Transmission-scanning electron microscopic observations of selected Eikenella corrodens strains

Probing bacterial interactions: integrated approaches combining atomic force microscopy, electron microscopy and biophysical techniques

Recent developments in the application of Atomic Force Microscopy (AFM) and other biophysical techniques for the study of bacterial interactions and adhesion are discussed in the light of established biological and microscopic approaches. Whereas molecular-biological techniques combined with electron microscopy allow the identification and localization of surface constituents mediating bacterial interactions, with AFM it has become possible to actually measure the forces involved in bacterial interactions. Combined with the flexibility of AFM in probing various types of physical interactions, such as electrostatic interactions, specific ligand-receptor interactions and the elastic forces of deformation and extension of bacterial surface polymers and cell wall, this provides prospects for the elucidation of the biophysical mechanism of bacterial interaction. However, because of the biochemical and a biophysical complexity of the bacterial cell wall, integrated approaches combining AFM with electron microscopy and biophysical techniques are needed to elucidate the mechanism by which a bacterium interacts with a host or material surface. The literature on electron microscopy of the bacterial cell wall is reviewed, with particular emphasis on the staining of specific classes of cell-wall constituents. The application of AFM in the analysis of bacterial surfaces is discussed, including AFM operating modes, sample preparation methods and results obtained on various strains. For various bacterial strains, the integration of EM and AFM data is discussed. Various biophysical aspects of the analysis of bacterial surface structure and interactions are discussed, including the theory of colloidal interactions and Bell's theory of cell-to-cell adhesion. An overview is given of biophysical techniques used in the analysis of the properties of bacterial surfaces and bacterial surface constituents and their integration with AFM. Finally, we discuss recent progress in the understanding of the role of bacterial interactions in medicine within the framework of the techniques and concepts discussed in the paper.

Contrast-enhanced immunoelectron microscopy for Helicobacter pylori

Since a method of contrast enhancement for immunoelectron microscopy has not been available in bacteriology, the morphological localization of proteins of Helicobacter pylori is not well known. In this report, we established a method of contrast enhancement in immunoelectron microscopy in this organism. Immunostained ultrathin sections are stained with a mixture of alcian blue and osmium tetroxide prior to staining with uranyl acetate. This method of staining provided good contrast enhancement of the bacterial cell wall and membrane without any loss of immunolabeled gold particles on the ultrathin section.

Eikenella corrodens phase variation involves a posttranslational event in pilus formation

The human pathogen Eikenella corrodens synthesizes type IV pili and exhibits a phase variation involving the irreversible transition from piliated to nonpiliated variants. On solid medium, piliated variants form small (S-phase), corroding colonies whereas nonpiliated variants form large (L-phase), noncorroding colonies. We are studying the molecular basis of this phase variation in the clinical isolate E. corrodens VA1. A genomic fragment encoding the major type IV pilin was cloned from the S-phase variant of strain VA1. Sequence analysis of the fragment revealed four tandemly arranged potential open reading frames (ORFs), designated pilA1, pilA2, pilB, and hagA. Both pilA1 and pilA2 predict a type IV pilin. The protein predicted by pilB shares sequence identity with the Dichelobacter nodosus FimB fimbrial assembly protein. The protein predicted by hagA resembles a hemagglutinin. The region containing these four ORFs was designated the pilA locus. DNA hybridization and sequence analysis showed that the pilA locus of an L-phase variant of strain VA1 was identical to that of the S-phase variant. An abundant pilA1 transcript initiating upstream of pilA1 and terminating at a predicted hairpin structure between pilA1 and pilA2 was detected by several assays, as was a less abundant read-through transcript encompassing pilA1, pilA2, and pilB. Transcription from the pilA locus was nearly indistinguishable between S- and L-phase variants. Electron microscopy and immunochemical analysis showed that S-phase variants synthesize, export, and assemble pilin into pili. In contrast, L-phase variants synthesize pilin but do not export and assemble it into pili. These data suggest that a posttranslational event, possibly involving an alteration in pilin export and assembly, is responsible for phase variation in E. corrodens.

Ruthenium red and the bacterial glycocalyx

Ruthenium red, a promising cationic reagent for electron microscopy (EM), has long been an important tool in histology. The reagent was initially used by botanists as a semispecific stain for pectic substances, but it has gradually been embraced by investigators in microbiology and the animal sciences as a stain for anionic glycosylated polymeric substances. Luft developed a reliable method and demonstrated that ruthenium red was a useful reagent for visualizing ultrastructural detail. Many investigators, using modifications of Luft's approach, have identified numerous applications for this important reagent. Ruthenium red has been used to show the ultrastructural detail of bacterial glycocalyces. Strong, sharp and consistent observations of this ultrastructural component of the bacterial cell have given a better understanding its fibrous anionic matrix. Any variations in staining owing to artifactual alteration of the fine delicate ultrastructural features have been overcome by incorporation of diamine lysine into ruthenium red methods, thus providing flexible processing times under less than ideal laboratory sampling conditions. Ruthenium red has broad utility in the biological sciences, and in combination with lysine, it is an excellent EM stain for enhanced visualization of bacterial glycocalyx from culture or from clinical specimens.

