Identification of regions on a 230-kilobase plasmid from enteroinvasive Escherichia coli that are required for entry into HEp-2 cells

The Intriguing Evolutionary Journey of Enteroinvasive E. coli (EIEC) toward Pathogenicity

Among the intestinal pathogenic Escherichia coli, enteroinvasive E. coli (EIEC) are a group of intracellular pathogens able to enter epithelial cells of colon, multiplicate within them, and move between adjacent cells with a mechanism similar to Shigella, the ethiological agent of bacillary dysentery. Despite EIEC belong to the same pathotype of Shigella, they neither have the full set of traits that define Shigella nor have undergone the extensive gene decay observed in Shigella. Molecular analysis confirms that EIEC are widely distributed among E. coli phylogenetic groups and correspond to bioserotypes found in many E. coli serogroups. Like Shigella, also in EIEC the critical event toward a pathogenic life-style consisted in the acquisition by horizontal gene transfer of a large F-type plasmid (pINV) containing the genes required for invasion, intracellular survival, and spreading through the intestinal mucosa. In Shigella, the ample gain in virulence determinants has been counteracted by a substantial loss of functions that, although important for the survival in the environment, are redundant or deleterious for the life inside the host. The pathoadaptation process that has led Shigella to modify its metabolic profile and increase its pathogenic potential is still in infancy in EIEC, although maintenance of some features typical of E. coli might favor their emerging relevance as intestinal pathogens worldwide, as documented by recent outbreaks in industrialized countries. In this review, we will discuss the evolution of EIEC toward Shigella-like invasive forms going through the epidemiology, including the emergence of new virulent strains, their genome organization, and the complex interactions they establish with the host.

Investigating the Relatedness of Enteroinvasive Escherichia coli to Other E. coli and Shigella Isolates by Using Comparative Genomics

Enteroinvasive Escherichia coli (EIEC) is a unique pathovar that has a pathogenic mechanism nearly indistinguishable from that of Shigella species. In contrast to isolates of the four Shigella species, which are widespread and can be frequent causes of human illness, EIEC causes far fewer reported illnesses each year. In this study, we analyzed the genome sequences of 20 EIEC isolates, including 14 first described in this study. Phylogenomic analysis of the EIEC genomes demonstrated that 17 of the isolates are present in three distinct lineages that contained only EIEC genomes, compared to reference genomes from each of the E. coli pathovars and Shigella species. Comparative genomic analysis identified genes that were unique to each of the three identified EIEC lineages. While many of the EIEC lineage-specific genes have unknown functions, those with predicted functions included a colicin and putative proteins involved in transcriptional regulation or carbohydrate metabolism. In silico detection of the Shigella virulence plasmid (pINV), which is essential for the invasion of host cells, demonstrated that a form of pINV was present in nearly all EIEC genomes, but the Mxi-Spa-Ipa region of the plasmid that encodes the invasion-associated proteins was absent from several of the EIEC isolates. The comparative genomic findings in this study support the hypothesis that multiple EIEC lineages have evolved independently from multiple distinct lineages of E. coli via the acquisition of the Shigella virulence plasmid and, in some cases, the Shigella pathogenicity islands.

Relationship among Shigella spp. and enteroinvasive Escherichia coli (EIEC) and their differentiation

Shigellosis produces inflammatory reactions and ulceration on the intestinal epithelium followed by bloody or mucoid diarrhea. It is caused by enteroinvasive E. coli (EIEC) as well as any species of the genus Shigella, namely, S. dysenteriae, S. flexneri, S. boydii, and S. sonnei. This current species designation of Shigella does not specify genetic similarity. Shigella spp. could be easily differentiated from E. coli, but difficulties observed for the EIEC-Shigella differentiation as both show similar biochemical traits and can cause dysentery using the same mode of invasion. Sequencing of multiple housekeeping genes indicates that Shigella has derived on several different occasions via acquisition of the transferable forms of ancestral virulence plasmids within commensal E. coli and form a Shigella-EIEC pathovar. EIEC showed lower expression of virulence genes compared to Shigella, hence EIEC produce less severe disease than Shigella spp. Conventional microbiological techniques often lead to confusing results concerning the discrimination between EIEC and Shigella spp. The lactose permease gene (lacY) is present in all E. coli strains but absent in Shigella spp., whereas β-glucuronidase gene (uidA) is present in both E. coli and Shigella spp. Thus uidA gene and lacY gene based duplex real-time PCR assay could be used for easy identification and differentiation of Shigella spp. from E. coli and in particular EIEC.

