Freeze-etching of bacteria

A Single-cell genome for Thiovulum sp

We determined a significant fraction of the genome sequence of a representative of Thiovulum, the uncultivated genus of colorless sulfur Epsilonproteobacteria, by analyzing the genome sequences of four individual cells collected from phototrophic mats from Elkhorn Slough, California. These cells were isolated utilizing a microfluidic laser-tweezing system, and their genomes were amplified by multiple-displacement amplification prior to sequencing. Thiovulum is a gradient bacterium found at oxic-anoxic marine interfaces and noted for its distinctive morphology and rapid swimming motility. The genomic sequences of the four individual cells were assembled into a composite genome consisting of 221 contigs covering 2.083 Mb including 2,162 genes. This single-cell genome represents a genomic view of the physiological capabilities of isolated Thiovulum cells. Thiovulum is the second-fastest bacterium ever observed, swimming at 615 μm/s, and this genome shows that this rapid swimming motility is a result of a standard flagellar machinery that has been extensively characterized in other bacteria. This suggests that standard flagella are capable of propelling bacterial cells at speeds much faster than typically thought. Analysis of the genome suggests that naturally occurring Thiovulum populations are more diverse than previously recognized and that studies performed in the past probably address a wide range of unrecognized genotypic and phenotypic diversities of Thiovulum. The genome presented in this article provides a basis for future isolation-independent studies of Thiovulum, where single-cell and metagenomic tools can be used to differentiate between different Thiovulum genotypes.

The tegument of Schistosoma mansoni: observations on the formation, structure and composition of cytoplasmic inclusions in relation to tegument function

Two major cytoplasmic inclusions, multilaminate vesicles and discoid granules, are present in the tegument of Schistosoma mansoni. These are produced at separate Golgi apparatuses in the tegument cell bodies and move up to the surface by a combination of diffusion and fluid flow. The discoid granules contain neutral mucopolysaccharide and are believed to break down to form the ground substance of the tegument. The multilaminate vesicles are rich in phospholipid and the contents, at least superficially, resemble unit membranes. The multilaminate vesicles are believed to contribute their contents to the multilaminate surface of the worm which appears to be continually replaced. These observations are related to current ideas on membrane turnover and the ability of the worm to acquire a disguise of host erythrocyte glycolipid.

Heliobacterium chlorum: cell organization and structure

The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.

Protein on the cell surface of the moderately halophilic phototrophic bacterium Rhodospirillum salexigens

A cell surface protein (Mr 68,000) of the moderately but obligately halophilic phototrophic bacterium Rhodospirillum salexigens was identified by two independent methods: first, by labeling the cell surface with radioactive iodine and lactoperoxidase, and second, by washing cells in 30% sucrose to remove proteins attached to the cell surface by ionic bonds. The identified protein very likely represents the outermost layer of the cell envelope of R. salexigens as observed by electron microscopy. The protein was isolated. Its isoelectric point was determined to be 4.4; the excess of acidic over basic amino acids was found to be 18.3 mol%; and its average hydrophobicity was 2.26 kJ per residue.

Understanding the artefact problem in freeze-fracture replication: a review

Freeze-fracture and freeze-etching techniques do not provide artefact-free images of native in vivo or in vitro cells and tissues. Each preparation stage can produce specific artefacts which must be recognized and understood if these methods are to contribute meaningful information to cell biology, This paper reviews the latest information available on artefacts in freeze-fracture replication (and etching) methods and points to possibilities for avoiding some of them. Different specimens show different sensitivity to artefactual changes and the final images must be interpreted carefully with regard to the multi-event process that has led to their production.

Ultrastructure of the Bacteroides nodosus cell envelope layers and surface

The surface structure and cell envelope layers of various virulent Bacteroides nodosus strains were examined by light microscopy and by electron microscopy by using negative staining, thin-section, and freeze-fracture-etch techniques. Three surface structures were described: pili and a diffuse material, both of which emerged from one or both poles of the bacteria (depending on the stage of growth and division), and large rodlike structures (usually 30 to 40 nm in diameter) associated with a small proportion of the bacterial population. No capsule was detected. The cell envelope consisted of four layers: a plasma membrane, a peptidoglycan layer, an outer membrane, and an outermost additional layer. The additional layer was composed of subunits, generally hexagonally packed with center-to-center spacing of 6 to 7 nm. The outer membrane and plasma membrane freeze-fractured through their hydrophobic regions revealing four fracture faces with features similar to those of other gram-negative bacteria. However, some unusual features were seen on the fracture faces of the outer membrane: large raised ring structure (11 to 12 nm in diameter) on cw 3 at the poles of the bacteria; complementary pits or ring-shaped depressions on cw 2; and small raised ring structures (7 to 8 nm in diameter) all over cw 2.

