Isolation of the membranes of an enterotoxigenic strain of Escherichia coli and distribution of enterotoxin activity in different subcellular fractions

The different ecological niches of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a water and food-borne pathogen that infects the small intestine of the human gut and causes diarrhoea. Enterotoxigenic E. coli adheres to the epithelium by means of colonization factors and secretes two enterotoxins, the heat labile toxin and/or the heat stable toxin that both deregulate ion channels and cause secretory diarrhoea. Enterotoxigenic E. coli as all E. coli, is a versatile organism able to survive and grow in different environments. During transmission and infection, ETEC is exposed to various environmental cues that have an impact on survivability and virulence. The ability to cope with exposure to different stressful habitats is probably shaping the pool of virulent ETEC strains that cause both endemic and epidemic infections. This review will focus on the ecology of ETEC in its different habitats and interactions with other organisms as well as abiotic factors.

Heat-Labile Enterotoxins

Heat-labile enterotoxins (LTs) of Escherichia coli are closely related to cholera toxin (CT), which was originally discovered in 1959 in culture filtrates of the gram-negative bacterium Vibrio cholerae. Several other gram-negative bacteria also produce enterotoxins related to CT and LTs, and together these toxins form the V. cholerae-E. coli family of LTs. Strains of E. coli causing a cholera-like disease were designated enterotoxigenic E. coli (ETEC) strains. The majority of LTI genes (elt) are located on large, self-transmissible or mobilizable plasmids, although there are instances of LTI genes being located on chromosomes or carried by a lysogenic phage. The stoichiometry of A and B subunits in holotoxin requires the production of five B monomers for every A subunit. One proposed mechanism is a more efficient ribosome binding site for the B gene than for the A gene, increasing the rate of initiation of translation of the B gene independently from A gene translation. The three-dimensional crystal structures of representative members of the LT family (CT, LTpI, and LTIIb) have all been determined by X-ray crystallography and found to be highly similar. Site-directed mutagenesis has identified many residues in the CT and LT A subunits, including His44, Val53, Ser63, Val97, Glu110, and Glu112, that are critical for the structures and enzymatic activities of these enterotoxins. For the enzymatically active A1 fragment to reach its substrate, receptor-bound holotoxin must gain access to the cytosol of target cells.

Protein selection and export via outer membrane vesicles

Outer membrane vesicles (OMVs) are constitutively produced by all Gram-negative bacteria. OMVs form when buds from the outer membrane (OM) of cells encapsulate periplasmic material and pinch off from the OM to form spheroid particles approximately 10 to 300nm in diameter. OMVs accomplish a diversity of functional roles yet the OMV's utility is ultimately determined by its unique composition. Inclusion into OMVs may impart a variety of benefits to the protein cargo, including: protection from proteolytic degradation, enhancement of long-distance delivery, specificity in host-cell targeting, modulation of the immune response, coordinated secretion with other bacterial effectors, and/or exposure to a unique function-promoting environment. Many enriched OMV-associated components are virulence factors, aiding in host cell destruction, immune system evasion, host cell invasion, or antibiotic resistance. Although the mechanistic details of how proteins become enriched as OMV cargo remain elusive, recent data on OM biogenesis and relationships between LPS structure and OMV-cargo inclusion rates shed light on potential models for OM organization and consequent OMV budding. In this review, mechanisms based on pre-existing OM microdomains are proposed to explain how cargo may experience differing levels of enrichment in OMVs and degrees of association with OMVs during extracellular export. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Alkaline pH Is a signal for optimal production and secretion of the heat labile toxin, LT in enterotoxigenic Escherichia coli (ETEC)

