Cytoskeletal composition of attaching and effacing lesions associated with enteropathogenic Escherichia coli adherence to HeLa cells

The cytoskeletal lesions associated with enteropathogenic Escherichia coli adhering to cultured HeLa epithelial cells were examined by immunofluorescence microscopy. The microfilament-associated proteins actin, alpha-actinin, talin, and ezrin were localized with adherent enteropathogenic E. coli, whereas tropomyosin, keratin and vimentin (intermediate filaments), tubulin (microtubules), and vinculin were not localized. These cytoskeletal structures differed significantly from those associated with Salmonella typhimurium internalization (invasion).

Haemophilus influenzae lipopolysaccharide disrupts confluent monolayers of bovine brain endothelial cells via a serum-dependent cytotoxic pathway

An in vitro blood-brain barrier (BBB) model consisting of primary cultures of bovine brain microvascular endothelial cells was used to examine the effect of Haemophilus influenzae type b (Hib) on the BBB. Whole bacteria and purified lipopolysaccharide (LPS; greater than 10 ng/ml) caused marked cytotoxicity on the bovine brain endothelial cells. This effect could be completely blocked by polymyxin B. Similar cytotoxic effects were observed with a cultured bovine pulmonary endothelial cell line. Serum was essential for the LPS-mediated cytotoxic effect, and human, horse, bovine, or fetal calf serum all had similar effects. The serum factor was not a complement component. A monoclonal antibody against CD14, a receptor involved in mediating the effect of LPS in monocytes, completely blocked the cytotoxic effect in both brain and pulmonary endothelial cells. These results suggest that Hib LPS disrupts an in vitro BBB model via a serum- and CD14-dependent pathway and that LPS has cytotoxic effects on bovine endothelial cells without the involvement of monocytic cells, an effect that may be important in gram-negative meningitis and in endotoxic shock.

Intracellular replication is essential for the virulence of Salmonella typhimurium

Salmonella typhimurium is a facultative intracellular parasite, capable of penetrating, surviving, and multiplying within diverse eukaryotic cell types, including epithelial and phagocytic cells. We have been studying intracellular replication of S. typhimurium and found that it is essential in the pathogenesis of this bacterium. A total of 45,000 independent mini-Mu MudJ transposon mutants in S. typhimurium SL1344 were screened in Madin-Darby canine kidney (MDCK) epithelial cells with a beta-lactam, cefotaxime, to enrich for mutants defective for intracellular replication. Ten different auxotrophic (purine, pyrimidine, purine/methionine, and valine/isoleucine) and three prototrophic replication-defective mutants (Rep-) were identified. All Rep- mutants showed no differences in aerobic and anaerobic growth patterns, motility, serum sensitivity, mouse macrophage survival, iron uptake, and phosphate requirements. All Rep- mutants were unable to multiply inside MDCK, HeLa, and Caco-2 epithelial cells. When required nutrients for various auxotrophs were supplemented, auxotrophs then replicated inside MDCK cells. Although the parental strain multiplies in large vacuoles inside MDCK cells that distort the host cells, MDCK cells infected with the Rep- mutants appeared relatively normal and few bacteria were seen inside vacuoles. The purine auxotrophs and the three prototrophic Rep- mutants were highly attenuated in mice, and oral and intraperitoneal LD50 levels were 3 to 4 orders of magnitude higher than the wild type level. The three prototrophs were invasive and persisted in the murine organs such as livers and spleens for at least 3 weeks. Therefore, these prototrophic genes are needed for intracellular replication and are essential to the virulence of S. typhimurium.