Recent evolution of bacterial pathogens: the gall-forming Pantoea agglomerans case

Pantoea agglomerans, a widespread epiphyte and commensal bacterium, has evolved into an Hrp-dependent and host-specific tumorigenic pathogen by acquiring a plasmid containing a pathogenicity island (PAI). The PAI was evolved on an iteron plasmid of the IncN family, which is distributed among genetically diverse populations of P. agglomerans. The structure of the PAI supports the premise of a recently evolved pathogen. This review offers insight into a unique model for emergence of new bacterial pathogens. It illustrates how horizontal gene transfer was the major driving force in the creation of the PAI, although a pathoadaptive mechanism might also be involved. It describes the crucial function of plant-produced indole-3-acetic acid (IAA) and cytokinines (CK) in gall initiation as opposed to the significant but secondary role of pathogen-secreted phytohormones. It also unveils the role of type III effectors in determination of host specificity and evolution of the pathogen into pathovars. Finally, it describes how interactions between the quorum sensing system, hrp regulatory genes, and bacterially secreted IAA or CKs affect gall formation and epiphytic fitness.

All blood, no stool: enterohemorrhagic Escherichia coli O157:H7 infection

Enterohemorrhagic Escherichia coli serotype O157:H7 is a pathotype of diarrheagenic E. coli that produces one or more Shiga toxins, forms a characteristic histopathology described as attaching and effacing lesions, and possesses the large virulence plasmid pO157. The bacterium is recognized worldwide, especially in developed countries, as an emerging food-borne bacterial pathogen, which causes disease in humans and in some animals. Healthy cattle are the principal and natural reservoir of E. coli O157:H7, and most disease outbreaks are, therefore, due to consumption of fecally contaminated bovine foods or dairy products. In this review, we provide a general overview of E. coli O157:H7 infection, especially focusing on the bacterial characteristics rather than on the host responses during infection.

The structures ofEscherichia coliO-polysaccharide antigens

Escherichia coli is usually a non-pathogenic member of the human colonic flora. However, certain strains have acquired virulence factors and may cause a variety of infections in humans and in animals. There are three clinical syndromes caused by E. coli: (i) sepsis/meningitis; (ii) urinary tract infection and (iii) diarrhoea. Furthermore the E. coli causing diarrhoea is divided into different 'pathotypes' depending on the type of disease, i.e. (i) enterotoxigenic; (ii) enteropathogenic; (iii) enteroinvasive; (iv) enterohaemorrhagic; (v) enteroaggregative and (vi) diffusely adherent. The serotyping of E. coli based on the somatic (O), flagellar (H) and capsular polysaccharide antigens (K) is used in epidemiology. The different antigens may be unique for a particular serogroup or antigenic determinants may be shared, resulting in cross-reactions with other serogroups of E. coli or even with other members of the family Enterobacteriacea. To establish the uniqueness of a particular serogroup or to identify the presence of common epitopes, a database of the structures of O-antigenic polysaccharides has been created. The E. coli database (ECODAB) contains structures, nuclear magnetic resonance chemical shifts and to some extent cross-reactivity relationships. All fields are searchable. A ranking is produced based on similarity, which facilitates rapid identification of strains that are difficult to serotype (if known) based on classical agglutinating methods. In addition, results pertinent to the biosynthesis of the repeating units of O-antigens are discussed. The ECODAB is accessible to the scientific community at http://www.casper.organ.su.se/ECODAB/.

