exposed for 1 h at 37 deg C to chemical treatments such as hydrogen bond-breaking agents, detergents, metal-chelating agents, proteases, and organic solvents. They were stable for 1 h at 37 deg C over the pH range 2.3 to 9.9 and in 1 M HCl and 1 M NaOH. The structural integrity of the spinae was also maintained when spinae were subjected to harsher treatments of autoclaving in 2% (wt/vol) sodium dodecyl sulfate and exposure to dithiothreitol at pH 9 for 1 h at 100 deg C. Partially dissociated spinae were obtained after 5 h at 100 deg C in 1 M HCl and 1 M NaOH. In acid, the tubular spinae became amorphous structures, with no helical striations visible. In alkali, the spinae had dissociated into irregular aggregates of disks. Since both high temperature and extremes of pH were required to achieve partial dissociation of the spinae, the strength of the structure presumably comes from covalent bonding.

Stability of pressure-extruded liposomes made from archaeobacterial ether lipids

Ether lipids were obtained from a wide range of archaeobacteria grown at extremes of pH, temperature, and salt concentration. With the exception of Sulfolobus acidocaldarius, unilamellar and/or multilamellar liposomes could be prepared from emulsions of total polar lipid extracts by pressure extrusion through filters of various pore sizes. Dynamic light scattering, and electron microscopy revealed homogeneous liposome populations with sizes varying from 40 to 230 nm, depending on both the lipid source and the pore size of the filters. Leakage rates of entrapped fluorescent or radioactive compounds established that those archaeobacterial liposomes that contained tetraether lipids were the most stable to high temperatures, alkaline pH, and serum proteins. Most ether liposomes were stable to phospholipase A2, phospholipase B and pancreatic lipase. These properties of archaeobacterial liposomes make them attractive for applications in biotechnology.

Analysis of the sodium dodecyl sulfate-stable peptidoglycan autolysins of select gram-negative pathogens by using renaturing polyacrylamide gel electrophoresis

For the first time, peptidoglycan autolysins from cellular fractions derived from sonicated cultures of Pseudomonas aeruginosa PAO1, Escherichia coli W7, Klebsiella pneumoniae CWK2, and Proteus mirabilis 19 were detected and partially characterized by zymogram analysis. Purified murein sacculi from P. aeruginosa PAO1 were incorporated into a sodium dodecyl sulfate (SDS)-polyacrylamide gel at a concentration of 0.05% (wt/vol) to serve as a substrate for the separated autolysins. At least 11 autolysin bands of various intensities with M(r)s ranging between 17,000 and 122,000 were detected in each of the homogenated cultures. Some of the autolysins of the four bacteria had similar M(r)s. The zymogram analysis was used to show that a number of the autolysins from E. coli were inhibited by the heavy metals Hg2+ and Cu2+, at 1 and 10 mM, respectively, high ionic strengths, and reagents known to affect the packing of lipopolysaccharides. The activity of an autolysin with an M(r) of 65,000 was also impaired by penicillin G, whereas it was enhanced by gentamicin. A preliminary screen to determine the relationship between penicillin-binding proteins (PBPs) and autolysins was carried out by using a dual assay in which radiolabelled penicillin V bands were visualized on an autolysin zymogram. Radiolabelled bands corresponding to PBPs 3, 4, 5, and 6 from E. coli and P. aeruginosa; PBPs 3, 4, and 6 from Proteus mirabilis; and PBP 6 from K. pneumoniae degraded the murein sacculi in the gels and were presumed to have autolytic activity, although the possibility of two distinct enzymes, each with one of the activities, comigrating in the SDS-polyacrylamide gels could not be excluded. Some radiolabelled bands possessed an Mr of <34,000 and coincided with similar low-Mr autolysin bands.

Minerals Associated with Biofilms Occurring on Exposed Rock in a Granitic Underground Research Laboratory

The concept of disposal of nuclear fuel waste in crystalline rock requires the effects of microbial action to be investigated. The Underground Research Laboratory excavated in a pluton of the Canadian Shield provides a unique opportunity to study these effects. Three biofilms kept moist by seepage through fractures in granitic rock faces of the Underground Research Laboratory have been examined. The biofilms contained a variety of gram-negative and gram-positive morphotypes held together by an organic extracellular matrix. Nutrient levels in the groundwater were low, but energy-dispersive X-ray spectroscopy has shown biogeochemical immobilization of several elements in the biofilms; some of these elements were concentrated from extremely dilute environmental concentrations, and all elements were chemically complexed together to form amorphous or crystalline fine-grained minerals. These were seen by transmission electron microscopy to be both associated with the surfaces of the bacteria and scattered throughout the extracellular matrix, suggesting their de novo development through bacterial surface-mediated nucleation. The biofilm consortia are thought to concentrate elements both by passive sorption and by energy metabolism. By Mössbauer spectroscopy and X-ray diffraction, one of the biofilms showed that iron was both oxidized and precipitated as ferrihydrite or hematite aerobically and reduced and precipitated as siderite anaerobically. We believe that some Archean banded-iron formations could have been formed in a manner similar to this, as it would explain the deposition of hematite and siderite in close proximity. This biogeochemical development of minerals may also affect the transport of material in waste disposal sites.

Preservation of surface lipids and determination of ultrastructure of Mycobacterium kansasii by freeze-substitution

The cell wall architecture of a slowly growing mycobacterium, Mycobacterium kansasii, was examined by freeze-substitution following growth in vitro. Freeze-substituted bacteria were marked by the presence of an electron-translucent space (or electron-transparent zone [ETZ] described by previous workers [T. Yamamoto, M. Nishiura, N. Harada, and T. Imaeda, Int. J. Lepr. 26:111-114, 1958]) surrounding the majority of cells. At least two morphotypes of mycobacteria were revealed by freeze-substitution. In the first, a relatively thin (11 +/- 2.3 to 3.5 +/- 3.1 nm), uniform ETZ surrounded intact cells which contained cytoplasm filled with well-stained ribosomes and a DNA nucleoid distributed throughout the cell. The second morphotype consisted of a small proportion of organisms that were distorted in shape and were surrounded by a much thicker (59 +/- 2.6 to 198 +/- 2.5 nm) ETZ in areas of the cell which appeared to have retracted from the space it had originally occupied, leaving depressions in the ETZ. The lipid nature of the ETZ was demonstrated because cells were devoid of an ETZ when organisms were freeze-substituted in the absence of osmium tetroxide in the substitution medium or treated with neutral lipid solvents (acetone or ethanol) before freeze-substitution. Moreover, thin-layer chromatography of acetone or ethanol extracts obtained from solvent-treated cells identified a lipid component which corresponded to the M. kansasii-specific phenolic glycolipid. In contrast, negligible amounts of glycolipids were detected in extracts obtained from control HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) buffer-treated cells, and these cells retained an ETZ. These results demonstrate that species-specific phenolic glycolipids are essential components in the architecture of the M. kansasii ETZ. Furthermore, we show that freeze-substitution is a reliable technique for the retention and precise preservation of lipid-containing polymers in the mycobacterial cell wall.

