Contact with epithelial cells induces the formation of surface appendages on Salmonella typhimurium

The enteric bacteria Salmonella typhimurium has the ability to invade (enter) nonphagocytic cells. The internalization process occurs as a result of an intimate interaction between the bacteria and the host cell, in which S. typhimurium triggers a cascade of host cell-signaling events leading to the formation of host cell membrane ruffles and bacterial uptake. Using high resolution scanning electron microscopy, we have observed that contact with cultured epithelial cells results in the formation of appendages on the surface of S. typhimurium. The formation of such appendages did not require de novo protein synthesis, and it was transient, since these surface structures were no longer present on bacteria that had initiated the internalization event. Salmonella mutants defective in the transient formation of these surface organelles were unable to enter into cultured epithelial cells, indicating that such structures are required for bacterial internalization.

Quantitative studies of invasion of rabbit ileal mucosa by Salmonella typhimurium strains which differ in virulence in a model of gastroenteritis

An asymmetric organ culture system in which ileal tissues, freshly removed from rabbits, can be maintained structurally and functionally for up to 4 h has been developed. The composition of the solutions used to maintain ileal tissue in vitro were as follows. The serosal surface was bathed in the World Health Organization (WHO) rehydration formulation: NaCl, 60 mM; NaHCO3, 30 mM; KCl, 20 mM; and glucose, 111 mM. The mucosal surface was bathed in the same solution with two important changes: all the sodium was replaced by choline, which is not absorbed, and tissue culture medium (consisting of commercial minimal essential medium to which was added fetal calf serum and glutamine to final concentrations of 10% [vol/vol] and 2.0 mM, respectively) was added to the choline-containing medium to a final concentration of 10% (vol/vol). The initial invasiveness (first 2 h) of seven strains of Salmonella typhimurium differing in virulence (defined in terms of clinical origin or the ability to induce fluid loss in monkeys or rabbit ileal loops) was assessed quantitatively in an in vitro invasion assay with the organ culture system. The virulent strains (TML, W118, and WAKE) were found to be about 25- to 100-fold more invasive than the avirulent strains (SL1027, M206, LT7, and Thax-1). Thus, a clear correlation between initial mucosal invasion and virulence of S. typhimurium in a model which is relevant to human gastroenteritis was established. This is the first time, to our knowledge, that quantitative studies of invasiveness have been carried out in vitro on freshly isolated functioning gut.

Electron holographic observation of thin biological filaments

A non-staining bacterial flagellum filament only 24 nm in diameter has been observed clearly by using a phase-shifting electron holographic technique. This method produces a sufficiently high contrast in-focus image that could never be obtained by ordinary transmission electron microscopic (TEM) methods.

Isolation, characterization and structure of bacterial flagellar motors containing the switch complex

A putative complex of the three switch proteins, FliG, FliM and FliN appears to be directly involved in torque generation and control of direction of rotation. We have developed a preparative procedure for flagellar motors that retains these proteins as evidenced by Western blots using anti-FliG, anti-FliM and anti-FliN antibodies. Immunogold labeling with these three antibodies shows that the three switch proteins are localized to the motor. Electron micrographs of frozen-hydrated preparations reveal a large, new component we have termed the "C ring complex" attached to the cytoplasmic face of the M ring. In a three-dimensional reconstruction of the cylindrically averaged structure, the M-S ring complex appears thicker and wider by the addition of extra material to the cytoplasmic surface of the M ring. In addition, extending into the cytoplasm from the thickened M ring is the C ring complex, a thin-walled cylinder having a length of 170 A and an outer diameter of 450 A compared to the 290 A diameter of the M ring. We provide evidence that the thickened M ring contains FliG and that the C ring complex may contain FliM and FliN. The large diameter of the C ring complex may permit interaction with the M ring and with the circlet of studs thought to be the MotA/MotB complex.

Purification and characterization of distinct type of mannose-sensitive fimbriae from Salmonella typhimurium

The fimbriae (E4) of a virulent strain of Salmonella typhimurium were purified by ion exchange chromatography in an FPLC system. They had a channelled appearance under transmission electron microscope and showed a major structural subunit of 17-kDa on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The purified fimbriae were found to agglutinate guinea pig erythrocytes, but this effect was inhibited in presence of D-mannose. Immune sera raised against the Mono-Q purified fimbriae (E4) showed cross-reactivity with the type-1 fimbriae (F1) composed of 21-kDa fimbrin subunit, purified by a different method from the same strain.