Enterotoxigenic Escherichia coli (ETEC) cause secretory diarrhea in children and travelers to endemic areas. ETEC spreads through the fecal-oral route. After ingestion, ETEC passes through the stomach and duodenum before it colonizes the lower part of the small intestine, exposing bacteria to a wide range of pH and environmental conditions. This study aimed to determine the impact of external pH and activity of the Cyclic AMP receptor protein (CRP) on the regulation of production and secretion of heat labile (LT) enterotoxin. ETEC strain E2863wt and its isogenic mutant E2863ΔCRP were grown in LBK media buffered to pH 5, 7 and 9. GM1 ELISA, cDNA and cAMP analyses were carried out on bacterial pellet and supernatant samples derived from 3 and 5 hours growth and from overnight cultures. We confirm that CRP is a repressor of LT transcription and production as has been shown before but we show for the first time that CRP is a positive regulator of LT secretion both in vitro and in vivo. LT secretion increased at neutral to alkaline pH compared to acidic pH 5 where secretion was completely inhibited. At pH 9 secretion of LT was optimal resulting in 600 percent increase of secreted LT compared to unbuffered LBK media. This effect was not due to membrane leakage since the bacteria were viable at pH 9. The results indicate that the transition to the alkaline duodenum and/or exposure to high pH close to the epithelium as well as activation of the global transcription factor CRP are signals that induce secretion of the LT toxin in ETEC.

Cytotoxic effects of Kingella kingae outer membrane vesicles on human cells

Kingella kingae is an emerging pathogen causing osteoarticular infections in pediatric patients. Electron microscopy of K. kingae clinical isolates revealed the heterogeneously-sized membranous structures blebbing from the outer membrane that were classified as outer membrane vesicles (OMVs). OMVs purified from the secreted fraction of a septic arthritis K. kingae isolate were characterized. Among several major proteins, K. kingae OMVs contained virulence factors RtxA toxin and PilC2 pilus adhesin. RtxA was also found secreted as a soluble protein in the extracellular environment indicating that the bacterium may utilize different mechanisms for the toxin delivery. OMVs were shown to be hemolytic and possess some leukotoxic activity while high leukotoxicity was detected in the non-hemolytic OMV-free component of the secreted fraction. OMVs were internalized by human osteoblasts and synovial cells. Upon interaction with OMVs, the cells produced increased levels of human granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 6 (IL-6) suggesting that these cytokines might be involved in the signaling response of infected joint and bone tissues during natural K. kingae infection. This study is the first report of OMV production by K. kingae and demonstrates that OMVs are a complex virulence factor of the organism causing cytolytic and inflammatory effects on host cells.

Type II Secretion in Escherichia coli

The type II secretion system (T2SS) is used by Escherichia coli and other gram-negative bacteria to translocate many proteins, including toxins and proteases, across the outer membrane of the cell and into the extracellular space. Depending on the bacterial species, between 12 and 15 genes have been identified that make up a T2SS operon. T2SSs are widespread among gram-negative bacteria, and most E. coli appear to possess one or two complete T2SS operons. Once expressed, the multiple protein components that form the T2S system are localized in both the inner and outer membranes, where they assemble into an apparatus that spans the cell envelope. This apparatus supports the secretion of numerous virulence factors; and therefore secretion via this pathway is regarded in many organisms as a major virulence mechanism. Here, we review several of the known E. coli T2S substrates that have proven to be critical for the survival and pathogenicity of these bacteria. Recent structural and biochemical information is also reviewed that has improved our current understanding of how the T2S apparatus functions; also reviewed is the role that individual proteins play in this complex system.

Virulence and immunomodulatory roles of bacterial outer membrane vesicles

Outer membrane (OM) vesicles are ubiquitously produced by Gram-negative bacteria during all stages of bacterial growth. OM vesicles are naturally secreted by both pathogenic and nonpathogenic bacteria. Strong experimental evidence exists to categorize OM vesicle production as a type of Gram-negative bacterial virulence factor. A growing body of data demonstrates an association of active virulence factors and toxins with vesicles, suggesting that they play a role in pathogenesis. One of the most popular and best-studied pathogenic functions for membrane vesicles is to serve as natural vehicles for the intercellular transport of virulence factors and other materials directly into host cells. The production of OM vesicles has been identified as an independent bacterial stress response pathway that is activated when bacteria encounter environmental stress, such as what might be experienced during the colonization of host tissues. Their detection in infected human tissues reinforces this theory. Various other virulence factors are also associated with OM vesicles, including adhesins and degradative enzymes. As a result, OM vesicles are heavily laden with pathogen-associated molecular patterns (PAMPs), virulence factors, and other OM components that can impact the course of infection by having toxigenic effects or by the activation of the innate immune response. However, infected hosts can also benefit from OM vesicle production by stimulating their ability to mount an effective defense. Vesicles display antigens and can elicit potent inflammatory and immune responses. In sum, OM vesicles are likely to play a significant role in the virulence of Gram-negative bacterial pathogens.