In vitro interaction of a polypeptide homologous to human Ro/SS-A antigen (calreticulin) with a highly conserved amino acid sequence in the cytoplasmic domain of integrin alpha subunits

We endeavored to identify proteins interacting with KLGFFKR, a highly conserved motif in the cytoplasmic domain adjacent to the transmembrane domain of the alpha subunit of integrins. We found that affinity chromatography of cell extracts with this peptide followed by elution with EDTA resulted in the isolation of a 60-kDa protein (p60). The N-terminal amino acid sequence of this 60-kDa polypeptide was found to be highly homologous to the Ro/SS-A antigen, a 60-kDa protein homologous to calreticulin and Aplysia "memory molecule". The binding of p60 was found to be specific for the KLGFFKR sequence since this polypeptide did not bind to a peptide with a scrambled amino acid sequence (KLRFGFK), and it was also specifically eluted from the KLGFFKR affinity matrix ith soluble KLGFFKR peptide but not with the scrambled peptide. Solid phase in vitro binding assays demonstrated specific interaction of p60 with integrin alpha 3 and alpha 5 subunits but not with the beta 1 subunit. Furthermore, p60 could be copurified with alpha 3 beta 1 following coincubation in vitro. These interactions could be inhibited by KLGFFKR peptide and also by EDTA, indicating sequence-specific and divalent cation dependent binding. Despite the fact that calreticulin is thought to be localized in the endoplasmic reticulum, a pool of Ro/SS A antigen homologous 60-kDa polypeptide was found to be present in the soluble cytoplasm, indicating the feasibility of an interaction of p60 with the integrin alpha subunits. Our data suggest that p60 (Ro/SS-A Ag) can specifically bind to integrin alpha subunits via the highly conserved KLGFFKR amino acid sequence.

Symmetric infection of rotavirus on polarized human intestinal epithelial (Caco-2) cells

When rotavirus infects the mature villus tip cells of the small intestine, it encounters a highly polarized epithelium. In order to understand this virus-cell interaction more completely, we utilized a cell culture-adapted rhesus rotavirus (RRV) to infect human intestinal (Caco-2) and Madin-Darby canine kidney (MDCK-1) polarized epithelial cells grown on a permeable support. Filter-grown Caco-2 cells and MDCK-1 cells, producing a transepithelial resistance of 300 to 500 and greater than 1,000 omega . cm2, respectively, were infected from either the apical or basolateral domain with RRV or Semliki Forest virus. Whereas Semliki Forest virus infection only occurred when input virions had access to the basolateral domain of MDCK-1 or Caco-2 cells, RRV infected MDCK-1 and Caco-2 monolayers in a symmetric manner. The effect of rotavirus infection on monolayer permeability was analyzed by measuring the transepithelial electrical resistance. Rotavirus infection on filter-grown Caco-2 cells caused a transmembrane leak at 18 h postinfection, before the development of the cytopathic effect (CPE) and extensive virus release. Electrical resistance was completely abolished between 24 and 36 h postinfection. Although no CPE could be detected on RRV-infected MDCK cells, the infection caused a transmembrane leak that totally abolished the electrical resistance at 18 to 24 h postinfection. Cell viability and the CPE analysis together with immunohistochemistry and immunofluorescence data indicated that the abolishment of resistance across the monolayer was due not to an effect on the plasma membrane of the cells but to an effect on the paracellular pathway limited by tight junctions. Attachment and penetration of rotavirus onto Caco-2 cells caused no measurable transmembrane leak during the first hour of infection.

Characterization of the oriT region of the IncFV plasmid pED208

DNA sequence analysis of a 2.2kb EcoRI-HindIII fragment from pED208, the derepressed form of the IncFV plasmid Folac, revealed sequences highly homologous to the oriT region, traM, and traJ genes of other IncF plasmids. The TraM protein was purified and immunoblots of fractionated cells containing pED208 or Folac showed that TraM was predominantly in the cytoplasm. Using DNA retardation assays and the DNase I footprinting technique, the TraM protein was found to bind to three large motifs in the oriT region: (I) an inverted repeat, (II) two direct repeats, and (III) the traM promoter region. These three footprint regions contained a Hinfl-like sequence (GANTC) that appeared 16 times, spaced 11-12 bp (or multiples thereof) apart, suggesting that TraM protein binds in a complex manner over this entire region.