Effect of sorbitan monolaurate polyoxyalkylene (Tween 20) on the ultrastructure of some bacteria

The ultrastructure of Listeria monocytogenes, Salmonella typhimurium, Pseudomonas pseudomallei and Pseudomonas aeruginosa treated with 0.5 and 1% Tween 20 was studied by means of transmission and scanning electron microscopy. Specific changes in the cell wall and cytoplasmic membrane were observed.

Sensing structural intermediates in bacterial flagellar assembly by export of a negative regulator

The ability of a regulatory protein to sense the integrity of the bacterial flagellar structure was investigated. In response to a defective hook-basal body complex, the anti-sigma 28 FlgM protein inhibits flagellin transcription. In cells with a functional hook-basal body complex, the flagellin genes are transcribed normally and the FlgM protein is expelled into the growth medium. In strains with a defective hook-basal body structure, FlgM is absent from the media. The presence of flagellin protein in the media is substantially reduced in strains carrying a FlgM-LacZ protein fusion, suggesting that the fusion is blocking the flagellar export apparatus. These results suggest that the FlgM protein assesses the integrity of the flagellar hook-basal body complex by itself being a substrate for export by the flagellar-specific export apparatus.

Salmonella induces the formation of filamentous structures containing lysosomal membrane glycoproteins in epithelial cells

Salmonella species invade and replicate within epithelial cells in membrane-bound vacuoles. In this report we show that upon infection of HeLa epithelial cells, Salmonella typhimurium residues in vacuoles that contain lysosomal membrane glycoproteins (lgps). Four to six hours after invasion, intracellular bacteria induce the formation of stable filamentous structures containing lgps that are connected to the bacteria-containing vacuoles. Formation of these lgp-rich structures requires viable intracellular bacteria and is blocked by inhibitors of vacuolar acidification. These structures are not present in uninfected cells or in cells infected with another invasive bacteria, Yersinia enterocolitica. Tracers added to the extracellular medium are not delivered to the Salmonella-induced filaments, suggesting that these structures are different from previously described tubular lysosomes. Initiation of intracellular bacterial replication correlates with formation of these lgp-containing filaments. Certain avirulent Salmonella mutants that are defective for intracellular replication fail to induce formation of these structures. These observations suggest that Salmonella-induced filaments containing lgps are linked to intracellular bacterial replication.

Chromosomal domains of supercoiling in Salmonella typhimurium

The chromosomes of enteric bacteria are divided into about 50 independently supercoiled domains. It is not known whether the net level of DNA supercoiling is similar in each domain, or whether the domains are differentially supercoiled. We have addressed this question genetically, using a supercoiling-sensitive promoter to probe the relative levels of supercoiling at defined points around the Salmonella typhimurium chromosome. We conclude that, within the limits of resolution of this approach, the level of supercoiling does not differ significantly between chromosomal domains, and that each domain responds in a similar fashion to factors that perturb supercoiling. These findings have implications for the organization of the bacterial genome.

The effect of sugars on the morphology of the bacterial flagellum

Using dark-field microscopy, we have found that certain sugars cause the normal-to-curly helical transition of bacterial flagella. Titration of flagella isolated from Salmonella typhimurium with 16 different carbohydrates showed that: (i) only certain sugars cause the transition. There is no obvious relationship between the simple physico-chemical properties of the sugar and whether the sugar causes the transition or not; (ii) the efficacies of sugars that do cause the transition differ markedly. For these sugars there is a relationship between efficacy and molecular size. These results suggest that the specific, though weak, binding of sugars to sites on flagella cause the morphological transition.