Morphology of freeze-etched Treponema refringens (Nichols)

The freeze-etch technique was used to study the morphology of Treponema refringens (Nichols). There is a single band of cytoplasmic fibrils which follows a path in the form of a right-handed helix with a periodicity of 1500 nm around the body of the treponeme just below the cytoplasmic membrane. There are two major fracture planes, one located in the interior of the outer envelope and the second in the interior of the cytoplasmic membrane. The "blebs" or "surface protuberances", which are quite prominent in negative-stained preparations, were not evident with freeze-etch preparation, indicating they are not a part of the normal structure of this organism. The outer envelope in untreated cells was observed to closely fit the body of the treponeme, whereas the outer envelope of glutaraldehyde-treated cells had a loose, wrinkled appearance. Thus the "loose-fitting" outer envelope generally described for treponemes is most likely an artifact of preparation for negative-staining and thin-sectioning.

The fine structure of Micrococcus radiophilus and Micrococcus radioproteolyticus

The radiation resistant bacteria Micrococcus radiophilus and M. radioproteolyticus were studied by thin sectioning and freez-etching techniques and the two species were found to be similar in the fine structure. The only significant difference was in the appearance of the surfaces of the cell walls in freeze-etched preparations. Since the two species, together with M. radiodurans, possess a unique cell wall structure and a cell wall peptidoglycan, which is different from that of other micrococci and Gram-positive cocci, it is recommended that they be reclassified into a new genus.

A structural glycoprotein, containing hydroxyproline, isolated from the cell wall of Chlamydomonas reinhardii

A soluble extract from purified cell walls of C. reinhardii has been separated by gel filtration into three fractions which together account for 94% of the cell wall. The major fraction (accounting for 70% of the extract) is a glycoprotein, with a molecular wt. in sodium perchlorate of 298,000, which can be split into 4 electrophoretically distinct species. It contains 35% protein with high levels of hydroxyproline, arabinose and galactose, and is capable of self assembly into crystalline structures identical to those found within the cell wall. The second fraction (25% of the extract) is a similar glycoprotein, but contains 24% protein, a higher proportion of mannose, and is incapable of self assembly. The third fraction (3-6% of the extract) is shown to be an adsorbed impurity from the growth medium used.

Outer membrane of Salmonella typhimurium: chemical analysis and freeze-fracture studies with lipopolysaccharide mutants

The outer membrane layer of the cell wall was isolated from wild-type Salmonella typhimurium LT2 as well as from its mutants producing lipopolysaccharides with shorter saccharide chains. Chemical analysis of these preparations indicated the following. (i) The number of lipopolysaccharide molecules per unit area was constant, regardless of the length of the saccharide side chain in lipopolysaccharide. (ii) In contrast, in "deep rough" (Rd or Re) mutants producing the lipopolysaccharides with very short saccharide chains, the amount of outer membrane protein per unit surface area decreased to about 60% of the value in the wild type. (iii) In the wild type, the amount of phospholipids is slightly less than what is needed to cover one side of the membrane as a monolayer. In comparison with the wild type, the outer membrane of Rd and Re mutants contains about 70% more phospholipids, which therefore must be distributed in both the outer and inner leaflets of the membrane. Freeze-fracture studies showed that the outer membrane of Re mutants were easily fractured, but fracture became increasingly difficult in strains producing lipopolysaccharides with longer side chains. The convex fracture face was always nearly smooth, but the concave fracture face or the outer half of the membrane was densely covered with particles 8 to 10 nm in diameter. The density of particles was decreased in Re mutants to the same extent as the reduction in proteins, suggesting the largely proteinaceous nature of particles. A model for the supramolecular structure of the outer membrane is presented on the basis of these and other results.