Phenotypic Characterization ofipaH+ Escherichia coli Strains Associated with Yolk Sac Infection

Seventy-six Escherichia coli serotypes possessing the ipaH gene typical of enteroinvasive E. coli (EIEC) strains were characterized. Biochemical identification of our strains shows positive reactions for lactose fermentation (100% of strains), lysine decarboxylase (98.7% of strains) and motility (67.1% of strains), properties that do not correspond with those described to the EIEC group. The serotypes agree with an initial classification. In this, some common O antigens identified among ipaH+ strains were O2 (n=20), OR (n=11) and non-determined O? (n=10). The O2:NM serotype was the most common. Sixty-six percent (n=50) of the ipaH+ E. coli strains were colicin producers, of them, 26 (34%) produced Col V and other colicins, 13 (17%) produced colicins other than Col V, and 11 (14.5%) produced Col V only. Trimethoprim/Sulfa (72%), ampicillin (64.5%), enrofloxacin (55.3%), and ciprofloxacin (47.4%) were the major antimicrobial resistance frequencies observed. Twenty-five different multiresistance patterns were observed, where sixty-six strains (86.8%) were included. A MIC test showed that most of the strains were sensitive to low gentamicin and kanamycin concentrations, whereas most of the strains were resistant to tetracycline. An invasiveness assay showed that the predominant alterations caused to HEp-2 cells were changes in shape and staining, and in most of the specimens, a partial monolayer detachment was also seen. Fifteen strains invaded more than 30% of the monolayer cells, causing the formation of intercellular bridges or filipoidal-like protrusions. The results suggest the existence of specific clone complexes derived from EIEC strains adapted to the avian host. To our knowledge, this is the first study that demonstrates the presence of extraintestinal invasive E. coli (ExIEC) strains.

Invasive ability of an Escherichia coli strain isolated from the ileal mucosa of a patient with Crohn's disease

Crohn's disease (CD) is an inflammatory bowel disease in which Escherichia coli strains have been suspected of being involved. We demonstrated previously that ileal lesions of CD are colonized by E. coli strains able to adhere to intestinal Caco-2 cells but devoid of the virulence genes so far described in the pathogenic E. coli strains involved in gastrointestinal infections. In the present study we compared the invasive ability of one of these strains isolated from an ileal biopsy of a patient with CD, strain LF82, with that of reference enteroinvasive (EIEC), enteropathogenic (EPEC), enterotoxigenic (ETEC), enteraggregative (EAggEC), enterohemorrhagic (EHEC), and diffusely adhering (DAEC) E. coli strains. Gentamicin protection assays showed that E. coli LF82 was able to efficiently invade HEp-2 cells. Its invasive level was not significantly different from that of EIEC and EPEC strains (P > 0.5) but significantly higher than that of ETEC (P < 0.03), EHEC (P < 0. 005), EAggEC (P < 0.004) and DAEC (P < 0.02) strains. Strain LF82 also demonstrated efficient ability to invade intestinal epithelial cultured Caco-2, Intestine-407, and HCT-8 cells. Electron microscopy examination of infected HEp-2 cells revealed the presence of numerous intracellular bacteria located in vacuoles or free in the host cell cytoplasm. In addition, the interaction of strain LF82 with epithelial cells was associated with the elongation of microvillar extensions that extruded from the host cell membranes and engulfed the bacteria. This internalization mechanism strongly resembles Salmonella- or Shigella-induced macropinocytosis. The use of cytochalasin D and colchicine showed that the uptake of strain LF82 by HEp-2 cells was mediated by both an actin microfilament-dependent mechanism and microtubule involvement. In addition, strain LF82 survived for at least 24 h in HEp-2 and Intestine-407 cells and efficiently replicated intracellularly in HEp-2 cells. PCR and hybridization experiments did not reveal the presence of any of the genetic determinants encoding EIEC, EPEC, or ETEC proteins involved in bacterial invasion. Thus, these findings show that LF82, which colonized the ileal mucosa of a patient with CD, is a true invasive E. coli strain and suggest the existence of a new potentially pathogenic group of E. coli, which we propose be designated adherent-invasive E. coli.

