Immuno-electron microscopic localization of lipopolysaccharide antigens on ultrathin sections of Salmonella typhimurium

The ultrastructural morphology of endotoxins and lipopolysaccharides

Endotoxins and LPS are constituents unique to the outer surface of gram-negative bacteria. Cell-associated endotoxins are now readily observable on the cell outer membrane with labelled monoclonal antibodies. These probes are not only more specific than those used in the past, but also easier to see. Interest in free endotoxin as a method to generate outer membrane proteins without contamination with other cell constituents is also increasing (Gamazo and Moriyon, 1987). The morphologic identification and characterization of LPS by electron microscopy has been facilitated recently by advances in chemical extraction and purification techniques. LPS, originally thought to be heterogenous, exists in forms that are dependent upon (1) the method of its extraction, (2) its chemical composition, and (3) the physical or chemical conditions of its environment. New models were proposed on the arrangement of LPS molecules in molecular aggregates (i.e. discs, vesicles or ribbons) and a schematic was presented on the dissociation from one morphologic type to another. Morphologic studies on endotoxins and LPS will continue in the future. Using molecular biological techniques, carbohydrate epitopes of LPS from one bacterial species will be expressed with increasing frequency in other bacterial species (Manning et al., 1986; Stein et al., 1988). Electron microscopy will help visualize the distribution of the 'new' LPS on the recipient cell surface. Labelled monoclonal antibodies will also differentiate host cell LPS from the recombinant LPS. As molecular model programming becomes more complex, new schematics will help visualize the arrangement of LPS in membranes to explain recombinant LPS structure as well as other characteristics (i.e. membrane permeability to various antibiotics).

Outer-membrane permeability of bacteria

Gram-negative bacteria evolved to survive under the conditions in which a number of hazardous compounds are abundant. The outer membrane which protects the cell interior acts as a barrier against such hazardous agents, yet the cells must incorporate the chemicals that are essential for the cellular activity. The devices that Gram-negative bacteria developed to incorporate such essence are the transmembrane pores. These pores could be subdivided into three categories: (1) pore made of porins has a weak solute selectivity; (2) pore made of lamB protein and tsx proteins hold intermediate solute specificity. and (3) pores for the diffusion of vitamin B12 and ferric ion-chelator complexes have a tight solute specificity. Porins are identified from a number of Gram-negatives and from the outer membrane of mitochondria of various sources. Studies on the diffusion properties of these outer-membrane proteins provided essential information to understand membrane transports.

Comparative immunoelectron microscopy with monoclonal antibodies on yellow fever virus-infected cells: pre-embedding labelling versus immunocryoultramicrotomy

To study the intra- and extracellular distribution of yellow fever virus 17D (YFV)-specific antigens, pre-embedding immunoelectron microscopy (IEM) and IEM on ultrathin frozen sections were carried out comparatively using monoclonal antibodies (MAB) and YFV-infected cells. In addition, three electron-dense marker systems (IgG-ferritin and IgG-gold and protein A-gold) were compared for their efficiency in detecting bound MAB. Pre-embedding immuno-labelling was performed in microtest plates followed by in situ embedding and immunocryoultramicrotomy was performed using pellets of sucrose-infused cells. In both procedures, cells were prefixed with different concentrations of glutaraldehyde (GA). In pre-embedding IEM virus-specific antigens could be detected on the envelopes of extracellular virions with YFV-neutralizing MAB. Using immunocryoultramicrotomy, neutralizing MAB bound to intracellular mature virions as well as to viral antigens incorporated into cytoplasmic membranes. A concentration of 1% GA destroyed antigenicity entirely, while with 0.25% and 0.1% GA immunoreactivity was retained for more than 3 mth. Some highly reactive MAB labelled antigen significantly in pre-embedding IEM, when used at concentrations of 1 ng/ml. Immunocryoultramicrotomy was 10-100 times less sensitive. On cryosections colloidal gold was the marker of choice, due to the fact that it showed less nonspecific sticking to intracellular components and that it was easily detectable on highly contrasted cryosections. Owing to their higher sensitivity, IgG-ferritin conjugates were preferred in pre-embedding IEM.