Localization and characterization of Xylella fastidiosa haemagglutinin adhesins

Xylella fastidiosa is a gram-negative, xylem-inhabiting, plant-pathogenic bacterium responsible for several important diseases including Pierce's disease (PD) of grapevines. The bacteria form biofilms in grapevine xylem that contribute to the occlusion of the xylem vessels. X. fastidiosa haemagglutinin (HA) proteins are large afimbrial adhesins that have been shown to be crucial for biofilm formation. Little is known about the mechanism of X. fastidiosa HA-mediated cell-cell aggregation or the localization of the adhesins on the cell. We generated anti-HA antibodies and show that X. fastidiosa HAs are present in the outer membrane and secreted both as soluble proteins and in membrane vesicles. Furthermore, the HA pre-proteins are processed from the predicted molecular mass of 360 kDa to a mature 220 kDa protein. Based on this information, we are evaluating a novel form of potential resistance against PD by generating HA-expressing transgenic grapevines.

Testing the efficacy and toxicity of adenylyl cyclase inhibitors against enteric pathogens using in vitro and in vivo models of infection

Enterotoxigenic Escherichia coli (ETEC) produces the ADP-ribosyltransferase toxin known as heat-labile enterotoxin (LT). In addition to the toxic effect of LT resulting in increases of cyclic AMP (cAMP) and disturbance of cellular metabolic processes, this toxin promotes bacterial adherence to intestinal epithelial cells (A. M. Johnson, R. S. Kaushik, D. H. Francis, J. M. Fleckenstein, and P. R. Hardwidge, J. Bacteriol. 191:178-186, 2009). Therefore, we hypothesized that the identification of a compound that inhibits the activity of the toxin would have a suppressive effect on the ETEC colonization capabilities. Using in vivo and in vitro approaches, we present evidence demonstrating that a fluorenone-based compound, DC5, which inhibits the accumulation of cAMP in intoxicated cultured cells, significantly decreases the colonization abilities of adenylyl cyclase toxin-producing bacteria, such as ETEC. These findings established that DC5 is a potent inhibitor both of toxin-induced cAMP accumulation and of ETEC adherence to epithelial cells. Thus, DC5 may be a promising compound for treatment of diarrhea caused by ETEC and other adenylyl cyclase toxin-producing bacteria.

Repression of the inner membrane lipoprotein NlpA by Rns in enterotoxigenic Escherichia coli

The expression of the inner membrane protein NlpA is repressed by the enterotoxigenic Escherichia coli (ETEC) virulence regulator Rns, a member of the AraC/XylS family. The Rns homologs CfaD from ETEC and AggR from enteroaggregative E. coli also repress expression of nlpA. In vitro DNase I and potassium permanganate footprinting revealed that Rns binds to a site overlapping the start codon of nlpA, preventing RNA polymerase from forming an open complex at nlpAp. A second Rns binding site between positions -152 and -195 relative to the nlpA transcription start site is not required for repression. NlpA is not essential for growth of E. coli under laboratory conditions, but it does contribute to the biogenesis of outer membrane vesicles. As outer membrane vesicles have been shown to contain ETEC heat-labile toxin, the repression of nlpA may be an indirect mechanism through which the virulence regulators Rns and CfaD limit the release of toxin.

Production and release of heat-labile toxin by wild-type human-derived enterotoxigenic Escherichia coli

Production and release of heat-labile toxin (LT) by wild-type enterotoxigenic Escherichia coli (ETEC) strains, isolated from diarrheic and asymptomatic Brazilian children, was studied under in vitro and in vivo conditions. Based on a set of 26 genetically diverse LT(+) enterotoxigenic E. coli strains, cell-bound LT concentrations varied from 49.8 to 2415 ng mL(-1). The amounts of toxin released in culture supernatants ranged from 0% to 50% of the total synthesized toxin. The amount of LT associated with secreted membrane vesicles represented <5% of the total toxin detected in culture supernatants. ETEC strains secreting higher amounts of LT, but not those producing high intracellular levels of cell-bound toxin, elicited enhanced fluid accumulation in tied rabbit ileal loops, suggesting that the strain-specific differences in production and secretion of LT correlates with symptoms induced in vivo. However, no clear correlation was established between the ability to produce and secrete LT and the clinical symptoms of the infected individuals. The present results indicate that production and release of LT by wild-type human-derived ETEC strains are heterogeneous traits under both in vitro and in vivo growth conditions and may impact the clinical outcomes of infected individuals.