Signal transduction pathways regulating differentiation and pathogenicity of Cryptococcus neoformans

The basidiomycetous yeast Cryptococcus neoformans is a human pathogen. Several phenotypes of this organism are defined as virulence traits including the polysaccharide capsule, melanin, and the ability to grow at 37 degreesC. The signaling pathways regulating the expression of these phenotypes and other important cellular processes are being defined on a molecular level. For example, the highly conserved signaling molecule calcineurin regulates high temperature growth in C. neoformans. A cryptococcal homolog of Saccharomyces cerevisiae STE12, the gene for a transcriptional regulator activated by the MAP kinase cascade, has also been identified. Additionally, the C. neoformans Galpha protein GPA1 and cAMP regulate mating, melanin production, encapsulation, and pathogenicity. This fungus is an excellent model to further dissect virulence-associated signaling pathways. The conserved role of Galpha proteins and cAMP-associated signaling pathways in fungal differentiation and pathogenicity is also reviewed.

Diarrheagenic Escherichia coli

Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler's diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (entero-pathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.

Calcineurin is required for virulence of Cryptococcus neoformans

Cyclosporin A (CsA) and FK506 are antimicrobial, immunosuppressive natural products that inhibit signal transduction. In T cells and Saccharomyces cerevisiae, CsA and FK506 bind to the immunophilins cyclophilin A and FKBP12 and the resulting complexes inhibit the Ca2+-regulated protein phosphatase calcineurin. We find that growth of the opportunistic fungal pathogen Cryptococcus neoformans is sensitive to CsA and FK506 at 37 degrees C but not at 24 degrees C, suggesting that CsA and FK506 inhibit a protein required for C. neoformans growth at elevated temperature. Genetic evidence supports a model in which immunophilin-drug complexes inhibit calcineurin to prevent growth at 37 degrees C. The gene encoding the C. neoformans calcineurin A catalytic subunit was cloned and disrupted by homologous recombination. Calcineurin mutant strains are viable but do not survive in vitro conditions that mimic the host environment (elevated temperature, 5% CO2 or alkaline pH) and are no longer pathogenic in an animal model of cryptococcal meningitis. Introduction of the wild-type calcineurin A gene complemented these growth defects and restored virulence. Our findings demonstrate that calcineurin is required for C. neoformans virulence and may define signal transduction elements required for fungal pathogenesis that could be targets for therapeutic intervention.

Characterization of Aeromonas trota strains that cross-react with Vibrio cholerae O139 Bengal

It has previously been shown that Vibrio cholerae O139 Bengal shares antigens with V. cholerae serogroups O22 and O155. We detected six surface water isolates of Aeromonas trota that agglutinated in polyclonal antisera to V. cholerae O139 and V. cholerae O22 but not in antiserum to V. cholerae O155. On the basis of agglutinin-absorption studies, the antigenic relationship between the cross-reacting bacteria were found to be in an a,b-a,c fashion, where a is the common antigenic epitope and b and c are unique epitopes. The antigen sharing between A. trota strains and V. cholerae O139 was confirmed in immunoblot studies. However, A. trota strains did not react with two monoclonal antibodies specific for V. cholerae O139 and, consequently, tested negative in the Bengal SMART rapid diagnostic test for V. cholerae O139 which uses one of the monoclonal antibodies. A polyclonal antiserum to a cross-reacting A. trota strain cross-protected infant mice against cholera on challenge with virulent V. cholerae O139. All A. trota strains were cytotoxic for HeLa cells, positive for adherence to HEp-2 cells, and weakly invasive for HEp-2 cells; one strain was heat-stable toxin positive in the suckling mouse assay; however, all strains were negative for cholera toxin-like enterotoxin. Studies on bacteria that share somatic antigen with V. cholerae O139 may shed further light on the genesis of V. cholerae O139.