Reduction of Cr(VI) by a Consortium of Sulfate-Reducing Bacteria (SRB III)

A consortium of bacteria with tolerance to high concentrations of Cr(VI) (up to 2,500 ppm) and other toxic heavy metals has been obtained from metal-refinishing wastewaters in Chengdu, People's Republic of China. This consortium consists of a range of gram-positive and gram-negative rods and has the capacity to reduce Cr(VI) to Cr(III) as amorphous precipitates which are associated with the bacterial surfaces. An endospore-producing, gram-positive rod and a gram-negative rod accumulate the most metallic precipitates, and, over time, 80 to 95% of Cr can be removed from concentrations ranging from 50 to 2,000 ppm (0.96 to 38.45 mM). Kinetic studies revealed a first-order constant for Cr removal of 0.1518 h -1 for an initial concentration of 1,000 ppm (19.3 mM), and the sorption isothermal data could be interpreted by the Freundlich relationship. The sorption was not entirely due to a passive interaction with reactive sites on the bacterial surfaces since gamma-irradiated, killed cells could not immobilize as much metal. When U or Zn was added with the Cr, it was also removed and could even increase the total amount of Cr immobilized. The consortium was tolerant to small amounts of oxygen in the headspace of tubes, but active growth of the bacteria was a requirement for Cr immobilization through Cr(VI) reduction, resulting in the lowering of E h . Our data suggest that the reduction was via H 2 S. This consortium has been named SRB III, and it may be useful for the bioremediation of fluid metal-refining wastes.

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.

Mineral Precipitation by Epilithic Biofilms in the Speed River, Ontario, Canada

Epilithic microbial communities, ubiquitously found in biofilms on submerged granite, limestone, and sandstone, as well as on the concrete support pillars of bridges, were examined in the Speed River, Ontario, Canada. Transmission electron microscopy showed that attached bacteria (on all substrata) were highly mineralized, ranging from Fe-rich capsular material to fine-grained (<1 μm) authigenic (primary) mineral precipitates. The authigenic grains exhibited a wide range of morphologies, from amorphous gel-like phases to crystalline structures. Energy-dispersive X-ray spectroscopy indicated that the most abundant mineral associated with epilithic bacteria was a complex (Fe, Al) silicate of variable composition. The gel-like phases were similar in composition to a chamositic clay, whereas the crystalline structures were more siliceous and had compositions between those of glauconite and kaolinite. The consistent formation of (Fe, Al) silicates by all bacterial populations, regardless of substratum lithology, implies that biomineralization was a surface process associated with the anionic nature of the cell wall. The adsorption of dissolved constituents from the aqueous environment contributed significantly to the mineral formation process. In this regard, it appears that epilithic microbial biofilms dominate the reactivity of the rock-water interface and may determine the type of minerals formed, which will ultimately become part of the riverbed sediment. Because rivers typically contain high concentrations of dissolved iron, silicon, and aluminum, these findings provide a unique insight into biogeochemical activities that are potentially widespread in natural waters.

Characterization of the Binding of Gallium, Platinum, and Uranium to Pseudomonas fluorescens by Small-Angle X-Ray Scattering and Transmission Electron Microscopy

Small-angle X-ray scattering (SAXS) was used to determine the binding of Ga, U, and Pt to Pseudomonas fluorescens in aqueous buffer. Atomic absorption spectrophotometry was used to quantify the heavy metals during bulk analysis, whereas transmission electron microscopy of whole mounts and thin sections was used to determine the locations of the cell-bound metal precipitates, as well as their sizes and physical structures. Energy-dispersive X-ray spectroscopy confirmed the compositions and identities of the precipitates and helped show that they were associated primarily with the envelope layers of the bacteria. Unlike Ga and Pt, which were located only at the cell surface, U was also found intracellularly in ∼ 10% of the cells. This cytoplasmic location ultimately killed and lysed the cells. Surface-bound Ga and U were spread over the entire cell envelope (outer membrane-peptidoglycan-plasma membrane complex), whereas Pt was associated only with the lipopolysaccharide-rich, external face of the outer membrane. SAXS confirmed these data and showed that the bacteria were metal-enshrouded particles that were 1.0 to 1.5 μm in diameter. SAXS also provided a statistically significant representation of the bound metal precipitates, which ranged in size from 10 nm to 1 μm. The correlation between the microscopic data and the scattering data was extremely good. Since SAXS is performed in an aqueous milieu, it yields a more representative picture of the physical state of the metal bound to cell surfaces.

Remobilization of Heavy Metals Retained as Oxyhydroxides or Silicates by Bacillus subtilis Cells

The stability of heavy metals bound as either silicates or oxyhydroxides by the surfaces of Bacillus subtilis with changes in pH and presence of competing cations and EDTA was studied. Wall-bound silicate was very stable. Of all conditions, a pH of 3.0 (by HNO 3 ) promoted the greatest release of Cd (87% [wt/wt]), Pb (77%), or Cu (54%). Bacterial Pb resisted extraction by EDTA, and Cd that was included in bacterial-silicates proved difficult to remobilize.

Characterization of the cell wall of the sheathed methanogen Methanospirillum hungatei GP1 as an S layer

The cell wall of Methanospirillum hungatei GP1 is a labile structure that has been difficult to isolate and characterize because the cells which it encases are contained within a sheath. Cell-sized fragments, 560 nm wide by several micrometers long, of cell wall were extracted by a novel method involving the gradual drying of the filaments in 2% (wt/vol) sodium dodecyl sulfate and 10% (wt/vol) sucrose in 50 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer containing 10 mM EDTA. The surface was a hexagonal array (a = b = 15.1 nm) possessing a helical superstructure with a ca. 2.5 degrees pitch angle. In shadowed relief, the smooth outer face was punctuated with deep pits, whereas the inner face was relatively featureless. Computer-based two-dimensional reconstructed views of the negatively stained layer demonstrated 4.0- and 2.0-nm-wide electron-dense regions on opposite sides of the layer likely corresponding to the openings of funnel-shaped channels. The face featuring the larger openings best corresponds to the outer face of the layer. The smaller opening was encircled by a stalk-like mass from which 2.2-nm-wide protrusions were resolved. The cell wall in situ was degraded at pH 9.6 at 56 degrees C but was unaffected at pH 7.4 at the same temperature. The cell wall was composed of two nonglycosylated polypeptides (114 and 110 kDa). The cell wall resembled an archaeal S layer and may function in regulating the passage of small (< 10-kDa) sheath precursor proteins.