Bacterial outer membrane vesicles and the host-pathogen interaction

Extracellular secretion of products is the major mechanism by which Gram-negative pathogens communicate with and intoxicate host cells. Vesicles released from the envelope of growing bacteria serve as secretory vehicles for proteins and lipids of Gram-negative bacteria. Vesicle production occurs in infected tissues and is influenced by environmental factors. Vesicles play roles in establishing a colonization niche, carrying and transmitting virulence factors into host cells, and modulating host defense and response. Vesicle-mediated toxin delivery is a potent virulence mechanism exhibited by diverse Gram-negative pathogens. The biochemical and functional properties of pathogen-derived vesicles reveal their potential to critically impact disease.

Release of Shiga toxin by membrane vesicles in Shigella dysenteriae serotype 1 strains and in vitro effects of antimicrobials on toxin production and release

Effects of various antimicrobials on in vitro Shiga toxin production and release by Shigella dysenteriae serotype 1 was investigated in this study with particular reference to the role of outer membrane vesicles in toxin release by the organism. Five antimicrobials, namely nalidixic acid, ciprofloxacin, norfloxacin, fosfomycin and mitomycin C, were chosen for the study and the toxin titre was measured by the reverse passive latex agglutination (RPLA) method using an available kit. Only mitomycin C was found to induce production of Shiga toxin in the bacteria and its release by outer membrane vesicles. The highest titre of toxin was obtained in vesicle fraction suggesting that the vesicles play an important role in the release of Shiga toxin from periplasmic space by the organism.

Bacterial surface association of heat-labile enterotoxin through lipopolysaccharide after secretion via the general secretory pathway

Heat-labile enterotoxin (LT) is an important virulence factor expressed by enterotoxigenic Escherichia coli. The route of LT secretion through the outer membrane and the cellular and extracellular localization of secreted LT were examined. Using a fluorescently labeled receptor, LT was found to be specifically secreted onto the surface of wild type enterotoxigenic Escherichia coli. The main terminal branch of the general secretory pathway (GSP) was necessary and sufficient to localize LT to the bacterial surface in a K-12 strain. LT is a heteromeric toxin, and we determined that its cell surface localization was mediated by the its B subunit independent of an intact G(M1) ganglioside binding site and that LT binds lipopolysaccharide and G(M1) concurrently. The majority of LT secreted into the culture supernatant by the GSP in E. coli associated with vesicles. Only a mutation in hns, not overexpression of the GSP or LT, caused an increase in vesicle yield, supporting a specific vesicle formation machinery regulated by the nucleoid-associated protein HNS. We propose a model in which LT is secreted by the GSP across the outer membrane, secreted LT binds lipopolysaccharide via a G(M1)-independent binding region on its B subunit, and LT on the surface of released outer membrane vesicles interacts with host cell receptors, leading to intoxication. These data explain a novel mechanism of vesicle-mediated receptor-dependent delivery of a bacterial toxin into a host cell.

Release of membrane vesicles containing endotoxic lipopolysaccharide in Escherichia coli O157:H7 clinical isolates

Membrane vesicles released by E. coli O157:H7 strains were investigated by immuno-electron microscopy using anti-O157 antibody. Anti-O157 antibody enhanced the negative-staining of vesicles and we found numerous small vesicles clearly formed around bacterial cells. An immunogold-electron microscopic examination confirmed that lipopolysaccharide (LPS) including the O-side chain is present on the surface of the vesicles. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of the purified vesicles showed that the vesicles contained LPS consisting of a lipid-A and an O polysaccharide. In addition, the endotoxic activity of the vesicle was confirmed by a limulus test. These results suggest that the vesicles may play an important role in the pathogenesis of Escherichia coli O157:H7.