Immunohistochemical localization of barley stripe mosaic virions in infected wheat cells

Barley stripe mosaic virus particles were localized in ultrathin sections with colloidal gold-labeled specific IgG or antiserum followed by gold-labeled goat anti-rabbit IgG. On the average, 1.5 gold particles were attached per virus rod. A statistical analysis of counts of gold and virus particles showed that the staining procedure was highly reproducible from experiment to experiment and after several independently prepared colloidal gold solutions. The procedure should be useful for the intracellular localization of any protein to which an antibody can be prepared.

An intermediate step in translocation of lipopolysaccharide to the outer membrane of Salmonella typhimurium

Evidence for transient localization of newly synthesized lipopolysaccharide at the periplasmic face of the inner membrane has been obtained by immunoelectron microscopic techniques. Salmonella typhimurium galE mutants in which O-antigen synthesis is dependent on addition of exogenous galactose were employed, and the distribution and fate of pulse-synthesized O antigen was examined by indirect ferritin labeling with anti-O-antigen IgG of spheroplasts prepared by treatment with lysozyme/EDTA. O-reactive lipopolysaccharide appeared rapidly at the exposed periplasmic face of the inner membrane after addition of galactose and was rapidly depleted upon termination of the pulse. Control experiments showed that secondary redistribution of lipopolysaccharide from outer membrane did not occur under the conditions employed for spheroplast formation and immunolabeling, and the pulse-chase kinetics were consistent with those expected for an intermediate in translocation of lipopolysaccharide to the outer membrane. In addition, undecaprenol-linked O antigen was detectable at the periplasmic face of the inner membrane within 30 sec after addition of galactose to a galE deep rough double mutant, and it accumulated stably in that location. The mutation in synthesis of the lipopolysaccharide core in the deep rough strain prevents transfer of O-antigen chains from undecaprenol phosphate to lipopolysaccharide. The result suggests that attachment of O antigen to lipopolysaccharide occurs on the extracytoplasmic side of the inner membrane and supports the conclusion that lipopolysaccharide is translocated to the outer membrane from the periplasmic, rather than the cytoplasmic, face of the inner membrane.

Advances in ultrastructural postembedment localization of antigens in Epon sections with peroxidase labelled antibodies

Procedures for ultrastructural postembedment immunolocalization of antigens were investigated by use of the indirect peroxidase labelled antibody method. Results were compared with those obtained with the ultrastructural preembedment technique. IgG globulins in IgG synthesizing cells of rat lymph nodes served as model. Formaldehyde fixation, ethanol dehydration and embedment in Epon 812 did not abolish immunoreactivity. Sufficient numbers of antigens remained for subsequent postembedment immunohistology. Antigen binding sites were readily localized in ultrathin sections. For this purpose, polymerized resin had to be partially removed. Sodium methoxide in methanol/benzene mixture rapidly dissolved ultrathin sections. Diluted alcoholic sodium hydroxide enabled reliable resin etching and subsequent immunostaining. Treatment of ultrathin sections with hydrogen peroxide alone, did not permit immunolocalization of IgG. Optimal antigen detection was attained with antibodies isolated by affinity chromatography and purified peroxidase conjugates. IgG was stained in the rough-surfaced endoplasmic reticulum, the perinuclear space and the Golgi apparatus; the subcellular distribution corresponded to that obtained with preembedment immunohistology. In the latter technique, substrate accumulation was more homogenous than in postembedment staining.

Diagnostic virology using electron microscopic techniques

This chapter illustrates the development of the use of electron microscopy in viral diagnosis. The field covered is confined to medical viral diagnosis, but parallel developments have taken place in both veterinary and botanical fields and techniques derived from both these sources are also included where relevant. It is reported that the scanning transmission mode of operation, which can induce image contrast changes electronically, may enhance studies with unstained sections and perhaps facilitate thin section immune electron microscopy (IEM). The application of negative stain IEM has been particularly useful for the study of the antigenic nature of some of the newly discovered noncultivable viruses. Viral antigens can also be detected in thin sections of infected cells by IEM with suitably labeled specific antibodies. Confirmation of viral infection by electron microscopy on tissues originally processed for light microscopy is also frequently useful.

Asymmetric localization of lipopolysaccharides on the outer membrane of Salmonella typhimurium

Treatment of intact cells of Salmonella typhimurium with galactose oxidase (EC 1.1.3.9) resulted in an extensive oxidation of the susceptible galactose residues in the lipopolysaccharides. Comparison with the extent of oxidation in isolated lipopolysaccharides indicated that most lipopolysaccharide molecules were located in the outer leaflet of the outer membrane in intact cells.