Ultrastructure of mycobacterial surfaces by freeze-substitution

The global status of tuberculosis has recently received much public attention, particularly in some developed countries that are now reporting an increase in cases after several years of decline. A number of factors have contributed to the resurgence in tuberculosis and other mycobacterial infections, including homelessness, increased urban overcrowding among the poor, increased drug abuse and the AIDS epidemic. In addition, the intrinsic nature of mycobacterial impermeability to some antibiotics, and their ability to survive in host environments has been attributed to the unique chemistry and architecture of their walls. A better understanding of these surface-related properties could in turn lead to the design of more effective chemotherapeutic agents or potential vaccine candidates. Using freeze-substitution, we offer a revised perspective on mycobacterial wall design and discuss its significance.

Surface action of gentamicin on Pseudomonas aeruginosa

The mode of action of gentamicin has traditionally been considered to be at the 30S ribosomal level. However, the inhibition of bacterial protein synthesis alone appears to be insufficient to entirely explain the bactericidal effects. Bacteriolysis is also mediated through perturbation of the cell surface by gentamicin (J.L. Kadurugamuwa, J.S. Lam, and T.J. Beveridge, Antimicrob. Agents Chemother. 37:715-721, 1993). In order to separate the surface effect from protein synthesis in Pseudomonas aeruginosa PAO1, we chemically conjugated bovine serum albumin (BSA) to gentamicin, making the antibiotic too large to penetrate through the cell envelope to interact with the ribosomes of the cytoplasm. Furthermore, this BSA-gentamicin conjugate was also used to coat colloidal gold particles as a probe for electron microscopy to study the surface effect during antibiotic exposure. High-performance liquid chromatography confirmed the conjugation of the protein to the antibiotic. The conjugated gentamicin and BSA retained bactericidal activity and inhibited protein synthesis on isolated ribosomes in vitro but not on intact cells in vivo because of its exclusion from the cytoplasm. When reacted against the bacteria, numerous gentamicin-BSA-gold particles were clearly seen on the cell surfaces of whole mounts and thin sections of cells, while the cytoplasm was devoid of such particles. Disruption of the cell envelope was also observed since gentamicin-BSA and gentamicin-BSA-gold destabilized the outer membrane, evolved outer membrane blebs and vesicles, and formed holes in the cell surface. The morphological evidence suggests that the initial binding of the antibiotic disrupts the packing order of lipopolysaccharide of the outer membrane, which ultimately forms holes in the cell envelope and can lead to cell lysis. It is apparent that gentamicin has two potentially lethal effects on gram-negative cells, that resulting from inhibition of protein synthesis and that resulting from surface perturbation; the two effects in concert make aminoglycoside drugs particularly effective antibiotics.

Proton motive force may regulate cell wall-associated enzymes of Bacillus subtilis

Bacterial metabolism excretes protons during normal metabolic processes. The protons may be recycled by chemiosmosis, diffuse through the wall into the medium, or bind to cell surface constituents. Calculations by Koch (J. Theor. Biol. 120:73-84, 1986) have suggested that the cell wall of gram-positive bacteria may serve as a reservoir of protons during growth and metabolism, causing the wall to have a relatively low pH. That the cell wall may possess a pH lower than the surrounding medium has now been tested in Bacillus subtilis by several independent experiments. When cultures of B. subtilis were treated with the proton conductors azide and carbonylcyanide m-chlorophenylhydrazone, the cells bound larger amounts of positively charged probes, including the chromium (Cr3+) and uranyl (UO2(2+) ions and were readily agglutinated by cationized ferritin. In contrast, the same proton conductors caused a decrease in the binding of the negatively charged probe chromate (CrO4(2-)). Finally, when levansucrase was induced in cultures by the addition of sucrose, the enzyme was inactive as it traversed the wall during the first 0.7 to 1.0 generation of growth. The composite interpretation of the foregoing observations suggests that the wall is positively charged during metabolism, thereby decreasing its ability to complex with cations while increasing its ability to bind with anions. This may be one reason why some enzymes, such as autolysins, are unable to hydrolyze their substrata until they reach the wall periphery or are in the medium.

Immunochemistry and localization of the enzyme disaggregatase in Methanosarcina mazei

The enzyme disaggregatase (Dag) from Methanosarcina mazei was studied immunochemically. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified Dag under reducing and nonreducing conditions revealed a single band with a 94-kDa molecular mass. Dag was found to be immunogenic in rabbits; a polyclonal antibody probe was prepared and used to detect the enzyme by slide immunoenzymatic assay, immunofluorescence, and immunoblotting in various species of Methanosarcina known to convert from packets to single cells, including M. mazei. The enzyme could not be detected in other members of the family Methanosarcinaceae that do not convert. By immunogold electron microscopy, Dag was mapped to the cell wall of packets and to the cell membrane of single cells of two M. mazei strains.

Mechanism of silicate binding to the bacterial cell wall in Bacillus subtilis

To investigate the chemical mechanism of silicate binding to the surface of Bacillus subtilis, we chemically modified cell wall carboxylates to reverse their charge by the addition of an ethylenediamine ligand. For up to 9 weeks, mixtures of Si, Al-Fe-Si, and Al-Fe-Si plus toxic heavy metals were reacted with these cells for comparison with control cells and abiotic solutions. In general, more Si and less metal were bound to the chemically modified surfaces, thereby showing the importance of an electropositive charge in cell walls for fine-grain silicate mineral development. The predominant reaction for this development was the initial silicate-to-amine complexation in the peptidoglycan of ethylenediamine-modified and control cell walls, although metal ion bridging between electronegative sites and silicate had an additive effect. The binding of silicate to these bacterial surfaces can thus be described as outer sphere complex formation because it occurs through electrostatic interaction.

Transmission electron microscopy, scanning tunneling microscopy, and atomic force microscopy of the cell envelope layers of the archaeobacterium Methanospirillum hungatei GP1

Methanospirillum hungatei GP1 possesses paracrystalline cell envelope components including end plugs and a sheath formed from stacked hoops. Both negative-stain transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) distinguished the 2.8-nm repeat on the outer surface of the sheath, while negative-stain TEM alone demonstrated this repeat around the outer circumference of individual hoops. Thin sections revealed a wave-like outer sheath surface, while STM showed the presence of deep grooves that precisely defined the hoop-to-hoop boundaries at the waveform nodes. Atomic force microscopy of sheath tubes containing entrapped end plugs emphasized the end plug structure, suggesting that the sheath was malleable enough to collapse over the end plugs and deform to mimic the shape of the underlying structure. High-resolution atomic force microscopy has revised the former idea of end plug structure so that we believe each plug consists of at least four discs, each of which is approximately 3.5 nm thick. PT shadow TEM and STM both demonstrated the 14-nm hexagonal, particulate surface of an end plug, and STM showed the constituent particles to be lobed structures with numerous smaller projections, presumably corresponding to the molecular folding of the particle.