Enterotoxigenic Escherichia coli secretes active heat-labile enterotoxin via outer membrane vesicles

Escherichia coli and other Gram-negative bacteria produce outer membrane vesicles during normal growth. Vesicles may contribute to bacterial pathogenicity by serving as vehicles for toxins to encounter host cells. Enterotoxigenic E. coli (ETEC) vesicles were isolated from culture supernatants and purified on velocity gradients, thereby removing any soluble proteins and contaminants from the crude preparation. Vesicle protein profiles were similar but not identical to outer membranes and differed between strains. Most vesicle proteins were resistant to dissociation, suggesting they were integral or internal. Thin layer chromatography revealed that major outer membrane lipid components are present in vesicles. Cytoplasmic membranes and cytosol were absent in vesicles; however, alkaline phosphatase and AcrA, periplasmic residents, were localized to vesicles. In addition, physiologically active heat-labile enterotoxin (LT) was associated with ETEC vesicles. LT activity correlated directly with the gradient peak of vesicles, suggesting specific association, but could be removed from vesicles under dissociating conditions. Further analysis revealed that LT is enriched in vesicles and is located both inside and on the exterior of vesicles. The distinct protein composition of ETEC vesicles and their ability to carry toxin may contribute to the pathogenicity of ETEC strains.

The release of outer membrane vesicles from the strains of enterotoxigenic Escherichia coli

The clinically isolated heat labile enterotoxin (LT)-producing strains of Escherichia coli can be separated into two groups, namely spontaneous LT-releasing strain and non-spontaneous LT-releasing strain, based on their phenotypes of spontaneous release of LT into the culture medium. The phenotype of spontaneous LT release was observed to correlate closely with the phenotype of the release of numerous small vesicles into the culture medium. Both morphological and biological examinations of the vesicle showed that the vesicle was released from the outer membrane. It can, therefore, be assumed that LT may be released from the cell at the time the vesicles form.

Intracellular distribution of heat-labile enterotoxin in a clinical isolate of Escherichia coli

The intracellular distribution of heat-labile enterotoxin in a human isolate of enterotoxigenic Escherichia coli varied significantly as a result of changing incubation time, media, and degree of aeration. Direct comparison with a K-12 plasmid recipient revealed a similar but less dramatic response to environmental factors.

[Cell wall proteins of Escherichia coli: a substance resembling the thermolabile enterotoxin (LT)]

In ultrasonic extracts of all 19 investigated non-enterotoxigenic E. coli strains a substance (LTLS) could be detected reacting positively in all tests which are commonly used to detect specifically E. coli thermolabile enterotoxin (LT). Culture supernatants of these strains in general did not contain LTLS in detectable amounts. LTLS can be found in the whole cell, however, the membrane fraction contains the highest quantities. Released LTLS appears mainly aggregated with components of the cell wall, especially with lipopolysaccharides. This fact in combination with the very low quantities produced by the bacteria renders very difficult purification of LTLS.

Mechanism of toxin secretion by Vibrio cholerae investigated in strains harboring plasmids that encode heat-labile enterotoxins of Escherichia coli

A genetically engineered Vibrio cholerae strain from which the cholera toxin genes had previously been deleted was used as a host in which to study the expression and secretion of related toxins and their subunits. Recombinant plasmids encoding heat-labile enterotoxins (LTs) from Escherichia coli of human and porcine origin were expressed in the V. cholerae host, and this resulted in the secretion of the LTs into the extracellular milieu. The secreted LTs were isolated and it was found that the A subunits of human and porcine LT were "unnicked" polypeptides, which indicates that nicking is not obligatory for toxin secretion. V. cholerae strains were also constructed that harbored plasmids encoding either the A or the B subunits of human LT (A+B-, or A-B+). Approximately 90% of the B subunits were secreted from the A-B+ strain, while all of the A subunits expressed by the A+B- strain remained cell associated. This implies that strains synthesizing both subunits assemble the A and B subunits prior to their secretion. We propose that the entry of the toxin into the secretory step of the export pathway is mediated by a secretory apparatus that recognizes structural domains within the B subunit of LT.

Cellular location of heat-labile enterotoxin in Escherichia coli

We demonstrated that both the A and B subunits of heat-labile enterotoxin from Escherichia coli are located in the periplasm. The toxin was shown to form aggregates in Tris-EDTA buffers which are routinely used for isolating membranes. The aggregates pellet upon centrifugation, and this may explain why several previous investigators have concluded that enterotoxin is associated with membranes.