Characterization of a Salmonella choleraesuis mutant that cannot multiply within epithelial cells

A mutant of Salmonella choleraesuis was identified that could invade (enter) and penetrate through polarized monolayers of Caco-2 and MDCK epithelial cells at normal levels but was defective for intracellular multiplication within these cells. It was also able to survive inside cultured J774 macrophage cells. These bacteria remained inside membrane-bound vacuoles, which coalesced at later times in the perinuclear region of the epithelial cell. This mutant exhibited slightly slower growth rates in rich or minimal media than the parental strain but was normal for iron usage, phosphate usage, and anaerobic growth and was a prototroph. The mutant was completely avirulent when administered orally or intravenously to susceptible mice. These results suggest that the ability to multiply within eukaryotic cells may contribute to S. choleraesuis virulence.

Cytoskeletal rearrangements accompanying salmonella entry into epithelial cells

Salmonella bacteria can enter (invade) eukaryotic cells, and exist as intracellular parasites. Confocal, light immunofluorescence and electron microscopy were used to examine various cytoskeletal components of cultured Madin Darby canine kidney (MDCK) and HeLa epithelial cells after infection with Salmonella typhimurium. These bacteria entered and remained within membrane-bound vacuoles and were surrounded by large (5–10 microns) dense structures composed of various cytoskeletal components. These structures consisted of extensive aggregations of polymerized actin, alpha-actinin and tropomyosin above and beside the invading bacterium in both epithelial cell lines. These structures were evident soon after bacterial addition (maximal at 20 min for HeLa cells, 60 min for MDCK cells), and disappeared later in the infection as the cytoskeletal components returned to a more normal distribution after bacterial internalization. Surprisingly, tubulin also aggregated above internalized Salmonella although bacterial entry or penetration through polarized monolayers was not disrupted by the microtubule-inhibiting agent nocadazole (this treatment actually enhanced tubulin accumulation around these organisms). There were little if any rearrangements in intermediate filaments composed of keratin or vimentin. Large amounts of talin also accumulated above and around invading Salmonella, but there was only a minor accumulation of vinculin around a few organisms. Pretreatment of epithelial cells with the microfilament inhibitor cytochalasin D blocked bacterial internalization but did not prevent accumulation of polymerized actin and alpha-actinin directly beneath uninternalized bacteria, yet prevented accumulation of the other cytoskeletal components. These results suggest that Salmonella bind to the surface and trigger a signal in epithelial cells that causes marked rearrangements in various cytoskeletal components, including recruitment of actin filaments and alpha-actinin, which then generates the force necessary for bacterial uptake.

Salmonella interactions with polarized human intestinal Caco-2 epithelial cells

Polarized monolayers of the human intestinal epithelial Caco-2 cell line were grown on permeable filters and infected apically with either Salmonella choleraesuis or Salmonella typhimurium. Both Salmonella species penetrated through the monolayer, requiring 2 h before appearing in the basolateral medium. Both species caused a loss in transepithelial resistance by 3-4 h, and the monolayer's integrity was completely disrupted by 6 h. Scanning and transmission electron microscopy revealed that the bacteria interacted with well-defined apical microvilli and caused disruptions in the brush border, including elongation and denuding of the microvilli. The cytoplasm was also disrupted locally, with blebs protruding from the apical surface. The bacteria entered (invaded) these cells and were enclosed in membrane-bound vacuoles within the cytoplasm. By 6 h there were many bacteria within most Caco-2 cells, and these organisms caused serious cytopathic consequences. These morphologic observations correlated well with animal infection models, indicating that this in vitro system will be useful to study pathogens that interact with human intestinal epithelia.

Alternative methods of attenuating Salmonella species for potential vaccine use

Because of their invasive properties, Salmonella species are uniquely suited for use as live oral vaccines, and mutants attenuated in general metabolic functions show promise as vaccine strains. We have identified a Salmonella choleraesuis transposon mutant which is deficient in abilities to attach to, invade or penetrate through animal cells. This mutant is avirulent in mice, yet is able to persist within the liver, spleen and gut-associated lymphoid tissue for periods which may be adequate for induction of secretory, humoral and cell-mediated immunity. This and other strains attenuated in factors specifically required for Salmonella pathogenesis may have potential as vaccine strains.