Inhibition of adhesion of enteroinvasive pathogens to human intestinal Caco-2 cells by Lactobacillus acidophilus strain LB decreases bacterial invasion

Salmonella typhimurium and enteropathogenic Escherichia coli (EPEC) were found to adhere to the brush border of differentiated human intestinal epithelial Caco-2 cells in culture, whereas Yersinia pseudotuberculosis and Listeria monocytogenes adhered to the periphery of undifferentiated Caco-2 cells. All these enterovirulent strains invaded the Caco-2 cells. Using a heat-killed human Lactobacillus acidophilus (strain LB) which strongly adheres both to undifferentiated and differentiated Caco-2 cells, we have studied inhibition of cell association with and invasion within Caco-2 cells by enterovirulent bacteria. Living and heat-killed Lactobacillus acidophilus strain LB inhibited both cell association and invasion of Caco-2 cells by enterovirulent bacteria in a concentration-dependent manner. The mechanism of inhibition of both adhesion and invasion appears to be due to steric hindrance of human enterocytic pathogen receptors by whole-cell lactobacilli rather than to a specific blockade of receptors.

Flagellar growth in a filament-less Salmonella fliD mutant supplemented with purified hook-associated protein 2

Bacterial flagellum consists of a basal body, a hook, HAP1 (hook-associated protein 1), HAP3, a long helical filament, and a cap (composed of HAP2), all connected in series. The mutant deficient in the HAP2 structural gene (fliD) of Salmonella typhimurium has flagella composed of only hook-HAP1-HAP3 and excretes flagellin monomers into the culture medium. However, when purified HAP2 was added to this mutant, the flagellin stopped leaking out and flagellar filaments grew. Turnover of HAP2 was not necessary for the growth of a filament. Therefore HAP2 facilitates the polymerization of endogenous flagellin, apparently without falling off the filament tip. This experimental system with exogenous HAP2 allowed us to synchronize filament growth; the average rate of filament growth can be estimated by measuring the length of grown filaments at various time periods in electron micrographs. The initial growth rate was about 30 nm/min, which corresponds to one flagellin per second.

Temperature-sensitive mutations in the phage P22 coat protein which interfere with polypeptide chain folding

Temperature-sensitive mutations in the coat protein of phage P22 severely restrict formation of infectious particles at restrictive temperature. A set of 25 temperature-sensitive strains, which had been localized to regions of the coat gene (Casjens, S., Eppler, K., Sampson, L., Parr, R., and Wyckoff, E. (1991) Genetics 127, 637-647), define 17 sites of single amino acid substitutions by DNA sequencing. Particles assembled from the mutant proteins at permissive temperature were not thermolabile at restrictive temperature, nor defective in the infectious process. At restrictive temperature, ts mutant polypeptide chains were synthesized at near wild-type rates. These inactive chains were not degraded within the cells. The ts chains failed to interact with scaffolding proteins to form the procapsid precursor shell, and they did not polymerize with each to form aberrant shells. Rather, the mutant coat protein accumulated as insoluble aggregates, with the amorphous morphology of inclusion bodies. The results indicate that the chains fail to reach the conformation needed for subunit-subunit or subunit-scaffolding interaction. These mutations appear to be of the class of temperature-sensitive folding mutations, which destabilize an intermediate in the intracellular folding pathway.

Nucleotide sequence of a 13.9 kb segment of the 90 kb virulence plasmid of Salmonella typhimurium: the presence of fimbrial biosynthetic genes

The 90kb plasmid resident in Salmonella typhimurium confers increased virulence in mice by promoting the spread of infection after invasion of the intestinal epithelium. The nucleotide sequence of a 13.9kb segment of this plasmid known to encode an outer membrane protein related in sequence to components of fimbrial biosynthesis in enteric bacteria was determined. This cloned segment between the repB and repC replicon regions programmed expression of abundant surface fimbriae in Escherichia coli and S. typhimurium cells. A 7kb region contained seven open reading frames, the protein products of five of which were related in sequence to regulatory, structural, and assembly proteins of adherence fimbriae/pili, such as the P and K88 pili. These five genes and two adjacent ones which were not markedly related to proteins in the data bases comprise the pef (plasmid-encoded fimbriae) locus. Transposon TnphoA insertions in four genes in the pef locus (pefA, pefC, orf5 and orf6) resulted in active PhoA fusions and blocked or reduced the surface presentation of fimbriae, indicating that the proteins encoded by these four genes are translocated at least across the cytoplasmic membrane and contribute to formation of the fimbrial structure. The differences in genetic organization and protein sequence relatedness from other fimbrial gene clusters suggest that the pef locus might encode a novel type of fimbria. Between the pef and the repB loci, there were five open reading frames, one of which (orf8) gave rise to active PhoA fusions but was not necessary for fimbrial expression. Two of the other proteins were homologous to transcription regulatory proteins and a third was the rck gene, which encodes an outer membrane protein that confers complement resistance to serum-sensitive hosts.