Synthesis and secretion of the plasmid-coded heat-labile enterotoxin of Escherichia coli in Vibrio cholerae

Both cholera toxin and heat-labile enterotoxin were made and secreted into culture supernatants by Vibrio cholerae containing the enterotoxin plasmid pCG86. Several regulatory mutations in V. cholerae that increased or decreased the synthesis of cholera toxin did not affect production of heat-labile enterotoxin. In contrast, a mutation in V. cholerae that interfered with the secretion of cholera toxin also decreased the secretion of heat-labile enterotoxin, indicating that they are processed by a common secretory pathway. Vibrio cholerae should be useful as a model system for analyzing the secretion of true extracellular proteins by Gram-negative bacteria.

Thermoactivation of a periplasmic heat-stable enterotoxin of Escherichia coli

Strains of Escherichia coli that host a plasmid that codes for the heat-stable (ST) enterotoxin showed 160 times more extracellular enterotoxin than intracellular activity. However, when washed bacteria were sonicated and incubated at between 50 and 85 degrees C, an activity similar to that of the ST enterotoxin was detected. No such effect was present in strains lacking the plasmid, in a plasmid ST- mutant, or in chromosomal mutants that lack a cyclic AMP-linked positive regulatory system which previously were shown to yield an ST- phenotype. The thermoactivation was inhibited by iodoacetamide and N-ethylmaleimide; chloramphenicol did not affect the phenomenon. The heat-activated ST-like enterotoxin was localized in the periplasmic space. The results are discussed in relation to the export of the toxin from the periplasm to the outside of the cell.

Assembly in vivo of enterotoxin from Escherichia coli: formation of the B subunit oligomer

An oligomer of the B subunit of heat-labile enterotoxin of Escherichia coli has been observed in minicells and in whole cells. There is a delay after synthesis of the B subunit before it appears in the oligomer. The delay is not due to slow processing of the precursor. A similar delay in oligomerization of the major outer membrane protein OmpF is also described.

Shigella dysenteriae 1 cytotoxin: periplasmic protein releasable by polymyxin B and osmotic shock

Treatment of Shigella dysenteriae 1 either with the antibiotic polymyxin B or by osmotic shock resulted in the release of 80 to 90% of the cytotoxin activity of the organism. Under the conditions employed, the release of toxin activity was accompanied by the appearance of a periplasmic enzyme, 5'-nucleotidase. There was no significant release of cytoplasmic contents, assessed by measurement of glucose-6-phosphate dehydrogenase activity. The release of cytotoxin and 5'-nucleotidase by polymyxin B were both dependent on the duration of incubation with, and the concentration of, the antibiotic. In terms of specific activity (cytotoxin activity per milligram of protein), the polymyxin B and osmotic shock extracts were 20- to 30-fold more active than crude toxin preparation derived from a whole-cell lysate. The data strongly support a periplasmic location for Shiga cytotoxin and the utility of the polymyxin B extraction to obtain starting material for toxin purification.

Cultural methods for the production of heat-stable enterotoxin by porcine strains of Escherichia coli and its detection by the infant mouse test

Casamino acids-yeast extract medium (CAY) and a tryptone-yeast extract medium (TY-1) were evaluated in testing for production of heat-stable enterotoxin (ST) by porcine strains of Escherichia coli using the infant mouse assay. More strains were ST-positive when grown in CAY medium than in TY-1 medium. Questionably or indeterminately ST positive strains were investigated in detail to determine whether or not they were weak ST producers. Growth in four different media and in different batches of CAY medium, inactivation of culture supernatant fluids at a lower temperature, addition of mitomycin C to growing cultures and preparation of periplasm-cytoplasm fractions of bacteria by sonication, all failed to yield ST positive samples. ST value limits (i.e. ratios of intestinal weight to remaining body weight of challenged mice), which clearly differentiated positive or negative strains for ST production, were set for CAY medium. A minimal salts-amino acids medium (MSA) was devised. Both in shake flask and fermenter cultures MSA medium gave better ST yields than CAY and a previously described defined medium.

Release of heat-labile enterotoxin subunits by Escherichia coli

Most of the heat-labile enterotoxin (LT) synthesized by Escherichia coli is cell associated; however, a small portion of LT (approximately 10%) is released by bacterial cells into the culture supernatant. The LT subunit B (LT-B) produced by a cloned LT-B gene (tox B) was released in amounts equal to the parent LT release. In contrast, no release of LT subunit A (LT-A) or its smaller derivatives was observed in strains containing cloned toxA genes. The data suggest that LT-B is necessary for the release of LT-A across the bacterial membrane.