Interaction of gentamicin with the A band and B band lipopolysaccharides of Pseudomonas aeruginosa and its possible lethal effect

The lipopolysaccharide (LPS) of Pseudomonas aeruginosa PAO1 possesses two distinct types of O polysaccharide, A and B band LPSs, but the majority of clinical isolates from cystic fibrosis patients who are infected with the organism possess only the A band as the major LPS antigen. The initial step in a series of events during the uptake of aminoglycoside antibiotics such as gentamicin is the ionic binding of the molecule to the cell surface. In an attempt to elucidate the role of A and B band LPSs of P. aeruginosa in this passive ionic binding of gentamicin to the outer membrane and its possible lethal effects, strains PAO1 (A+B+) and LPS isogenic derivatives (A+B-,A-B+,A-B-) were treated with the antibiotic. Ionic binding of gentamicin appeared to be subtly different in PAO1 and its LPS derivatives; a lethal dose of drug was bound to all strains, although the degree of binding varied with each strain. The outer membrane affinity for gentamicin was higher in strains possessing the B band than in strains with A band LPS, and these B band strains were more prone to antibiotic-induced killing. Strains with both A and B band LPSs bound the most gentamicin of all strains, and this binding caused an almost 50% loss in viability. Ionic binding of aminoglycoside antibiotucs to the outer membrane of cell surfaces must not only weaken th cell surface (R. E. W. Hancock, Annu. Rev. Microbiol. 38:237-264, 1984; N. L. Martin and T. J. Beveridge, Antimicrob. Agents Chemother. 29:1079-1087, 1986; S. G. Walker and T. J. Beveridge, Can. J. Microbiol. 34:12-18, 1988) but it must also be more important in cell death than was originally thought.

Freeze-fracture planes of methanogen membranes correlate with the content of tetraether lipids

Methanospirillum hungatei GP1 contained 50% of its ether core lipids (polar lipids less head groups) as tetraether lipids, and its plasma membrane failed to fracture along its hydrophobic domain during freeze-etching. The membrane of Methanosaeta ("Methanothrix") concilii did not contain tetraether lipids and easily fractured to reveal typical intramembranous particles. Methanococcus jannaschii grown at 50 degrees C contained 20% tetraether core lipids, which increased to 45% when cells were grown at 70 degrees C. The frequency of membrane fracture was reduced as the membrane-spanning tetraether lipids approached 45%. As the tetraether lipid content increased, and while fracture was still possible, the particle density in the membrane increased; these added particles could be tetraether lipid complexes torn from the opposing membrane face. The diether membrane (no tetraether lipid) of Methanococcus voltae easily fractured, and the intramembranous particle density was low. Protein-free liposomes containing tetraether core lipids (ca. 45%) also did not fracture, whereas those made up exclusively of diether lipids did split, indicating that tetraether lipids add considerable vertical stability to the membrane. At tetraether lipid concentrations below 45%, liposome bilayers fractured to reveal small intramembranous particles which we interpret to be tetraether lipid complexes.

Participation of a cyanobacterial S layer in fine-grain mineral formation

Cyanobacteria belonging to the Synechococcus group are ubiquitous inhabitants of diverse marine and freshwater environments. Through interactions with the soluble constituents of their aqueous habitats, they inevitably affect the chemistry of the waters they inhabit. Synechococcus strain GL24 was isolated from Fayetteville Green Lake, New York, where it has a demonstrated role in the formation of calcitic minerals. In order to understand the detailed interactions which lead to mineral formation by this organism, we have undertaken detailed ultrastructural studies of its cell surface and the initial events in mineral growth using a variety of electron microscopic and computer image enhancement techniques. Synechococcus strain GL24 has a hexagonally symmetrical S layer as its outermost cell surface component. The constituent protein(s) of this structure appears as a double band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with M(r)s of 104,000 and 109,000. We demonstrate that the S layer acts as a template for fine-grain gypsum and calcite formation by providing discrete, regularly arranged nucleation sites for the critical initial events in the mineralization process. To our knowledge, this is the first time that a bacterial S layer has been shown to have a role in mineral formation in a natural environment, and this report provides conclusive evidence for the specific involvement of bacterial surfaces in natural mineral formation processes.

The membrane-induced proton motive force influences the metal binding ability of Bacillus subtilis cell walls

Bacillus subtilis 168 is a gram-positive bacterium whose cell wall contains the highly electronegative polymers peptidoglycan (chemotype A1 gamma) and glycerol-based teichoic acid to produce a surface with a net negative charge with high metal binding capacity. During metabolism, a membrane-induced proton motive force continuously pumps protons into the wall fabric. As a result, a competition between protons and metal ions for anionic wall sites occurs, and less metal is bound in living cells than in nonliving cells or those in which the plasma membrane has been uncoupled. This was shown by using two metallic ions, UO2(2+) and Sc3+, on control cells, cells uncoupled with either carbonyl cyanide m-chlorophenylhydrazone or NaN3, or cells killed by gamma radiation. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma atomic-emission spectroscopy showed that more metal was retained in the walls of nonliving cells and those with deenergized membranes than in their living counterparts.

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.

Detection of growth sites in and protomer pools for the sheath of Methanospirillum hungatei GP1 by use of constituent organosulfur and immunogold labeling

Methanospirillum hungatei GP1 integrated approximately 9% of cellular [35S]cysteine into its sheath. Autoradiography of sodium dodecyl sulfate-polyacrylamide gels revealed that [35S]cysteine was confined to the proteins released by the sodium dodecyl sulfate-beta-mercaptoethanol-EDTA solubilization method (G. Southam and T. J. Beveridge, J. Bacteriol. 173:6213-6222, 1991) and was not present in the proteins released by treatment with phenol (G. Southam and T. J. Beveridge, J. Bacteriol. 174:935-946, 1992). Limited labeling of exposed sulfhydryl groups on hoops produced from sheath material suggested that most organosulfur groups occur within hoops and therefore help contribute to resilience. Electron microscopic autoradiography demonstrated that sheath growth, which is most active at the sites of cell division (spacer region), occurs through the de novo development of hoops. For growth to occur in the spacer region, sheath precursors must transverse several periodic envelope layers, including the cell wall (a single layer) and the various lamellae of the spacer plug (T. J. Beveridge, G. D. Sprott, and P. Whippey, J. Bacteriol. 173:130-140, 1991).

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.