Characterization of virulence genes of enteroinvasive Escherichia coli by TnphoA mutagenesis: identification of invX, a gene required for entry into HEp-2 cells

While enteroinvasive Escherichia coli (EIEC) and shigellae are genotypically nearly identical, a difference has been reported in the infective dose to humans: EIEC is 10,000-fold less infectious than shigellae. A possible basis for this difference lies in the inherent invasiveness of these bacteria toward epithelial cells. Thus, despite the high degree of homology between the invasion plasmids of EIEC and shigellae, substantial differences in genetic organization and/or sequence may exist. We have undertaken a systematic genetic analysis of the EIEC plasmid pSF204, using transposon mutagenesis. Congo red-negative TnphoA insertion mutants (Pcr- PhoA-) and TnphoA fusion mutants (PhoA+) were isolated and screened for the ability to invade cultured HEp-2 cells. Most invasion-negative (Inv-) mutations mapped to a 30-kb segment of the invasion plasmid, including homologs of the Shigella flexneri ipa, mxi, and spa genes. Inv- PhoA+ fusions in the EIEC ipaC, mxiG, mxiJ, mxiM, and mxiD homologs and in a proposed new gene, named invX, located downstream of the spa region were identified and characterized. This analysis indicates the presence of the ipaC, mxiG, mxiJ, mxiM, mxiD, and invX gene products in the EIEC cell envelope and demonstrates a strict requirement for these genetic loci in invasion. Overall, our results suggest a high degree of genetic, structural, and functional homology between the EIEC and S. flexneri large invasion plasmids.

Pathogenesis of Providencia alcalifaciens-induced diarrhea

Providencia alcalifaciens is a member of the family Enterobacteriaceae. There are reports that P. alcalifaciens can cause diarrhea, but the mechanism(s) by which it causes diarrhea is known. We studied P. alcalifaciens isolated from a child and two adults with diarrhea for enteropathogenicity. The three isolates did not exhibit any characteristic adherence to cultured HEp-2 cell monolayers, and they did not produce enterotoxins, cytotoxins, or keratoconjunctivitis in the Sereny test. Two isolates invaded cultured HEp-2 cell monolayers, producing localized bacterial clusters and actin condensation. The pattern of actin condensation was different from that produced by enteropathogenic Escherichia coli but similar to that produced by Shigella flexneri. Invasion and actin condensation were poor for the third isolate. Histology of adult rabbit small intestinal loops inoculated with all three isolates revealed bacterial attachment to, penetration of, and microulcer formation on the surface epithelium and hyperemia, edema, and polymorphonuclear cell infiltration of lamina propria. All the isolates produced diarrhea in rabbits with removable intestinal ties, and some of these rabbits developed hindlimb paralysis. Intestinal histology of the rabbits with removable intestinal ties which developed diarrhea showed changes similar to that in adult rabbits on which ileal loop assays had been performed. Transmission electron microscopy of intestinal tissues also confirmed tissue penetration by the isolates. Nerve tissue histology of two rabbits that developed hindlimb paralysis showed focal mononuclear cell infiltration around peripheral nerve sheaths. It is concluded that some strains of P. alcalifaciens are enteropathogenic and that they cause diarrhea by invading the intestinal mucosal epithelium. However, the relevance to human disease of the hindlimb paralysis observed in this animal model is not clear.