Molecular cloning of Vibrio cholerae enterotoxin genes in Escherichia coli K-12

Hybridization probes derived from the A and B subunit genes of the heat-labile enterotoxin (LT) of Escherichia coli were used to analyze DNA from Vibrio cholera strain 569B for cholera toxin gene sequences. Southern blot analysis indicated that the cholera toxin A and B subunit genes were each duplicated in the strain. One of the two toxin subunit gene pairs was cloned as a 5.1-kilobase DNA insert in plasmid pBR322. E. coli cells carrying the recombinant plasmid pJM17 were shown to produce cholera toxin, which was found to be largely cell associated. Protein chemical analysis indicated that the toxin was in its unnicked form and required additional proteolytic processing by trypsin to exhibit full toxicity in tissue culture. The alteration in E. coli of the secretion and proteolytic processing of cholera toxin parallels that previously observed for LT. An in vitro generated insertion mutation in the A subunit gene on pJM17 was shown to abolish production of the A chain but still allow production of the B chain. These observations, together with restriction mapping data, have demonstrated that the cholera toxin and LT genes are very similar in their genetic organization.

Probing for enterotoxigenicity among the salmonellae: an evaluation of biological assays

Sixty-eight Salmonella strains representing 39 serotypes were variously screened for enterotoxigenicity by using the Chinese hamster ovary (CHO), Y1 adrenal, and Vero cell tests, rabbit skin tests for delayed permeabiltity factor (DPF) and rapid permeability factor (RPF), the rabbit ileal loop test, and the infant mouse test. An iron-sufficient medium, YT-1, and a deferrated medium, DF, were compared. Of the culture supernatant fluids of strains grown in DF medium, 66% yielded positive reactions in the CHO cell test compared with only 10% with TY-1 medium. The corresponding performances with supernatant fluids of DF medium cultures in Y1 adrenal and Vero cell tests were 85 and 69% positive, respectively. The overall agreement between the Y1 adrenal or CHO cell test and the rabbit skin test for DPF, i.e., positive or negative in both tests, was about 70%. Positivity in DPF tests was a better predictor of positivity in either the Y1 adrenal or rabbit ileal loop test than vice versa. CHO cell, DPF, and rabbit ileal loop reactivities of unheated culture filtrates were each neutralized by anticholera antitoxin. Only four strains gave positive reactions in the infant mouse test, whereas up to 66% were positive for RPF in rabbit skin, based on positivity in Ty-1 or DR medium or both. DPF and RPF were produced by 35% of the strains. Of the 28 isolates from human stools, 82 and 92% and all of 11 strains tested were positive in the DPF, Y1 adrenal cell, and rabbit ileal loop tests, respectively. The corresponding data for 17 sewage isolates, representing 17 different serotypes rarely isolated from human stools in Sweden, were 63 and 69% and 8 of 8 tested. On the basis of this investigation, rabbit skin tests for both DPF and RPF provide the most reliable means of screening for enterotoxigenicity among salmonellae.

Outer-membrane vesicles released by normally growing Escherichia coli contain very little lipoprotein

The lipoprotein content of the outer-membrane medium vesicles, which are released from Escherichia coli during normal growth, was compared to the lipoprotein content of the corresponding cellular outer membranes. It was found that the medium vesicles contained only 35% free lipoprotein and almost none of the bound lipoprotein when compared with cellular outer membranes. Medium vesicles also had reduced amounts of protein II and a protein V (Mr = 16 000), while they contained large amounts of pore-forming proteins I and lamB. A mechanism is proposed in which outer membrane vesicles are formed when the outer membrane expands faster than the underlying peptidoglycan layer. The lack or enrichment of individual proteins in medium vesicles may be determined by their interactions with the peptidoglycan-bound lipoprotein complex.