Formation of unilamellar liposomes from total polar lipid extracts of methanogens

Unilamellar liposomes were formed by controlled detergent dialysis of mixed micelles consisting of acetone-insoluble total polar lipids extracted from various methanogens and the detergent n-octyl-beta-D-glucopyranoside. The final liposome populations were studied by dynamic light scattering and electron microscopy. Unilamellar liposomes with mean diameters smaller than 100 nm were obtained with lipid extracts of Methanococcus voltae, Methanosarcina mazei, Methanosaeta concilii, and Methanococcus jannaschii (grown at 50 degrees C), whereas larger (greater than 100-nm) unilamellar liposomes were obtained with lipid extracts of M. jannaschii grown at 65 degrees C. These liposomes were shown to be closed intact vesicles capable of retaining entrapped [14C]sucrose for extended periods of time. With the exception of Methanospirillum hungatei liposomes, all size distributions of the different liposome populations were fairly homogeneous.

A new mercury-penicillin V derivative as a probe for ultrastructural localization of penicillin-binding proteins in Escherichia coli

The precise ultrastructural localization of penicillin-binding protein (PBP)-antibiotic complexes in Escherichia coli JM101, JM101 (pBS96), and JM101(pPH116) was investigated by high-resolution electron microscopy. We used mercury-penicillin V (Hg-pen V) as a heavy-metal-labeled, electron-dense probe for accurately localizing PBPs in situ in single bacterial cells grown to exponential growth phase. Biochemical data derived from susceptibility tests and bacteriolysis experiments revealed no significant differences between Hg-pen V and the parent compound, penicillin V, or between strains. Both antibiotics revealed differences in the binding affinities for PBPs of all strains. Deacylation rates for PBPs were slow despite the relatively low binding affinities of antibiotics. Cells bound most of the Hg-pen V added to cultures, and the antibiotic-PBP complex could readily be seen by electron microscopy of unstained whole mounts as distinct, randomly situated electron-dense particles. Fifty to 60% of the antibiotic was retained by cells during processing for conventional embedding so that thin sections could also be examined. These revealed similar electron-dense particles located predominantly on the plasma membrane and less frequently in the cytoplasm. Particles positioned on the plasma membranes were occasionally shown to protrude into the periplasmic space, thereby reflecting the high resolution of the Hg-pen V probe. Moreover, some particles were observed free in the periplasm, suggesting, for the first time, that a proportion of PBPs may not be restricted to the plasma membrane but may be tightly associated with the peptidoglycan for higher efficiency of peptidoglycan assembly. All controls were devoid of the electron-dense particles. The presence of electron-dense particles in cells of the wild-type JM101, demonstrated that our probe could identify PBPs in naturally occurring strains without inducing PBP overproduction.

Characterization of novel, phenol-soluble polypeptides which confer rigidity to the sheath of Methanospirillum hungatei GP1

Treatment of the Methanospirillum hungatei GP1 sheath with 90% (wt/vol) phenol resulted in the solubilization of a novel phenol-soluble group of polypeptides. These polypeptides were purified by the removal of insoluble material by ultracentrifugation and represented approximately 19% of the mass of the sheath. The phenol-insoluble material resembled untreated sheath but had lost its rigidity and cylindrical form. Recombination of phenol-soluble and phenol-insoluble fractions by dialysis to remove phenol resulted in cylindrical reassembly products. Although bona fide sheath (complete with the 2.8-nm lattice) was not produced, a role for the phenol-soluble polypeptides in the maintenance of sheath rigidity is implied. The phenol-soluble polypeptides have limited surface exposure as detected by antibodies on intact sheath; therefore, they are not responsible for the 2.8-nm repeat occurring on the outer face of the sheath. However, longitudinal and transverse linear labeling by protein A-colloidal gold on the outer and inner faces, respectively, occurred with monoclonal antibodies specific to the phenol-soluble polypeptides. Restricted surface exposure of phenol-soluble polypeptides on the sheath highlighted molecular defects in sheath architecture. These lattice faults may indicate sites of sheath growth to accommodate cell growth or division (longitudinal immunogold label) and filament division (transverse immunogold label). The identification of a second group of polypeptides within the infrastructure of the sheath suggests that the sheath is a trilaminar structure in which phenol-soluble polypeptides are sandwiched between sodium dodecyl sulfate-beta-mercaptoethanol-EDTA-soluble polypeptides (G. Southam and T. J. Beveridge, J. Bacteriol. 173:6213-6222, 1991) (phenol-insoluble material).

Influence of oxidation state on iron binding by Bacillus licheniformis capsule

We examined ferric (Fe3+) and ferrous (Fe2+) iron binding by the anionic gamma-glutamyl capsule polymer of Bacillus licheniformis ATCC 9945. The addition of FeCl3 to B. licheniformis capsule under aerobic conditions resulted in flocculation due to the capsule-induced formation of amorphous, rust-colored ferrihydrite. Significant binding of iron, which could be attributed to binding by both the anionic capsule and the ferrihydrite precipitate, occurred. In contrast, the addition of FeCl2 to B. licheniformis capsule under anaerobic conditions resulted in significantly less iron being bound and no color change or flocculation occurring. Capsule-bound ferric iron could be partially released upon addition of several reducing agents. From these observations, it can be concluded that the oxidation state of iron significantly influences its tendency to be bound by anionic bacterial polymers such as capsules.

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.

Dissolution and immunochemical analysis of the sheath of the archaeobacterium Methanospirillum hungatei GP1

The sheath of Methanospirillum hungatei GP1 was degraded by three dissolution techniques, which produced a range of soluble products. By using 0.05 M L-arginine buffer (pH 12.6) at 90 degrees C for 10 min, 74% (dry weight) of the sheath was dissolved; however, the solubilized polypeptides were extensively degraded. Treatment with 2% beta-mercaptoethanol and 2% sodium dodecyl sulfate at 90 degrees C in 0.05 M 2(N-cyclohexylamino)ethanesulfonic acid (CHES) buffer (pH 9.0) solubilized 42% (dry weight) of the sheath as a group of polypeptides of 30 to 40 kDa. At 100 degrees C for 2 h, 5% beta-mercaptoethanol, 2% sodium dodecyl sulfate (SDS), and 20 mM EDTA released 74% of the sheath's mass as a group of polypeptides of 10 to 40 kDa. All solubilized products were examined by SDS-polyacrylamide gel electrophoresis, and a range of high- and low-molecular-weight polypeptides was identified. None were glycoproteins. Hoops, which comprise the sheath's structure, were seen by electron microscopy after all of the attempted dissolutions. Monoclonal antibodies were produced against the 10- to 40-kDa range of solubilized products and against the approximately 40-kDa polypeptides, and polyclonal antiserum was produced against an 18-kDa polypeptide. These immunological markers were used in Western immunoblotting and protein A-colloidal gold-antibody probing by electron microscopy to identify the structural location of the various polypeptides. Native sheath, which possesses 2.8-nm particles on its outer surface (M. Stewart, T.J. Beveridge, and G.D. Sprott, J. Mol. Biol. 183:509-515, 1985; P.J. Shaw, G.J. Hills, J.A. Henwood, J.E. Harris, and D.B. Archer, J. Bacteriol. 161:750-757, 1985), presented a gentle wave-form surface in platinum-shadowed specimens. In contrast, the inner face of the sheath was highlighted by ridges lying perpendicular to the longitudinal axis of the sheath and likely corresponded to hoop boundaries. Both the polyclonal and monoclonal antibodies were specific for different faces; polyclonal antibodies labeled the inner face, whereas monoclonal antibodies labeled the outer face. Accordingly, the apparent asymmetry of structure between the two faces of the sheath can be correlated by our immunochemical probing with a distinct asymmetry in the distribution of exposed polypeptides between the faces. The possible implications of this asymmetry for growth and maturation of the sheath are explained.