Salmonella as an intracellular parasite

Salmonella species are facultative intracellular parasites, capable of penetrating (invading), surviving, and often multiplying within diverse eukaryotic cell types, including epithelial and phagocytic cells. These processes are essential for virulence, and involve both bacterial and host cell products. The use of cultured eukaryotic cells and other model systems has facilitated the study of bacterial-host cell interactions, and has led to a better understanding of the genetic and molecular basis of Salmonella pathogenicity.

Common themes in microbial pathogenicity

A bacterial pathogen is a highly adapted microorganism which has the capacity to cause disease. The mechanisms used by pathogenic bacteria to cause infection and disease usually include an interactive group of virulence determinants, sometimes coregulated, which are suited for the interaction of a particular microorganism with a specific host. Because pathogens must overcome similar host barriers, common themes in microbial pathogenesis have evolved. However, these mechanisms are diverse between species and not necessarily conserved; instead, convergent evolution has developed several different mechanisms to overcome host barriers. The success of a bacterial pathogen can be measured by the degree with which it replicates after entering the host and reaching its specific niche. Successful microbial infection reflects persistence within a host and avoidance or neutralization of the specific and nonspecific defense mechanisms of the host. The degree of success of a pathogen is dependent upon the status of the host. As pathogens pass through a host, they are exposed to new environments. Highly adapted pathogenic organisms have developed biochemical sensors exquisitely designed to measure and respond to such environmental stimuli and accordingly to regulate a cascade of virulence determinants essential for life within the host. The pathogenic state is the product of dynamic selective pressures on microbial populations.

Epithelial cell surfaces induce Salmonella proteins required for bacterial adherence and invasion

Salmonella bacteria are capable of entering (invading) and multiplying within eukaryotic cells. Stable adherence to and invasion of epithelial cells by S. choleraesuis and S. typhimurium were found to require de novo synthesis of several new bacterial proteins. This inducible event appears to be a coordinately regulated system dependent on trypsin- and neuraminidase-sensitive structures present on the epithelial cell surface. Mutants of S. choleraesuis and S. typhimurium were unable to synthesize these proteins and did not stably adhere to nor invade eukaryotic cells. Two such S. typhimurium mutants were avirulent in mice, an indication that these proteins are required for Salmonella virulence.

Passage of Salmonella through polarized epithelial cells: role of the host and bacterium

Salmonella are intracellular parasites which enter their hosts by penetrating the intestinal epithelial barrier. We examined the interaction of S. choleraesuis and S. typhimurium with Madin Darby canine kidney (MDCK) and human larynx (HEp-2) epithelial cells to characterize bacterial adherence, invasion and penetration through epithelial monolayers. Epithelial cell microfilaments were required for bacterial internalization and surrounded the bacteria as they were internalized. The bacteria entered membrane-bound vacuoles inside epithelial cells where they replicated. When polarized MDCK cell monolayers were infected, we found that Salmonella could pass through this barrier and enter medium bathing the opposite surface, although most bacteria remained within the monolayer. Synthesis of several Salmonella proteins was induced by the presence of epithelial cell surfaces, and these proteins were required for bacterial adherence and invasion. This induction was stimulated by trypsin- and neuraminidase-sensitive structures on epithelial cells.

Identification and characterization of TnphoA mutants of Salmonella that are unable to pass through a polarized MDCK epithelial cell monolayer