Binding of an Escherichia coli double-stranded DNA virus PRD1 to a receptor coded by an IncP-type plasmid

IncP plasmid RP1 Tra regions are needed to assemble the receptor for lipid-containing double-stranded DNA bacteriophage PRD1 on the cell surface. Using radioactively labeled phage and electron microscopic techniques, we showed that the surfaces of Salmonella typhimurium(RP1) and Escherichia coli(RP1) cells contained approximately 50 and 20 PRD1 binding sites, respectively. Expression of the receptor was growth phase dependent and was highest at late logarithmic or early stationary phase. The PRD1-resistant RP1 transposon mutants isolated were all Tra-, and the transposons were located in both the Tra1 and Tra2 regions.

Effect of low concentrations of quinolone antibiotics on bacterial virulence mechanisms

Recent studies have shown that exposure to quinolone antibiotics at or below the minimal inhibitory concentration (MIC) results in reduction in the level of production or total elimination of certain factors that contribute to the virulence of bacteria. This study was designed to determine whether low concentrations of enoxacin, lomefloxacin, and ciprofloxacin altered the morphology or affected the production of various virulence factors in several different genera of bacteria. The factors studied were nuclease and a toxin production in Staphylococcus aureus, cell size, pili and fimbriae production, and adherence of Salmonella typhimurium, Escherichia coli, and Pseudomonas aeruginosa to urinary epithelial cells and dog bladder cells, and the major virulence factor in Yersinia pseudotuberculosis. In addition, the effect of growth in low levels of enoxacin on phagocytosis of S. aureus by human polymorphonuclear leukocytes (PMNs) was studied. Following exposure to subinhibitory levels of quinolones tested, significant reduction in activity or complete elimination was seen in all of those factors measured. Minor differences were noted in the efficiency of elimination among the three quinolones tested. At as low as 1/8 MIC there is significant enhancement of phagocytic activity by human PMNs. These data suggest that exposure to quinolones at concentrations below the MIC disrupts the regulatory mechanisms that control cell morphology, adherence as well as exocellular enzyme production and plasmid maintenance. This may mean that certain virulent organisms that survive exposure to quinolone antibiotics may be less likely to produce or maintain the disease state in susceptible hosts.

Observation of biological materials by X-ray photoelectron-conversion contact microscopy

Dried biological specimens, such as fossil diatoms, collagen, nerve tissue and spicule of Trepang, were observed by X-ray photoelectron-conversion contact microscopy. A spatial resolution of 0.2 microns was attained. The fossil diatom image shows a clear difference below and above the carbon K-absorption edge (4.46 nm).

Ultrastructure of the surface film of bacterial colonies

The structure of the surface of colonies of various Gram-negative and Gram-positive bacteria was examined by transmission electron microscopy. The results indicate that bacterial colonies in the course of their development produce a film which becomes thicker with increased duration of growth. The basic part of the film is an elementary membrane, which is a stable structure with a large surface area. The inner and outer surfaces of the film membrane are covered by amorphous layers. These layers are thicker in the surface film of Gram-negative bacterial colonies than in those of Gram-positive bacteria. Membrane vesicles from the bacterial colonies take part in the formation of the surface film. The presence of the film on the surface of the colonies of different bacteria suggests that this structure may play an important role.