Characterization of an amorphous and soluble hemagglutinin from Yersinia pseudotuberculosis

Yersinia pseudotuberculosis which were screened out depending on auto-agglutination and Ca2+ dependency, were examined for their production of hemagglutinin (HA), and its purification and characterization were performed. The HA with a broad reactivity with various mammalian erythrocytes was recovered from the culture supernatant of these strains grown at 37 C but not 25 C. HAs from two strains, R148R and T1040, were purified by salt precipitation, gel filtration and anion-exchange chromatography by HPLC. Both purified HAs were cysteine-deficient acidic protein with an apparent molecular weight in the range of 15,000 to 16,000. N-terminal amino acid sequences of the first 25 residues were found to share 12% identity with that of afimbrial adhesin from enterotoxigenic Escherichia coli 2230. Immunoelectron microscopy and immunodiffusion test with polyclonal antiserum raised against the purified R148RHA demonstrated that the HA was associated with the amorphous aggregates which were detached from bacteria. These results suggest that the HA of Y. pseudotuberculosis belongs to a third type of HA produced by the yersinial species.

Adhesion to and invasion of cultured human cells by Bartonella bacilliformis

Bartonella bacilliformis was tested for its ability to adhere to and invade tissue culture cell monolayers. The parasite was able to efficiently bind and penetrate human dermal fibroblasts, human laryngeal epithelium, and human umbilical vein endothelial cells. Exposure of the organism to immune serum prepared against a crude Bartonella extract containing cell wall and membranous material resulted in decreased ability of the parasite to invade host cells. There was also an overall reduction in the invasiveness of bartonellae and total host cell association when human laryngeal epithelial cells and human umbilical vein endothelial cells were preexposed to cytochalasin D, indicating an active involvement of host cells in the uptake of bartonellae. Transmission electron microscopy revealed the presence of bartonellae inside and outside intracellular vacuoles. These data suggest that a surface-associated factor is involved in the invasion process and that internalization of the parasite by host cells involves a microfilament-dependent process similar to phagocytosis.

Binding of class II Escherichia coli enterotoxins to mouse Y1 and intestinal cells

The binding of class II Escherichia coli heat-labile enterotoxins (LT) to Y1 tissue-cultured cells and mouse intestinal cells was studied and compared with that of class I toxins, including cholera enterotoxin. All radioiodinated (125I) toxins retained their biological activities in both model systems, but only LTIIb could be shown to bind specifically to target cells. LTIIa could inhibit the binding of both class I and LTIIb toxins, a finding which correlates with its ability to bind to multiple gangliosides. LTIIb could not inhibit the binding of the other enterotoxins. The binding and activity of class II toxins could not be modulated by prior exposure of target cells to the B subunit of LTI.

Molecular cloning of epithelial cell invasion determinants from enterotoxigenic Escherichia coli

Although penetration of the epithelial mucosa has not been identified as a virulence mechanism in enterotoxigenic Escherichia coli (ETEC), we have found that this pathogen is capable of invading human intestinal cell lines. Classical ETEC strain H10407 was most invasive for epithelial cell lines derived from ileocecal and colonic tissues. An ETEC cosmid library was screened for clones that could direct E. coli HB101 to invade cultured human ileocecal epithelial cells (HCT 8 cells). Three invasive recombinant cosmid clones were isolated. These cosmids could direct HB101 invasion at an efficiency that was equal to or greater than that of the parent ETEC strain. The invasion cosmids also allowed for enhanced HCT 8 cell adherence by HB101. Electron micrographs of ETEC and recombinant HB101 strains revealed intracellular bacteria contained within endocytic vacuoles. Restriction endonuclease mapping and hybridization analyses showed that the three ETEC clones represent two separate invasion systems present in the parent ETEC strain and that both systems are chromosomally encoded. The parent ETEC strain and one cloned invasion system did not invade HeLa cells. Interestingly, one cloned invasion system was capable of directing HB101 to invade HeLa cells. Invasion of HCT 8 cells by recombinant HB101 strains and the parent ETEC strain was inhibited by cytochalasin D, indicating that the wild-type and both cloned invasion systems trigger an actin polymerization-dependent uptake process. It is not known whether the invasive phenotype of ETEC is relevant for enterotoxigenic disease. However, the parent ETEC strain, as well as recombinant HB101 strains, was capable of transcytosis through polarized HCT 8 monolayers. This transcytosis suggests that ETEC may cross the gut epithelium in vivo and that this invasion may have a previously unrecognized role in the disease process.