Surface protein mutants of the invasive Salmonella species, S. choleraesuis, were generated using the transposon TnphoA. 626 alkaline phosphatase (PhoA+) fusion mutants were identified and screened for their ability to pass through (transcytose) polarized epithelial monolayers of Madin Darby canine kidney (MDCK) cells grown on membrane filters. Forty two mutants were unable to pass through this barrier. All of these transcytosis mutants were unable to adhere to or invade MDCK monolayers, yet these mutations were not in the genes encoding type 1 pili or mannose-resistant haemagglutination (MRHA). These transcytosis mutants could be grouped into six classes. Class 1 mutants had altered lipopolysaccharide (LPS) O side-chain structures while Class 2 mutants had defects in their LPS core. Mutants belonging to Classes 5 and 6 did not decrease the transepithelial electrical resistance of polarized MDCK cell monolayers, in contrast to the parental strain and the other mutants (Classes 1, 2, 3 and 4). Mutants belonging to Class 1 were less virulent in mice, while Class 2 (defective core) and Classes 4 and 5 (normal LPS) mutant strains were avirulent in mice. Mutants from Classes 3 and 6 were as virulent in mice as S. choleraesuis. These results suggest that the ability to pass through epithelial barriers may be an important virulence characteristic of Salmonella. These data indicate that bacterial adherence, internalization and monolayer transcytosis are closely linked events. It was also demonstrated that a mutant with decreased rates of intracellular replication still passed through the monolayer at rates similar to wild-type S. choleraesuis.

Virulence factors associated with Salmonella species

Salmonella species can cause diseases ranging in severity from acute gastroenteritis to typhoid fever. These bacteria are considered as intracellular pathogens and produce several products which are required for entry and survival in the intracellular environment, in addition to factors necessary for existence in the gastrointestinal tract and the outside environment. The virulence factors used by these bacteria to cause these diseases are complex, and only recently have we begun to characterize these factors and determine the contribution they make to Salmonella virulence.

Comparison of the invasion strategies used by Salmonella cholerae-suis, Shigella flexneri and Yersinia enterocolitica to enter cultured animal cells: endosome acidification is not required for bacterial invasion or intracellular replication

Strains of Escherichia, Salmonella, Shigella and Yersinia actively enter eukaryotic cells. Several techniques were used to compare and contrast the invasion mechanisms of Salmonella cholerae-suis, Yersinia enterocolitica and Shigella flexneri. Three animal cell lines (CHO, HEp-2 and MDCK) were examined for susceptibility to bacterial entry by these strains. Levels of intracellular bacteria varied widely between cell lines, but CHO cells were the most susceptible to bacterial invasion, HEp-2 invasion levels were intermediary, whereas polarized MDCK cells were invaded to a lesser extent. This illustrates that tissue culture models can be optimized to study bacterial invasion and intracellular replication. We used these tissue culture models to examine the interactions between host cells and these invasive bacteria. The use of lysosomotropic agents (methylamine and ammonium chloride), cationic ionophores (monensin) and acidification-defective CHO cell lines demonstrated that endosome acidification is not required for bacterial invasion or intracellular replication. Drugs which inhibited microfilament formation (cytochalasins B and D) prevented internalization of S. cholerae-suis, Y. enterocolitica and S. flexneri, indicating that invasion is a microfilament-dependent event. The microtubule inhibitors, colchicine, vincristine and vinblastine, did not affect bacterial internalization.

Two unusual pilin sequences from different isolates of Pseudomonas aeruginosa

The pilin genes of two Pseudomonas aeruginosa strains isolated from two different patients with cystic fibrosis were cloned and sequenced. The predicted protein sequences of these two pilins had several unusual features compared with other published P. aeruginosa pilin sequences.

Penetration of Salmonella through a polarized Madin-Darby canine kidney epithelial cell monolayer

Many intracellular parasites are capable of penetrating host epithelial barriers. To study this process in more detail we examined the interactions between the pathogenic bacteria Salmonella choleraesuis and polarized epithelial monolayers of Madin-Darby canine kidney (MDCK) cells grown on membrane filters. Association of bacteria with the MDCK cell apical surface was an active event, requiring bacterial RNA and protein synthesis, and was blocked by low temperatures. Salmonella were internalized within a membrane-bound vacuole and exhibited penetration through, but not between MDCK cells. A maximum of 14 Salmonella per MDCK cell crossed the monolayer per hour to the basolateral surface yet the monolayer remained viable and impermeable to Escherichia coli. Apical S. choleraesuis infection resulted in an increase in paracellular permeability but the MDCK intercellular contacts were not significantly disrupted. Basolateral S. choleraesuis infection was inefficient, and only small numbers of S. choleraesuis penetrated to the apical medium.