Ultrastructure, inferred porosity, and gram-staining character of Methanospirillum hungatei filament termini describe a unique cell permeability for this archaeobacterium

By light microscopy, Methanospirillum hungatei GP1 stains gram positive at the terminal ends of each multicellular filament and gram negative at all regions in between. This phenomenon was studied further by electron microscopy and energy-dispersive X-ray spectroscopy of Gram-stained cells, using a platinum compound to replace Gram's iodine (J. A. Davies, G. K. Anderson, T. J. Beveridge, and H. C. Clark, J. Bacteriol. 156:837-845, 1983). Crystal violet-platinum precipitates could be found only in the terminal cells of each filament, which suggested that the multilamellar plugs at the filament ends were involved with stain penetration. When sheaths were isolated by sodium dodecyl sulfate-dithiothreitol treatment, the end plugs could be ejected and their layers could be separated from one another by 0.1 M NaOH treatment. Each plug consisted of at least three individual layers; two were particulate and possessed 14.0-nm particles hexagonally arranged on their surfaces with a spacing of a = b = 18.0 nm, whereas the other was a netting of 12.5-nm holes with spacings and symmetry identical to those of the particulate layers. Optical diffraction and computer image reconstruction were used to clarify the structures of each layer in an intact plug and to provide a high-resolution image of their interdigitated structures. The holes through this composite were three to six times larger than those through the sheath. Accordingly, we propose that the terminal plugs of M. hungatei allow the access of larger solutes than does the sheath and that this is the reason why the end cells of each filament stain gram positive whereas more internal cells are gram negative. Intuitively, since the cell spacers which partition the cells from one another along the filament contain plugs identical in structure to terminal plugs, the diffusion of large solutes for these cells would be unidirectional along the filament-cell axis.

Metal-Binding Characteristics of the Gamma-Glutamyl Capsular Polymer of Bacillus licheniformis ATCC 9945

The metal-binding affinity of the anionic poly-γ- d -glutamyl capsule of Bacillus licheniformis was investigated by using Na + , Mg 2+ , Al 3+ , Ca 2+ , Cr 3+ , Mn 2+ , Fe 3+ , Ni 2+ , and Cu 2+ . Purified capsule was suspended in various concentrations of the chloride salts of the various metals, and after dialysis the bound metals were analyzed either by graphite furnace atomic absorption spectroscopy or by inductively coupled plasma-mass spectrometry. Exposure of purified capsule to excess concentrations of Na + revealed it to contain 8.2 μmol of anionic sites per mg on the basis of Na binding. This was confirmed by titration of the capsule with HCl and NaOH. Other metal ions were then added in ionic concentrations equivalent to 25, 50, 75, 100, 200, and 400% of the available anionic sites. The binding characteristics varied with the metal being investigated. Addition of Cu 2+ , Al 3+ , Cr 3+ , or Fe 3+ induced flocculation. These metal ions showed the greatest affinity for B. licheniformis capsule in competitive-binding experiments. Flocculation was not seen with the addition of other metal ions. With the exception of Ni 2+ and Fe 3+ all capsule-metal-binding sites readily saturated. Ni 2+ had low affinity for the polymer, and its binding was increased at high metal concentrations. Fe 3+ binding resulted in the development of rust-colored ferrihydrite which itself could bind additional metal. Metal-binding characteristics of B. licheniformis capsule appear to be influenced by the chemical and physical properties of both the capsule and the metal ions.

High-resolution topography of the S-layer sheath of the archaebacterium Methanospirillum hungatei provided by scanning tunneling microscopy

The inner and outer surfaces of the sheath of Methanospirillum hungatei GP1 have been imaged for the first time by using a bimorph scanning tunneling microscope (STM) on platinum-coated or uncoated specimens to a nominal resolution in height of ca. 0.4. nm. Unlike more usual types of microscopy (e.g., transmission electron microscopy), STM provided high-resolution topography of the surfaces, giving good depth detail which confirmed the sheath to be a paracrystalline structure possessing minute pores and therefore impervious to solutes possessing a hydrated radius of greater than 0.3 nm. STM also confirmed that the sheath consisted of a series of stacked hoops approximately 2.5 nm wide which were the remnants of the sheath after treatment with 2% (wt/vol) sodium dodecyl sulfate-2% (vol/vol) beta-mercaptoethanol (pH 9.0). No topographical infrastructure could be seen on the sides of the hoops. This research required the development of a new long-range STM capable of detecting small particles such as bacteria on graphite surfaces as well as a new "hopping" STM mode which did not deform the poorly conducting bacterial surface during high-resolution topographical analysis.