Survey of Escherichia coli septicemia over a six-year period

Escherichia coli was the most frequent species isolated from blood cultures in the Hospital Covadonga of Oviedo (Spain) over a six-year period (474 episodes, 15.3% of the total septicemias and 2.7 episodes per 1,000 patients). Escherichia coli strains were susceptible in greater than 95% of episodes to cefoxitin, cefotaxime, gentamicin, tobramycin and amikacin. In a series of 72 episodes, microbiological features and host factors were studied. No endemic stains were found. Type 1 fimbria was detected in 73.6% of strains and P-fimbriae in 12.5%, without correlation between P-fimbria and urinary infection; 84.7% of the strains were resistant to decomplement human serum; 61.1% produced aerobactin and 20.8% were hemolytic. Factors such as age, hospital location, metastatic focus and surgical treatment were significantly correlated with morbidity and mortality. The global mortality rate was 18%, and in 8.3% of cases was directly associated with septicemia.

A plasmid-encoded surface protein found in chronic-disease isolates of Coxiella burnetti

The cbbE' gene codes for the E' protein of Coxiella burnetii and was detected in genomic DNA from all known human isolates of the biotzere strain but not in DNA from the other five strains of C. burnetti. The biotzere strain is strictly associated with chronic disease in humans. Extrinsic iodination of biotzere strain cells radiolabeled a 55-kDa protein which comigrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with the E' protein synthesized in vitro from recombinants containing the cbbE' gene. The 125I-labeled 55-kDa protein was immunoprecipitated with polyclonal anti-E' antiserum, confirming its identity as E'. Predicted secondary structure of the E' polypeptide shows six regions of beta-sheet structure and an alpha-helix near the C terminus with adequate lengths to span a membrane. The predicted hydropathy profile of E' is similar to profiles of known outer membrane proteins and corroborates the biochemical data, indicating that the protein is located in the outer membrane of C. burnetii.

Characterization of three proteins expressed from the virulence region of plasmid pSDL2 in Salmonella dublin

Infection of both cattle and humans with Salmonella dublin can result in septicemia and death. Like many nontyphoid Salmonella species that cause disease, S. dublin contains a cryptic plasmid (pSDL2) that is required for the full expression of virulence. Transposon mutagenesis of pSDL2 defined a 4.1-kb EcoRI region that is necessary for the development of a systemic infection in BALB/c mice. This EcoRI fragment was cloned into an expression vector (pEL11), and three proteins produced from this region with apparent molecular weights of 30,500, 76,000, and 27,000 were identified. Because bacterial proteins that play a role in virulence are often associated with the outer membrane, we were interested in establishing whether the proteins expressed from the EcoRI fragment are located in the membrane. Transposon mutagenesis of pEL11 with TnphoA defined the order of the genes along the fragment and suggested that the proteins may be exported out of the cytoplasm. Sucrose gradient cell fractionation was done to identify the cellular location of each of the three proteins. The 30-kDa protein was identified in the outer membrane fraction, and the 76-kDa protein was located in the cytosolic fraction. The 27-kDa protein was identified in both the cytosolic and the outer membrane fractions. The outer membrane contained less than 10% of the activity of enzymes known to be located in the cytoplasm, periplasm, and inner membrane. Sequence data of the 4.1-kb EcoRI region revealed that both the 30- and the 27-kDa proteins lack a typical signal sequence for export out of the cytoplasm (M. Krause, C. Roudier, J. Fierer, J. Harwood, and D. G. Guiney, Mol. Microbiol. 5:307, 1991). The outer membrane location of these proteins suggests that they may be exported out of the cytoplasm by an unusual mechanism.