[Electron microscopic studies of fimbriae and lectin phagocytosis of Salmonella typhimurium variety copenhagen (STMVC)]

On Salmonella typhimurium variatio copenhagen (STMVC) strains, isolated from pigeons with acute Salmonellosis, two types of fimbriae can be identified, depending on different growth conditions. In addition to the common 7 nm fimbriae we were able to demonstrate in the electron microscope, thin, curled fimbriae 3 nm in diameter which are characterized by a mannose-resistant hemagglutination. The simultaneous expression of both types of fimbriae on a single bacterial cell can be induced by transferring a microcolony from agar medium (3 nm fimbriae), to the surface of a nutrient broth (7 nm fimbriae) and continuing the incubation. One selected strain expresses only the thin fimbriae on nearly all the bacteria on an agar medium. These thin fimbriae seem to play a role in the lectinophagocytosis in macrophage cultures. The attachment of Salmonellae mediated by these fimbriae as well as the internalization of fimbriated cells by macrophages is shown in the electron microscope and discussed in respect to infection and immunization.

Size of the export channel in the flagellar filament of Salmonella typhimurium

The size of the putative export channel in the bacterial flagellar filament appears small (25 A) in studies done by electron microscopy but large (60 A) in studies done by X-ray diffraction. We have undertaken additional studies by electron microscopy to examine some of the possible causes of the difference. A comparison of three-dimensional image reconstructions of native and reconstituted filaments rules out the presence or absence of flagellin monomers in the export channel as the source of the variation in apparent channel size. The channel seen in reconstructions from both kinds of filaments is 25 A in diameter. The difference in the previous studies is more probably a result of artifacts introduced in either the X-ray or the electron microscopical methodology. Comparisons of three-dimensional reconstructions from images of filaments embedded in various stains (anionic, cationic and neutral) and in ice, taken at a range of defocuses, rule out the two most likely sources of artifact in electron microscopy (i.e., staining artifacts and defocus phase contrast). Based on these studies we suggest that the channel seen in the image reconstructions is free of exported flagellin monomers, that its true diameter is about 25 A, and, therefore, that the flagellin monomer must be unfolded to pass along it.

Domain organization of the subunit of the Salmonella typhimurium flagellar hook

The deduced amino acid sequences of the family of axial proteins of the bacterial flagellum possess N and C-terminal heptad repeats of hydrophobic amino acid residues, which suggests that these proteins all fold to form bundles of alpha-helices (e.g. coiled coils). There is evidence that flagellin, which is one of the axial proteins, has an axially oriented bundle of alpha-helices that gives rise to the inner, rod-shaped domains seen in electron density maps. We present evidence that a second member of the family, the hook subunit, also has such an axially oriented, rod-shaped domain. In three-dimensional reconstructions from electron micrographs of the helical hook of Salmonella typhimurium, the rod-shaped domain has a diameter of 18 A, which is that expected for a coiled coil. The corresponding domain in the flagellin subunit of the filament, however, is larger, having a diameter of 24 A suggesting a bundle of three or more alpha-helices. In addition to the rod-shaped domain, the hook has two other domains. At a radius of 55 A is the middle spheroidal domain about 25 A in diameter and at a radius of 75 A is the outer ellipsoidal domain about 20 A by 30 A by 40 A. The flagellin subunit also has a middle and an outer domain although they appear different from those of the hook. This is no doubt a result of the lack of any sequence similarity of the hook and flagellin subunits, apart from the N and C-terminal heptad repeats. Along the hook axis, there is a 25 A wide channel, which presumably serves in the export of hook and flagellin subunits in the assembly of the filament. There is a comparably sized channel in the filaments as deduced from electron micrographs. Thus, electron microscopy consistently finds a small channel, whereas in X-ray diffraction studies of the filament, the channel size appeared to be about 60 A. At a diameter of 60 A, the channel could pass the flagellin or hook subunit in its completely folded state, but if the channel is only 25 A in diameter, the subunit would have to be at least partially unfolded in order to pass through the channel.