Purification and characterization of Eikenella corrodens type IV pilin

Eikenella corrodens is a gram-negative human pathogen associated with periodontal diseases and soft-tissue infections. Pilin was purified by association-dissociation and fast protein liquid chromatography; it had an apparent molecular mass of about 14.8 kDa and an N-terminal amino acid sequence reflective of type IV pilins. Antibodies to the purified protein reacted with pili on whole cells. This is the first report of purification of type IV pili/pilin from this organism. Other type IV pili are important virulence factors; we are currently investigating the biological role of pili in E. corrodens.

Scanning electron microscopy of Eikenella corrodens colony morphology variants

Eikenella corrodens is a gram-negative, human pathogen which exhibits colony morphology variation. Scanning electron microscopy (SEM) was used to examine large (non-corroding) and small (corroding) colony variants from the type strain (ATCC 23834) and two clinical isolates (strains VA1 and CM1). Large colonies were large, flatter, and appeared relatively featureless compared to small colonies and had even, smooth colony margins. Small colonies were more raised from the medium surface, and often had a central raised region surrounded by flatter border. Cells on the surface of large colonies were more regularly arranged at the colony edge, and end-to-end rows of cells around the colony were seen in some strains. Cells in the center of the upper surface of small colonies were usually randomly arranged. Within cross sections of small colonies, cells were arranged randomly or perpendicular to the medium; in large colonies, cells were random or arranged horizontally. Amorphous, "slime" material was often seen covering groups of cells in large and small variants. An unusual variant, possibly a mutant, which combined features of both colony types was isolated.

Sequence divergence in two tandemly located pilin genes of Eikenella corrodens

Eikenella corrodens normally inhabits the human respiratory and gastrointestinal tracts but is frequently the cause of abscesses at various sites. Using the N-terminal portion of the Moraxella nonliquefaciens pilin gene as a hybridization probe, we cloned two tandemly located pilin genes of E. corrodens 31745, ecpC and ecpD, and expressed the two pilin genes separately in Escherichia coli. A comparison of the predicted amino acid sequences of E. corrodens 31745 EcpC and EcpD revealed considerable divergence between the sequences of these two pilins and even less similarity to EcpA and EcpB of E. corrodens type strain ATCC 23834. EcpC from E. corrodens 31745 displayed high degrees of homology to the pilins of Neisseria gonorrhoeae and Pseudomonas aeruginosa. EcpD from E. corrodens 31745 showed the highest homologies with the pilin of one of the three P. aeruginosa classes, whereas EcpA and EcpB of strain ATCC 23834 most closely resemble Moraxella bovis pilins. These findings raise interesting questions about potential genetic transfer between different bacterial species, as opposed to convergent evolution.

Comparison of alcian blue and ruthenium red effects on preservation of outer envelope ultrastructure in methanotrophic bacteria

Alcian blue (AB) and ruthenium red (RR) effects on ultrastructural preservation of the bacterial cell envelope of methanotrophs are compared. A previous successful method with RR that enhanced preservation of outer envelope layers in two representative methanotroph species is applied to other genera and species of methanotropic bacteria. Alcian blue is substituted for RR in this en bloc protocol. The effect of AB on preservation of these layers is assessed at the ultrastructural level and compared to RR for all species examined. Further, comparison with freeze etch and a fixation in the absence of either RR or AB is made. Both RR and AB are found to aid preservation and help visualize additional components of the cell envelope which are lost or minimized in a standard fixation not employing these cationic reagents. For some species, images obtained are similar between RR and AB procedures and agree with images seen by freeze etch. For other species, AB preserves extended filamentous material that is partially condensed even with the use of RR. Thus, use of AB improves the preservation of outer envelope structure in these organisms equally or more effectively than use of RR.