The Yersinia enterocolitica inv gene product is an outer membrane protein that shares epitopes with Yersinia pseudotuberculosis invasin

An essential virulence attribute for Yersinia enterocolitica and Yersinia pseudotuberculosis is the ability to invade the intestinal epithelium of mammals. The chromosomal invasin gene (inv) has been cloned from both of these Yersinia species, and the Y. pseudotuberculosis invasin has been well characterized (R. R. Isberg, D. L. Voorhis, and S. Falkow, Cell 50:769-778, 1987). Here we constructed TnphoA translational fusions to the Y. enterocolitica inv gene to identify, characterize, and localize the inv protein product in Escherichia coli. The cloned Y. enterocolitica inv locus encoded a unique protein of ca. 92 kilodaltons when expressed in minicells. A protein of comparable size was detected in immunoblots by using monoclonal antibodies directed against the Y. pseudotuberculosis invasin. This protein, which we also refer to as invasin, promoted both attachment to and invasion of cultured epithelial cells. These two functions were not genetically separable by insertional mutagenesis. We determined that the Y. enterocolitica invasin was localized on the outer membrane and that it was exposed on the bacterial cell surface, which may have implications for how invasin functions to mediate invasion.

Nucleotide sequence of the Yersinia enterocolitica ail gene and characterization of the Ail protein product

The ability to enter (invade) eucaryotic cells is a property common to many pathogenic bacteria. Yersinia enterocolitica is a facultative intracellular pathogen whose primary site of multiplication is the reticuloendothelial system. In an effort to understand how Y. enterocolitica crosses the intestinal epithelial cell layer, we previously reported the cloning of two loci from Y. enterocolitica that individually conferred an invasive phenotype to the normally noninvasive Escherichia coli HB101. One of these loci, ail, is encoded by a region of DNA that is less than 650 base pairs. We have identified the ail gene product in maxicells as a 17-kilodalton membrane-associated protein. The Ail protein has been purified, and its N-terminal sequence has been determined. The nucleotide sequence of the ail gene revealed a single unique open reading frame of 178 amino acids. Comparison of amino acid sequences deduced from the gene and obtained by analysis of the purified protein identified the first 23 amino acids as a signal sequence. The site(s) at which transcription initiates on the ail gene was identified by primer extension analysis and shown to be identical in E. coli and Y. enterocolitica. Two small open reading frames downstream of ail were found and shown to exhibit considerable identity to the proposed IS3 transposase.

Cloning and molecular characterization of genes whose products allow Salmonella typhimurium to penetrate tissue culture cells

Invasion of the intestinal epithelium is thought to be an important step in the pathogenesis of Salmonella infections. Using an in vitro system, we have isolated a genetic locus, inv, that confers to a noninvasive strain of Salmonella typhimurium the ability to penetrate tissue culture cells. Highly virulent S. typhimurium strains carrying inv mutations were defective for entry into Henle-407 cells while remaining unaffected in their ability to attach to cultured cells. When administered perorally to BALB/c mice, inv mutants of S. typhimurium had higher 50% lethal doses (LD50) than their wild-type parent strains. To the contrary, there were no differences in the observed LD50 when strains were administered intraperitoneally. In addition, inv mutants presented decreased ability to colonize the Peyer's patches, the small intestinal wall, and the spleen when administered perorally, although when administered intraperitoneally, they showed no difference in their ability to colonize the spleen compared to the wild-type parent strain.

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.

Identification of an Escherichia coli gene homologous to virR, a regulator of Shigella virulence

Virulence in Shigella spp., as well as in strains of enteroinvasive Escherichia coli, is regulated by growth temperature. Previously, virR had been identified as the gene controlling the temperature-regulated expression of Shigella virulence. Since Shigella spp. and E. coli are also known to share greater than 90% DNA sequence homology, we sought to determine if nonpathogenic E. coli K-12 C600 contains a gene homologous to the Shigella flexneri 2a gene virR. Through the use of transduction and molecular cloning of strain C600 chromosomal DNA we have shown that E. coli K-12 does indeed contain a gene functionally homologous to the virR of S. flexneri.