Identification of different forms of the murein-bound lipoprotein found in isolated outer membranes of Escherichia coli

The identification of the free and murein-bound forms of the Escherichia coli lipoprotein on dodecylsulphate-polyacrylamide gels was systematically investigated by analyzing the low-molecular-weight proteins (Mr less than 20 000) of both cytoplasmic and outer membranes. The free form of the lipoprotein was identified on 15% polyacrylamide gels as the fastest migrating component (Mr = 7200-7500) of isolated outer membranes; it could be separated from a small cytoplasmic membrane protein (Mr = 6500) which was probably identical to the dicyclohexylcarbodiimide binding proteolipid of the membrane-bound ATPase. Lysozyme treatment of both outer membranes and murein sacculi failed to convert the murein-bound lipoprotein into a fragment of uniform size; instead the bound form appeared as a series of bands consisting of lipoprotein bound to one, two,...eight murein subunits. The composition of this ladder depended on the method used to isolate outer membranes. Beside these lipoprotein bands the outer membrane contained two other proteins, III and V; the relation of these proteins to previously described proteins is discussed.

K88-mediated binding of Escherichia coli outer membrane fragments to porcine intestinal epithelial cell brush borders

We have examined the interactions between various radiolabeled membrane fractions obtained from an enterotoxigenic Escherichia coli strain and brush borders isolated from porcine intestinal epithelial cells. Outer membrane fragments containing the K88 attachment factor bound tightly to brush borders, whereas cytoplasmic membrane vesicles did not. Three different types of outer membrane preparations were tested: (i) cellular outer membranes isolated from lysozyme spheroplasts, (ii) medium vesicles or outer membrane fragments released into the medium during growth, and (iii) periplasmic vesicles, or outer membrane fragments which were released from the cells during spheroplast formation and were therefore isolated in the periplasmic fraction. Of these fractions, which were heterogeneous, it was always the outer membrane subfraction which bound tightly to brush borders. This binding, which was K88 dependent, may have some physiological significance in view of the association between outer membrane fragments and enterotoxin. Thus, released outer membrane fragments equipped with attachment factors may function as enterotoxin carriers which increase the efficiency with which enterotoxin can be delivered to intestinal epithelial cells.

Cholera toxin is synthesized in precursor form on free polysomes in Vibrio cholerae 569B

Membrane-bound and free polysomes have been isolated from Vibrio cholerae 569B. Nacent polypeptide chains were completed in a cell-free translation mixture containing Escherichia coli S-300 extracts and [3H]leucine or [35S]methionine. Cholera toxin-related polypeptides synthesized in vitro were immunologically detected after treatment with either anti-subunit A or anti-subunit B serum. Immunoreactive translation products were removed from reaction mixtures with formalinized Cowan's strain of Staphylococcus aureus, electrophoresed on sodium dodecyl sulfate-polyacrylamide gels, and visualized by fluorography. Anti-subunit A serum precipitated two major polypeptide species (molecular weights 52,000 and 45,000) from translation mixtures programed with free polysomes, whereas anti-subunit B serum precipitated only the 45,000-molecular-weight polypeptide. No cholera toxin-related polypeptides were detectable in translation mixtures programed with membrane-bound polysomes. Purified subunit A and cholera toxin competed for anti-subunit A binding sites and blocked the immunoprecipitation of the 35S-labeled 52,000- and 45,000-dalton polypeptides from in vitro translation mixtures. The data presented suggest that cholera toxin is synthesized in the cytoplasm in a precursor form on free polysomes and is secreted post-translationally.

Some characteristics of the outer membrane material released by growing enterotoxigenic Escherichia coli

The high-molecular-weight material released into the medium by Escherichia coli AP1, an enterotoxigenic strain of porcine origin, has been isolated and resolved into two clearly distinct fractions, based on sucrose density gradient and differential centrifugation, chemical analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and freeze-fracture electron microscopy. These two fractions, referred to as "medium vesicles" and "medium lipopolysaccharides", were compared with the cellular outer and cytoplasmic membranes, the periplasmic fraction, and the cytoplasmic fraction. The medium vesicles closely resembled outer membrane and accounted for 3 to 5% of the total cellular outer membrane. They contained most of the heat-labile enterotoxin (LT) activity released into the medium by E. coli AP1. The medium lipopolysaccharide consisted mostly of lipopolysaccharide and a small amount of outer membrane and contained relatively little LT activity. Based on experiments with E. coli K-12 strains, in which about 5% of the newly synthesized outer membrane is lost from areas of outer membrane synthesis, it is proposed that enterotoxigenic E. coli strains release LT as part of such newly synthesized outer membrane fragments and that released outer membrane fragments may function as physiologically significant LT carriers.