Variable sensitivity of oral isolates of Eikenella corrodens to serum bactericidal activity: role of antibody

Eikenella corrodens is a facultatively anaerobic Gram-negative bacterium which is among the predominant cultivable microflora of periodontal lesions characterized by loss of attachment level. In the present study, we examined the potential role of complement-mediated killing in host defense against this periodontopathic organism. Seven clinical isolates obtained from human subgingival plaque and one reference strain of E. corrodens were characterized with respect to (a) susceptibility to the bactericidal properties of pooled human serum and (b) the role of the classical and/or alternative pathway(s) of complement in effecting killing of sensitive strains. Six strains, including the reference strain, were found to be variably serum-sensitive, exhibiting 1-12.5% survival after two hr of incubation in the presence of 20% pooled human serum. The remaining two isolates were serum-resistant. Both serum-resistant and serum-sensitive strains consumed complement via the classical pathway in normal but not in hypogammaglobulinemic serum, thus ruling out an antibody-independent mechanism of classical pathway activation. Four of six serum-sensitive strains exhibited little or no loss of viability following incubation with serum depleted of the classical pathway component Clq. One strain which was resistant to killing by normal human serum was, nevertheless, highly susceptible to complement-mediated killing in the presence of rabbit immune serum. Two additional serum-sensitive strains were killed, albeit to a lesser extent, in Clq-depleted serum, indicative of a role of the alternative pathway in killing of some serum-sensitive strains. These results indicate a potential role for complement-mediated killing in host defense against Gram-negative periodontal bacteria such as E. corrodens. However, the ultimate contribution of this immune defense mechanism may be defined, at least in part, by the presence of a humoral response to key bacterial membrane constituents.

Ruthenium red staining of vaginal epithelial cells and adherent bacteria

The vaginal epithelium of the rhesus monkey is a keratinized stratified squamous epithelium throughout the menstrual cycle and early pregnancy. The superficial cells have a thickened cell envelope, surface microridges, and numerous adherent bacteria. During later pregnancy the cells mucify and have a typical cell membrane, microvilli, and no adherent bacteria. In the present study we have extended these observations by examining vaginal surface structures after ruthenium red staining. Throughout the cycle, the superficial cells have a thin layer of stained material closely associated with the cell membrane, but in some cases a much thicker mucous blanket was observed. During later pregnancy the epithelial cells had a moderately thick, somewhat clumped ruthenium red-positive material associated with the cell membrane. Glycocalyx components of the surface of many bacteria also stained with ruthenium red. The adherence of many types of bacteria to the vaginal epithelial cells appears to be effected by the interaction of polyanionic components on the surface of both the bacterial and epithelial cells.

Isolation and characterization of the outer membrane and lipopolysaccharide from Eikenella corrodens

The chemical composition of the outer membrane fractions (OMFs) of Eikenella corrodens strains 23834 and 470 as well as the strain 23834 lipopolysaccharide (LPS) was determined. The OMFs were obtained by Triton X-100 treatment of the heavier membrane fraction from sucrose density centrifugation of the total membrane fraction. The resulting OMFs of strains 23834 and 470, free of cytoplasmic membrane components, were found to contain 69.6 and 75.0% (wt/wt) protein, 4.8 and 9.2% lipid, 4.6 and 4.7% carbohydrate, and 2.0 and 4.6% muramic acid, respectively. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis both OMFs contained one major peptide determined to be 33,500 daltons for the strain 23834 OMF, and 37,500 daltons for the strain 470 OMF. Analysis of the OMF fatty acids revealed hexadecanoic, hexadecenoic, octadecenoic, and lesser amounts of octadecanoic acids. Transmission electron microscopic examination of the OMFs revealed typical large sheets of membrane. Structures (10 nm in diameter) resembling pores were also evident. The E. corrodens LPS was found to be composed of 34.5% (wt/wt) carbohydrate and 25.0% lipid A. Only minute amounts of 2-keto-3-deoxyoctonate and heptose could be detected. Fatty acid analysis revealed primarily octadecanoic and hexadecanoic acids, with lesser amounts of octadecenoic acid. No hydroxy fatty acids were detected. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed the E. corrodens LPS to resemble other smooth-type LPSs. Transmission electron microscopic examination revealed a vesicle-like morphology. The E. corrodens LPS appears not to be a "classical," i.e., enteric, type of LPS.

Morphology and ultrastructure of oral strains of Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus

Selected human oral and nonoral strains of the genera Actinobacillus and Haemophilus were examined by transmission and scanning electron microscopy. The strains examined were morphologically identical to recognized Actinobacillus actinomycetemcomitans, Haemophilus aphrophilus, and Haemophilus paraphrophilus. By transmission electron microscopy, the cells were typically gram negative in morphology, with several strains possessing some extracellular ruthenium red-staining polymeric material. Numerous vesicular structures, morphologically identical to lipopolysaccharide vesicles, were seen to originate from and be continuous with the surface of the outer membrane. Large numbers of these vesicles were also found in the external environment. Scanning electron microscopic observations revealed that both actinobacilli and haemophili possessed surface projections and an amorphous surface material which connected and covered adjacent cells.