The expression of Pseudomonas aeruginosa PAK pilin gene mutants in Escherichia coli

Previous work has demonstrated the expression of the cloned pilin gene of Pseudomonas aeruginosa PAK within Escherichia coli and has pinpointed this protein's localization exclusively to the cytoplasmic membrane (Finlay et al., 1986). To define regions of the pilin subunit necessary for its stability and transport within E. coli, we constructed six mutants of the pilin gene and studied their expression and localization using a T7 promoter system. Two of the mutants have either a 4- or 8-amino-acid deletion at the N-terminus and both were stably expressed and transported primarily to the cytoplasmic membrane of E. coli. The other four mutants are C-terminal truncations having between 36 and 56 amino acids of the N-terminal region of the unprocessed pilin. Studies with these truncated mutants revealed that only the first 36 residues of the unprocessed pilin subunit were required for insertion into the E. coli membrane.

Nucleotide sequence of the tra YALE region from IncFV plasmid pED208

The pED208 plasmid is a 90-kilobase conjugative plasmid which is the derepressed form of Fo lac plasmid (IncFV). A 3.3-kilobase HindIII-PstI fragment from the pED208 plasmid was cloned and sequenced and was found to contain four open reading frames which were highly homologous to the traA, traL, traE, and traY gene products of the F plasmid. The pED208 traA propilin protein was 119 amino acids in length, consisting of a leader sequence of 55 amino acids and a mature pilin subunit of 64 residues. The leader sequence contained a hydrophobic region followed by a classic signal peptidase cleavage site (Ala-Ser-Ala-55). F and pED208 pilin proteins shared 27 conserved residues and had similar predicted secondary structures. The pED208 traA and traL genes were separated by a single base pair, and no ribosome binding site preceded the traL gene. The pED208 traY gene contained an IS2 insertion element in orientation II 180 nucleotides (60 residues) upstream of the traY stop codon. This insertion of IS2 resulted in a predicted fusion peptide of 69 residues for traY which may provide the observed traY activity. Since IS2 is absent in the wild-type plasmid, Fo lac, derepression and concomitant multipiliation may be due to the insertion of IS2 providing constitutive expression of the pED208 tra operon.

Origin of transfer of IncF plasmids and nucleotide sequences of the type II oriT, traM, and traY alleles from ColB4-K98 and the type IV traY allele from R100-1

The complete nucleotide sequences of the ColB4-K98 (ColB4) plasmid transfer genes oriT, traM, and traY as well as the traY gene of R100-1 are presented and compared with the corresponding regions from the conjugative plasmids F, R1, and R100. The sequence encoding the oriT nick sites and surrounding inverted repeats identified in F was conserved in ColB4. The adenine-thymine-rich sequence following these nick sites was conserved in R1 and ColB4 but differed in F and R100, indicating that this region may serve as the recognition site for the traY protein. A series of direct repeats unique to the ColB4 plasmid was found in the region of dyad symmetry following this AT-rich region. This area also encodes 21-base-pair direct repeats which are homologous to those in F and R100. The traM gene product may bind in this region. Overlapping and following these repeats is the promoter(s) for the traM protein. The traM protein from ColB4 is similar to the equivalent products from F, R1, and R100. The traY protein from ColB4 is highly homologous to the R1 traY gene product, while the predicted R100-1 traY product differs at several positions. These differences presumably define the different alleles of traM and traY previously identified for IncF plasmids by genetic criteria. The translational start codons of the ColB4 and R100-1 traY genes are GUG and UUG, respectively, two examples of rare initiator codon usage.

Nucleotide sequences of five IncF plasmid finP alleles

The nucleotide sequences of five finP alleles from various IncF plasmids (finP types I to V) as well as of three finP mutations were determined and compared. The finP gene specificity could be attributed to a variable, six-to-seven-nucleotide loop located between inverted repeats, and the sequence data were consistent with the product of finP being an RNA molecule rather than a protein. The finP mutations interrupted a proposed finP promoter or destabilized a predicted stem-and-loop structure in the finP RNA molecule.