The nature of the attachment of a regularly arranged surface protein to the outer membrane of an Acinetobacter sp

Acinetobacter 199A carries on the outer surface of its outer membrane a layer of regularly arranged protein subunits. The isolated surface protein assembles into the same regular array even in the absence of the underlying outer membrane. Cl- minus is required for this self-assembly. Evidence is presented that the interaction of the surface protein with the outer membrane involves the linking of a carboxyl group in the surface protein to a negatively charged group in the outer membrane protein, via a divalent cation. The surface protein could be detached from the outer membrane by the protein perturbant urea, by the chelating agent EDTA and by replacing Mg-2+ with Na+. It could not be detached by treatment with phospholipases A anc D or the detergents Tween 80 and sodium deoxycholate. The conditions favourable for reattachment of surface protein to the cell wall were the presence of divalent cations and a pH of 3-5. Conversion of carboxyl groups in the surface protein to amine with carbodiimide and ethylene diamine interfered with reattachment. The surface protein did not attach to isolated cell wall lipid or lipopolysaccharide.

Location of the fracture faces within the cell envelope of Acinetobacter species strain MJT-F5-5

The cell wall of the gram-negative bacterium Acinetobacter species strain MJT/F5/5 shows in thin section an external "additional" layer, an outer membrane, an intermediate layer, and a dense layer. Negatively stained preparations showed that the additional layer is composed of hexagonally arranged subunits. In glycerol-treated preparations, freeze-etching revealed that the cell walls consist of four layers, with the main plane of fracture between layers cw 2 and cw 3. The surface of [Formula: see text] 2 consisted of densely packed particles, whereas [Formula: see text] 3 appeared to be fibrillar. In cell envelopes treated with lysozyme by various methods, the removal of the dense layer has detached the outer membrane and additional layer from the underlying layers, as shown in thin sections. When freeze-etched in the absence of glycerol, these detached outer membranes with additional layers fractured to reveal both the faces [Formula: see text] 2 and [Formula: see text] 3 with their characteristic surface structures, and, in addition, both the external and internal etched surfaces were revealed. This experiment provided conclusive evidence that the main fracture plane in the cell wall lies within the interior of the outer membrane. This and other evidence showed that the corresponding layers in thin sections and freeze-etched preparations are: the additional layer, cw 1; the outer membrane, cw (2 + 3); and the intermediate and dense layers together from cw 4. Because of similarities in structure between this Acinetobacter and other gram-negative bacteria, it seemed probable that the interior of the outer membrane is the plane most liable to fracture in the cell walls of most gram-negative bacteria.

Freeze-etching of the cell envelope of an Acinetobacter species which carries a regular array of surface subunits

Freeze-etching was applied to preparations, with and without glycerol, of Acinetobacter sp. strain MJT/F5/199A, consisting of intact cells after normal growth or after incubation with chloramphenicol, spheroplasts, and isolated cell walls and outer membranes. Etched preparations show that a regular array of subunits forms the surface of normal cells. Near the zones of constriction in dividing cells, blebs and irregularities are seen, and some blebs, consisting of both surface subunits and outer membrane, are released from the cells. The cross-fractured cell envelope shows four layers which are related to the structures seen in section as follows: cw1, which is not visible in section, contains the surface subunits; cw2 consists of all or part of the outer membrane; cw3 includes the intermediate and dense, peptidoglycan-containing layers; within these cell wall layers is the plasma membrane. Internal fracture of the plasma membrane occurs under all conditions tested, but the fracture plane in the cell wall is only revealed in chloramphenicol-treated cells or normal cells freeze-fractured with glycerol present; the characteristic fracture faces are not seen in spheroplasts or isolated outer membranes. The concave fracture face cw2 consists of densely packed granules, while the convex face cw3 is fibrillar. The probable location of this fracture plane is discussed. After incubation with chloramphenicol, the outer surface of the cells is obscured by extracellular material, the dense peptidoglycan-containing layer is increased in thickness, and the cytoplasm contains rounded bodies bounded by one or more unit membranes.

Microflora of soil as viewed by freeze-etching

The indigenous microflora of soil were released from the soil materials and concentrated without the occurrence of growth by use of a blending-simple centrifugation procedure. The cell concentrate was then frozen-etched and viewed by transmission electron microscopy. Criteria were established for detecting microbial cells among the residual soil debris. The freeze-etching of the soil cell concentrate provided results on cell size distributions in agreement with those obtained by thin sectioning. However, the blending-simple centrifugation procedure for cell release and concentration from soil allowed the observation of large cells (>/=1.0 mum in diameter) which apparently are missed by the "exhaustive centrifugal washing" cell separation-concentration procedure. The procedure of blending-simple centrifugation combined with the viewing of frozenetched preparations allowed evaluations of the soil microflora for cellular diameters, length-width ratios, shapes, and structure.