Antigenic determinants of murein lipoprotein and its exposure at the surface of Enterobacteriaceae

Lipoproteins: Structure, Function, Biosynthesis

The Lpp lipoprotein of Escherichia coli is the first identified protein with a covalently linked lipid. It is chemically bound by its C-terminus to murein (peptidoglycan) and inserts by the lipid at the N-terminus into the outer membrane. As the most abundant protein in E. coli (10⁶ molecules per cell) it plays an important role for the integrity of the cell envelope. Lpp represents the type protein of a large variety of lipoproteins found in Gram-negative and Gram-positive bacteria and in archaea that have in common the lipid structure for anchoring the proteins to membranes but otherwise strongly vary in sequence, structure, and function. Predicted lipoproteins in known prokaryotic genomes comprise 2.7% of all proteins. Lipoproteins are modified by a unique phospholipid pathway and transferred from the cytoplasmic membrane into the outer membrane by a special system. They are involved in protein incorporation into the outer membrane, protein secretion across the cytoplasmic membrane, periplasm and outer membrane, signal transduction, conjugation, cell wall metabolism, antibiotic resistance, biofilm formation, and adhesion to host tissues. They are only found in bacteria and function as signal molecules for the innate immune system of vertebrates, where they cause inflammation and elicit innate and adaptive immune response through Toll-like receptors. This review discusses various aspects of Lpp and other lipoproteins of Gram-negative and Gram-positive bacteria and archaea.

Immunoproteomic analysis to identify Shiga toxin-producing Escherichia coli outer membrane proteins expressed during human infection

Shiga-toxin producing Escherichia coli (STEC) is the etiologic agent of acute diarrhea, dysentery, and hemolytic-uremic syndrome (HUS). There is no approved vaccine for STEC infection in humans, and antibiotic use is contraindicated, as it promotes Shiga toxin production. In order to identify STEC-associated antigens and immunogenic proteins, outer membrane proteins (OMPs) were extracted from STEC O26:H11, O103, O113:H21, and O157:H7 strains, and commensal E. coli strain HS was used as a control. SDS-PAGE, two-dimensional-PAGE analysis, Western blot assays using sera from pediatric HUS patients and controls, and matrix-assisted laser desorption ionization-tandem time of flight analyses were used to identify 12 immunogenic OMPs, some of which were not reactive with control sera. Importantly, seven of these proteins have not been previously reported to be immunogenic in STEC strains. Among these seven proteins, OmpT and Cah displayed IgG and IgA reactivity with sera from HUS patients. Genes encoding these two proteins were present in a majority of STEC strains. Knowledge of the antigens produced during infection of the host and the immune response to those antigens will be important for future vaccine development.

"Gently rough": the vaccine potential of a Salmonella enterica regulatory lipopolysaccharide mutant

BACKGROUND

An alternative to multivalent vaccines could be to construct strains capable of conferring broad protection through shared antigens. Down-regulation of immunodominant major antigens has been proposed to enhance the immunogenicity of conserved antigens.

METHODS

The protection provided by an aroA as well as structural and regulatory lipopolysaccharide (LPS) mutants of Salmonella enterica serovar Typhimurium against homologous and heterologous challenges was assessed in the murine model of typhoid. The reactivity and cross-reactivity of the immune sera raised was tested by enzyme-linked immunospot assay and immunoblots. Conserved outer membrane proteins were identified by mass spectrometry.

RESULTS

Unlike any structural LPS mutants, the regulatory mutant lacking RfaH was finely balanced between safety and immunogenicity, and its vaccine potential was comparable to that of the well-characterized DeltaaroA mutant. Loss of the transcriptional antiterminator RfaH resulted in a heterogeneous length of LPS chains, designated here as the "gently rough" phenotype. Our study also provides evidence that the rough phenotype enhances the immunogenicity of minor antigens, which may improve cross-protection against heterologous bacteria. A panel of conserved antigens shared by members of the Enterobacteriaceae family was identified as abundant porins and lipoprotein antigens.

CONCLUSIONS

Fine-tuned down-regulation of immunodominant epitopes can create live vaccine strains that are not only desirably attenuated but that also exhibit an improved cross-protective potential.

Outer membrane protein A, peptidoglycan-associated lipoprotein, and murein lipoprotein are released by Escherichia coli bacteria into serum

Complexes containing lipopolysaccharide (LPS) and three outer membrane proteins (OMPs) are released by gram-negative bacteria incubated in human serum and into the circulation in an experimental model of sepsis. The same OMPs are bound by immunoglobulin G (IgG) in the cross-protective antiserum raised to Escherichia coli J5 (anti-J5 IgG). This study was performed to identify the three OMPs. The 35-kDa OMP was identified as outer membrane protein A (OmpA) by immunoblotting studies using OmpA-deficient bacteria and recombinant OmpA protein. The 18-kDa OMP was identified as peptidoglycan-associated lipoprotein (PAL) based on peptide sequences from the purified protein and immunoblotting studies using PAL-deficient bacteria. The 5- to 9-kDa OMP was identified as murein lipoprotein (MLP) based on immunoblotting studies using MLP-deficient bacteria. The studies identify the OMPs released into human serum and into the circulation in an experimental model of sepsis as OmpA, PAL, and MLP.