Radial mass density functions of vitrified helical specimens determined by scanning transmission electron microscopy: their potential use as substitutes for equatorial data

Using STEM dark field images, we have determined linear mass densities and radial density profiles of vitrified helical particles. The samples studied are: TMV, RNA-free helical polymers of TMV coat protein (TMV-P), Salmonella typhimurium bacterial flagellar filaments and Escherichia coli pili. The difference between the profiles obtained for TMV and TMV-P shows a maximum at a radius of about 4 nm, corresponding to the RNA in TMV. Of the peaks that are resolved in X-ray diffraction analysis we can resolve the ones for TMV at radii of approximately 4.2 and approximately 6.7 nm and a shoulder at approximately 7.8 nm. Density peaks in bacterial flagellar filaments appear at radii of approximately 4.2, approximately 6.5, approximately 8.5, and approximately 10.5 nm. Accurate mass data can be obtained if the filaments are embedded in ice layers of uniform thickness; their diameters need to be similar to that of the mass standard (TMV) when these data are measured in a comparative manner. Ice layers are often not uniform, and thickness variations are well revealed in STEM dark field. The signal-to-noise ratio and contrast for the transverse projections are lower than those measured for freeze-dried specimens: half an order and one order of magnitude, respectively. The thinnest uniformly thick ice layer still containing a single layer of particles is approximately 10-15 nm thicker than the particles. Radial mass density functions that are directly determined in STEM may have a potential use as substitutes for the unreliable equatorial data in helical reconstructions of TEM bright field images of vitrified specimens.

Acetyladenylate or its derivative acetylates the chemotaxis protein CheY in vitro and increases its activity at the flagellar switch

CheY, a key protein in the mechanism of bacterial chemotaxis, is known to interact with the flagellar switch and thereby cause clockwise rotation. This activity of CheY was significantly increased by producing acetyladenylate (AcAMP) within cytoplasm-free bacterial envelopes containing purified CheY. This was achieved by including in the envelopes the enzyme acetyl-CoA synthetase (ACS) and ATP, and adding acetate externally. The fraction of clockwise-rotating envelopes, tethered to glass by their flagella, increased from 14% to 58% by the presence of AcAMP (or its derivative). In parallel experiments carried out with [14C]acetate under similar conditions, CheY became acetylated: [1-14C]acetate was as effective as [2-14C]acetate in labeling CheY, and ACS-dependent labeling of CheY by [alpha-32P]ATP was not detected. The switch proteins, FliG, FliM, and FliN, isolated to purity, were not acetylated. The acetylation was specific for CheY and dependent on its native conformation. The acetylated form the CheY was estimated to be more active than its nonacetylated form by 4-5 orders of magnitude. Acetylated CheY was stable in the presence of the strong nucleophiles hydroxylamine or ethanolamine, indicative of N-acetylation. There was a correlation between the activity of CheY in vivo and its ability to be acetylated in vitro. Thus, proteins with a single substitution at their active site, CheY57DE and CheY109KR, are not active in vivo and accordingly were not acetylated in vitro; in contrast, the protein CheY13DK is active in vivo and was normally acetylated in vitro. The possibility that CheY acetylation plays a role in bacterial chemotaxis is discussed.

Processing images of helical structures: a new twist

Helical macromolecular assemblies are particularly difficult to study by X-ray diffraction but are quite well suited to analysis by electron microscopy. Most of our information about helical macromolecular assemblies has come from the electron microscope but has been limited to about 25 A resolution. With the use of low-dose electron cryomicroscopy, one can obtain structural data to near atomic resolution on two-dimensional crystals, but the problem is to extract the information from the noise. In this paper we present methods to extract signal from low-dose electron cryomicrographs of helically symmetric structures. We apply these methods to extract 10 A data from the bacterial flagellar filament.

Conformational switching in the flagellar filament of Salmonella typhimurium

The flagellar filament of the mutant Salmonella typhimurium strain SJW814 is straight, and has a right-handed twist like the filament of SJW1655. Three-dimensional reconstructions from electron micrographs of ice-embedded filaments reveal a flagellin subunit that has the same domain organization as that of SJW1655. Both show slight changes from the domain organization of the subunits from SJW1660, which possesses a straight, left-handed filament. This points to the possible role of changes in subunit conformation in the left-to-right-handed structural transition in filaments. Comparison of the left and right-handed filaments shows that the subunit's orientation and intersubunit bonding appear to change. The orientation of the subunit in the SJW814 filament is intermediate between that of SJW1655 and SJW1660. Its intermediate orientation may explain why the filaments of SJW1655 and SJW1660 are locked in one conformation, whereas the filament of SJW814 can be induced to switch by, for example, changes in pH and ionic strength.