Remobilization of toxic heavy metals adsorbed to bacterial wall-clay composites

Significant quantities of Ag(I), Cu(II), and Cr(III) were bound to isolated Bacillus subtilis 168 walls, Escherichia coli K-12 envelopes, kaolinite and smectite clays, and the corresponding organic material-clay aggregates (1:1, wt/wt). These sorbed metals were leached with HNO3, Ca(NO3)2, EDTA, fulvic acid, and lysozyme at several concentrations over 48 h at room temperature. The remobilization of the sorbed metals depended on the physical properties of the organic and clay surfaces and on the character and concentration of the leaching agents. In general, the order of remobilization of metals was Cr much less than Ag less than Cu. Cr was very stable in the wall, clay, and composite systems; pH 3.0, 500 microM EDTA, 120-ppm [mg liter-1] fulvic acid, and 160-ppm Ca remobilized less than 32% (wt/wt) of sorbed Cr. Ag (45 to 87%) and Cu (up to 100%) were readily removed by these agents. Although each leaching agent was effective at mobilizing certain metals, elevated Ca or acidic pH produced the greatest overall mobility. The organic chelators were less effective. Lysozyme digestion of Bacillus walls remobilized Cu from walls and Cu-wall-kaolinite composites, but Ag, Cr, and smectite partially inhibited enzyme activity, and the metals remained insoluble. The extent of metal remobilization was not always dependent on increasing concentrations of leaching agents; for example, Ag mobility decreased with some clays and some composites treated with high fulvic acid, EDTA, and lysozyme concentrations. Sometimes the organic material-clay composites reacted in a manner distinctly different from that of their individual counterparts; e.g., 25% less Cu was remobilized from wall- and envelope-smectite composites than from walls, envelopes, or smectite individually in 500 microM EDTA. Alternatively, treatment with 160-ppm Ca removed 1.5 to 10 times more Ag from envelope-kaolinite composites than from the individual components. The particle size of the deposited metal may account for some of the stability changes; those metals that formed large, compact aggregates (Cr and Ag) as seen by transmission electron microscopy were less likely to be remobilized. In summary, it is apparent that remobilization of toxic heavy metals in sediments, soils, and the vadose zone is a complicated issue. Predictions based on single inorganic or organic component systems are too simplistic.

Isolation, characterization, and cellular insertion of the flagella from two strains of the archaebacterium Methanospirillum hungatei

In high (45 mM)-phosphate medium, Methanospirillum hungatei strains GP1 and JF1 grew as very long, nonmotile chains of cells that did not possess flagella. However, growth in lower (3 or 30 mM)-phosphate medium resulted in the production of mostly single cells and short chains that were motile by means of two polar tufts of flagella, which transected the multilayered terminal plug of the cell. Electron microscopy of negatively stained whole mounts revealed a flagellar filament diameter of approximately 10 nm. Flagellar filaments were isolated from either culture fluid or concentrated cell suspensions that were subjected to shearing. Flagellar filaments were sensitive to treatment with both Triton X-100 and Triton X-114 at concentrations as low as 0.1% (vol/vol). The filaments of both strains were composed of two flagellins of Mr 24,000 and 25,000. However, variations in trace element composition of the medium resulted in the production of a third flagellin in strain JF1. This additional flagellin appeared as a ladderlike smear on sodium dodecyl sulfate-polyacylamide gels with a center of intensity of Mr 35,000 and cross-reacted with antisera produced from filaments containing only the Mr-24,000 and -25,000 flagellins. On sodium dodecyl sulfate-polyacrylamide gels, all flagellins stained by the thymol-sulfuric acid and Alcian blue methods, suggesting that they were glycosylated. This was further supported by chemical deglycosylation of the strain JF1 flagellins, which resulted in a reduction in their apparent molecular weight on sodium dodecyl sulfate-polyacylamide gels. Heterologous reactions to sera raised against the flagella from each strain were limited to the Mr-24,000 flagellins.

Evaluation of freeze-substitution and conventional embedding protocols for routine electron microscopic processing of eubacteria

Freeze-substitution and more conventional embedding protocols were evaluated for their accurate preservation of eubacterial ultrastructure. Radioisotopes were specifically incorporated into the RNA, DNA, peptidoglycan, and lipopolysaccharide of two isogenic derivatives of Escherichia coli K-12 as representative gram-negative eubacteria and into the RNA and peptidoglycan of Bacillus subtilis strains 168 and W23 as representative gram-positive eubacteria. Radiolabeled bacteria were processed for electron microscopy by conventional methods with glutaraldehyde fixation, osmium tetroxide postfixation, dehydration in either a graded acetone or ethanol series, and infiltration in either Spurr or Epon 812 resin. A second set of cells were simultaneously freeze-substituted by plunge-freezing in liquid propane, substituting in anhydrous acetone containing 2% (wt/vol) osmium tetroxide, and 2% (wt/vol) uranyl acetate, and infiltrating in Epon 812. Extraction of radiolabeled cell components was monitored by liquid scintillation counting at all stages of processing to indicate retention of cell labels. Electron microscopy was also used to visually confirm ultrastructural integrity. Radiolabeled nucleic acid and wall components were extracted by both methods. In conventionally embedded specimens, dehydration was particularly damaging, with ethanol-dehydrated cells losing significantly more radiolabeled material during dehydration and subsequent infiltration than acetone-treated cells. For freeze-substituted specimens, postsubstitution washes in acetone were the most deleterious step for gram-negative cells, while infiltration was more damaging for gram-positive cells. Autoradiographs of specimens collected during freeze-substitution were scanned with an optical densitometer to provide an indication of freezing damage; the majority of label lost from freeze-substituted cells was a result of poor freezing to approximately one-half of the cell population, thus accounting for the relatively high levels of radiolabel detected in the processing fluids. These experiments revealed that gram-positive and gram-negative cells respond differently to freezing; these differences are discussed with reference to wall structure. It was apparent that the cells frozen first (ie., the first to contact the cryogen) retained the highest percentage of all radioisotopes, and the highest level of cellular infrastructure, indicative of better preservation. The preservation of these select cells was far superior to that obtained by more conventional techniques.

Effect of chemical fixatives on accurate preservation of Escherichia coli and Bacillus subtilis structure in cells prepared by freeze-substitution

Five chemical fixatives were evaluated for their ability to accurately preserve bacterial ultrastructure during freeze-substitution of select Escherichia coli and Bacillus subtilis strains. Radioisotopes were specifically incorporated into the peptidoglycan, lipopolysaccharide, and nucleic acids of E. coli SFK11 and W7 and into the peptidoglycan and RNA of B. subtilis 168 and W23. The ease of extraction of radiolabels, as assessed by liquid scintillation counting during all stages of processing for freeze-substitution, was used as an indicator of cell structural integrity and retention of cellular chemical composition. Subsequent visual examination by electron microscopy was used to confirm ultrastructural conformation. The fixatives used were: 2% (wt/vol) osmium tetroxide and 2% (wt/vol) uranyl acetate; 2% (vol/vol) glutaraldehyde and 2% (wt/vol) uranyl acetate; 2% (vol/vol) acrolein and 2% (wt/vol) uranyl acetate; 2% (wt/vol) gallic acid; and 2% (wt/vol) uranyl acetate. All fixatives were prepared in a substitution solvent of anhydrous acetone. Extraction of cellular constituents depended on the chemical fixative used. A combination of 2% osmium tetroxide-2% uranyl acetate or 2% gallic acid alone resulted in optimum fixation as ascertained by least extraction of radiolabels. In both gram-positive and gram-negative organisms, high levels of radiolabel were detected in the processing fluids in which 2% acrolein-2% uranyl acetate, 2% glutaraldehyde-2% uranyl acetate, or 2% uranyl acetate alone were used as fixatives. Ultrastructural variations were observed in cells freeze-substituted in the presence of different chemical fixatives. We recommend the use of osmium tetroxide and uranyl acetate in acetone for routine freeze-substitution of eubacteria, while gallic acid is recommended for use when microanalytical processing necessitates the omission of osmium.