[Lipopeptides as natural adjuvants for vaccines from Gram-negative bacteria]

Bacterial cell wall components such as lipopolysaccharide, a variety of membrane proteins, murein, and lipoprotein can act as immunoadjuvants for bacterial vaccines, thus enhancing protection from bacterial infections. Synthetically prepared N-terminal parts of the lipoprotein from Enterobacteria carrying three fatty acid residues or lipopeptide analogs containing one to four aminoacids bound to S-glycerylcysteine act as potent immunoadjuvants in vivo in combination with or covalently linked to antigens. Here we demonstrate that the supplementation of Salmonella vaccines with these synthetic lipopeptides significantly enhances their vaccine efficiency in mice. Variations in the native lipopeptide structure regarding chain length and amino acid sequence of the peptide moiety, as well as modifications of the lipoamino acid, lead to reduction or even complete loss of the adjuvant activity. The immunoadjuvant properties of the lipopeptides as described here are mediated by an enhancement of the humoral immune response.

Purification and immunological characterization of a major low-molecular-weight lipoprotein from Borrelia burgdorferi

Borrelia burgdorferi resembles gram-negative bacteria in having both cellular and outer membranes. We previously showed that a lipopolysaccharide (LPS)-like material could be extracted from B. burgdorferi with phenol-chloroform-petroleum ether (PCP). The PCP extract of B. burgdorferi exhibited biological activity in several in vitro assays (e.g., mitogenicity, pyrogenicity, and cytokine release). These activities suggested the presence of endotoxin. The PCP extract of B. burgdorferi, however, also contained a small amount of protein. Preliminary studies showed that monoclonal antibody prepared against this protein inhibited the mitogenic activity of the PCP extract toward murine spleen cells. The current study was therefore undertaken to characterize this protein and to establish methods for its separation from the LPS. The PCP-extracted protein consisted of a single, low-molecular-weight lipoprotein (apparent M(r), 10,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) (SDS-PAGE). By protein analysis, it accounted for 2% of the dry weight of defatted cells, thus making it a major constituent of the spirochete. It was purified from the LPS by initial extraction into 10% Triton X-100 followed by immunoaffinity chromatography in the presence of detergent. On removal of the LPS, the purified lipoprotein formed aggregates stable to SDS-PAGE which were detectable on Western blots (immunoblots) probed with either the monoclonal antibody or polyclonal antiserum. From a plot of the aggregate molecular weight versus aggregate size, a monomer molecular weight of 7,500 was obtained. Indirect immunofluorescence with the monoclonal antibody showed that the lipoprotein was exposed at the surface of the spirochete in only a small percentage of cells. The lipoprotein was present in several strains of B. burgdorferi but absent in other Borrelia spp., treponemes, and gram-negative human pathogens, indicating species specificity.

A conserved domain on enterobacterial porin protein analysed by monoclonal antibody

Broadly cross-reactive monoclonal antibodies (MAbs) against enterobacterial outer membrane (OM) porin (Po) protein were isolated after immunization of BALB/c mice with whole cells of E. coli 055:B5. MAbs (n = 6) of the IgG class but of four different isotypes were studied. Based on a competition ELISA, all of the MAbs were directed against one and the same Po protein domain (Po I). The MAbs cross-reacted with 72 of 74 strains from 10 different genera of the Enterobacteriaceae. One Morganella and one Salmonella strain showed no cross-reactivity. Also, nine strains of various Neisseria spp. cross-reacted while 21 strains of various other nonenteric Gram-negative bacteria showed no cross-reactivity. The Po I sites were inaccessible in intact homologous bacteria but partially accessible in the OM. Digestion of OM with lysozyme or lysostaphin affected the accessibility of the Po I sites in OMs of various enterobacteria. Lysostaphin strongly enhanced the immunoaccessibility, whereas lysozyme had lesser effects. The enzymes also affected the binding by Neisseria OMs of the anti-Po I MAb. The Po I site was immunogenic both in humans and rabbits. The data indicate that Po I is an important Po protein domain, and that the effects of peptidoglycan-degrading enzymes must be considered in studies of Po protein domains.

Protection against experimental Pseudomonas aeruginosa infection by recombinant P. aeruginosa lipoprotein I expressed in Escherichia coli

Lipoprotein I (OprI) is one of the major proteins of the outer membrane of Pseudomonas aeruginosa. OprI is a candidate for a vaccine against P. aeruginosa, because it cross-reacts antigenically in all serotype strains of the International Antigenic Typing Scheme. We recently cloned and expressed the gene coding for OprI in Escherichia coli. This heterologously expressed OprI was used successfully to immunize mice against P. aeruginosa. In addition, OprI from serogroup 12 of P. aeruginosa was highly purified by preparative isoelectric focusing and used for immunization of mice. Both vaccines protected the mice against a challenge with a four- to fivefold 50% lethal dose of P. aeruginosa.