Mechanism of gram variability in select bacteria

Gram stains were performed on strains of Actinomyces bovis, Actinomyces viscosus, Arthrobacter globiformis, Bacillus brevis, Butyrivibrio fibrisolvens, Clostridium tetani, Clostridium thermosaccharolyticum, Corynebacterium parvum, Mycobacterium phlei, and Propionibacterium acnes, using a modified Gram regimen that allowed the staining process to be observed by electron microscopy (J. A. Davies, G. K. Anderson, T. J. Beveridge, and H. C. Clark, J. Bacteriol. 156:837-845, 1983). Furthermore, since a platinum salt replaced the iodine mordant of the Gram stain, energy-dispersive X-ray spectroscopy could evaluate the stain intensity and location by monitoring the platinum signal. These gram-variable bacteria could be split into two groups on the basis of their staining responses. In the Actinomyces-Arthrobacter-Corynebacterium-Mycobacterium-Propionibacterium group, few cells became gram negative until the exponential growth phase; by mid-exponential phase, 10 to 30% of the cells were gram negative. The cells that became gram negative were a select population of the culture, had initiated septum formation, and were more fragile to the stress of the Gram stain at the division site. As cultures aged to stationary phase, there was a relatively slight increase toward gram negativity (now 15 to 40%) due to the increased lysis of nondividing cells by means of lesions in the side walls; these cells maintained their rod shape but stained gram negative. Those in the Bacillus-Butyrivibrio-Clostridium group also became gram negative as cultures aged but by a separate set of events. These bacteria possessed more complex walls, since they were covered by an S layer. They stained gram positive during lag and the initial exponential growth phases, but as doubling times increased, the wall fabric underlying the S layer became noticeably thinner and diffuse, and the cells became more fragile to the Gram stain. By stationary phase, these cultures were virtually gram negative.

Characterization of a dynamic S layer on Bacillus thuringiensis

The surfaces of three Bacillus thuringiensis strains possess an S layer composed of linear arrays of small particles arranged with p2 symmetry and with a = 8.5 nm, b = 7.2 nm, and gamma = 73 degrees. Platinum shadows of whole cells and S-layer fragments revealed the outer surface of the array to be smooth and the inner surface to be corrugated. Treatment with 2 M guanidine hydrochloride at pH 2.5 to 4 best removed the S layer for chemical characterization; it was a relatively hydrophilic 91.4-kilodalton protein with a pI of 5, no detectable carbohydrate, cysteine, methionine or tryptophan, and 21.2% nonpolar residues. No N-terminal homology with other S-layer proteins was evident. Antibody labeling experiments confirmed that the amount of S layer was proportional to the growth phase in broth cultures. Late-exponential- and stationary-growth-phase cells typically sloughed off fragments of S layer, and this may be the result of wall turnover. Indigenous autolytic activity in isolated walls rapidly digested the wall fabric, liberating soluble S-layer protein. At the same time, proteases frequently reduced the molecular weight of the 91.4-kilodalton protein, but these polypeptides could still be identified as S-layer components by immunoblotting. As cultures were serially subcultured, the frequency of appearance of the S layer diminished, and it was eventually lost. The dynamic nature of this S layer makes it atypical of most previously identified S layers and made it unusually difficult to characterize.

Bacterial sorption of heavy metals

Four bacteria, Bacillus cereus, B. subtilis, Escherichia coli, and Pseudomonas aeruginosa, were examined for the ability to remove Ag+, Cd2+, Cu2+, and La3+ from solution by batch equilibration methods. Cd and Cu sorption over the concentration range 0.001 to 1 mM was described by Freundlich isotherms. At 1 mM concentrations of both Cd2+ and Cu2+, P. aeruginosa and B. cereus were the most and least efficient at metal removal, respectively. Freundlich K constants indicated that E. coli was most efficient at Cd2+ removal and B. subtilis removed the most Cu2+. Removal of Ag+ from solution by bacteria was very efficient; an average of 89% of the total Ag+ was removed from the 1 mM solution, while only 12, 29, and 27% of the total Cd2+, Cu2+, and La3+, respectively, were sorbed from 1 mM solutions. Electron microscopy indicated that La3+ accumulated at the cell surface as needlelike, crystalline precipitates. Silver precipitated as discrete colloidal aggregates at the cell surface and occasionally in the cytoplasm. Neither Cd2+ nor Cu2+ provided enough electron scattering to identify the location of sorption. The affinity series for bacterial removal of these metals decreased in the order Ag greater than La greater than Cu greater than Cd. The results indicate that bacterial cells are capable of binding large quantities of different metals. Adsorption equations may be useful for describing bacterium-metal interactions with metals such as Cd and Cu; however, this approach may not be adequate when precipitation of metals occurs.

Physicochemical interaction of Escherichia coli cell envelopes and Bacillus subtilis cell walls with two clays and ability of the composite to immobilize heavy metals from solution

Isolated Escherichia coli K-12 cell envelopes or Bacillus subtilis 168 cell walls were reacted with smectite or kaolinite clay in distilled deionized water (pH 6.0); unbound envelopes or walls were separated by sucrose density gradient centrifugation, and the extent of adsorption was calculated. At saturation, both clays adsorbed approximately 1.0 mg (dry weight) of envelopes or walls per mg (dry weight) of clay. Clays showed a preference for edge-on orientation with both walls and envelopes, which was indicative of an aluminum polynuclear bridging mechanism between the wall or envelope surface and the clay edge. The addition of heavy metals increased the incidence of planar surface orientations, which suggested that multivalent metal cation bridging was coming into play and was of increasing importance. The metal-binding capacity of isolated envelopes, walls, clays, and envelope-clay or wall-clay mixtures was determined by atomic absorption spectroscopy after exposure to aqueous 5.0 mM Ag+, Cu2+, Cd2+, Ni2+, Pb2+, Zn2+, and Cr3+ nitrate salt solutions at pHs determined by the buffering capacity of wall, envelope, clay, or composite system. The order of metal uptake was walls greater than envelopes greater than smectite clay greater than kaolinite clay for the individual components, and walls plus smectite greater than walls plus kaolinite greater than envelopes plus smectite greater than envelopes plus kaolinite for the mixtures. On a dry-weight basis, the envelope-clay and wall-clay mixtures bound 20 to 90% less metal than equal amounts of the individual components did.(ABSTRACT TRUNCATED AT 250 WORDS)