Lipoproteins in bacteria

Covalent modification of membrane proteins with lipids appears to be ubiquitous in all living cells. The major outer membrane (Braun's) lipoprotein of E. coli, the prototype of bacterial lipoproteins, is first synthesized as a precursor protein. Analysis of signal sequences of 26 distinct lipoprotein precursors has revealed a consensus sequence of lipoprotein modification/processing site of Leu-(Ala, Ser)-(Gly, Ala)-Cys at -3 to +1 positions which would represent the cleavage region of about three-fourth of all lipoprotein signal sequences in bacteria. Unmodified prolipoprotein with the putative consensus sequence undergoes sequential modification and processing reactions catalyzed by glyceryl transferase, O-acyl transferase(s), prolipoprotein signal peptidase (signal peptidase II), and N-acyl transferase to form mature lipoprotein. Like all exported proteins, the export of lipoprotein requires functional SecA, SecY, and SecD proteins. Thus all precursor proteins are exported through a common pathway accessible to both signal peptidase I and signal peptidase II. The rapidly increasing list of lipid-modified proteins in both prokaryotic as well as eukaryotic cells indicates that lipoproteins comprise a diverse group of structurally and functionally distinct proteins. They share a common structural feature which is derived from a common biosynthetic pathway.

Antibody response to defined domains on enterobacterial outer membrane proteins in healthy persons and patients with bacteraemia

Monoclonal antibody (MAb)-based competitive enzyme immunoassays (cELISAs) were elaborated to measure antibodies against MAb-defined domains on three different enterobacterial outer membrane (OM) proteins in sera from healthy individuals (n = 30) and in paired serum samples from patients (n = 45) with bacteraemia caused by enterobacteria or by various nonenteric bacteria (n = 15). The MAb-defined domains were Hm I and Hm II on the heat-modifiable (Hm) protein, PALp I and PALp II on the peptidoglycan-associated lipoprotein (PALp), and BLp I on Braun's lipoprotein (BLp). All MAbs have shown broad cross-reactivity with and specificity for enterobacteria. Sera from healthy individuals and from patients with infections caused by nonenteric bacteria contained low levels of MAb-blocking antibodies. Bacteraemia caused by enterobacteria resulted in generation of antibodies against the MAb-defined domains in many of the patients. Thus, 40% and 69% showed a positive BLp I cELISA with the first and second serum samples, respectively. Of the second serum samples, 20-38% showed positive Hm and PALp cELISAs. The BLp I cELISA showed higher diagnostic sensitivity than the previously described indirect ELISA for IgG antibodies against E. coli 055 OM protein antigens. Assays using the MAbs as competitors showed that the patients bacteraemic with enterobacteria, also generated antibodies against other domains on the OM proteins. The cELISAs may be useful in the diagnosis and management of patients with serious infections caused by enterobacteria. In this regard, the BLp I cELISA showed the most promising results.

Enhancement of protection against Salmonella infection in mice mediated by a synthetic lipopeptide analogue of bacterial lipoprotein in S. typhimurium vaccines

Vaccines consisting of acetone-killed Salmonella typhimurium were supplemented with a synthetically prepared lipopeptide derivative of bacterial lipoprotein, Pam3Cys-Ser-Ser-Asn-Ala. NMRI mice were immunized with these vaccines, receiving two intraperitoneal injections and were challenged intraperitoneally with graded doses of S. typhimurium C5. The protective capacity of the supplemented vaccines was compared with that of the unsupplemented bacterial vaccine, and with the effectiveness of the supplementing component alone. The LD50 served as a criterion for protective capacity. The results showed that 90% of the S. typhimurium S-form vaccine could be replaced by the adjuvant lipopeptide without a recognizable decrease in protective immunizing capacity. A similar but less pronounced enhancement of protection was obtained with a R-mutant vaccine supplemented with the lipopeptide; by supplementing the standard vaccine dose with lipopeptide an increase in protection was also achieved. Lipopeptide alone was not effective in protecting mice from infection with S. typhimurium.

Monoclonal antibodies against three different enterobacterial outer membrane proteins. Characterization, cross-reactivity, and binding to bacteria

BALB/c mice were immunized with whole-cells of Escherichia coli 055:B5 or Proteus mirabilis NCTC 60 to produce broadly cross-reacting monoclonal antibodies (MAbs) against outer membrane (OM) proteins. A total of 10 anti-OM MAbs of the IgG class were selected. These included 5 MAbs against the heat-modifiable (Hm) protein, 3 against the peptidoglycan-associated lipoprotein (PALp), and 2 against Braun's lipoprotein (BLp). Based on competition ELISA, the MAbs defined 2 Hm protein binding sites (Hm I and Hm II), 2 PALp sites (PALp I and PALp II), and one BLp site (BLp I). The MAbs showed broad cross-reactivity against 74 strains of 10 different genera of the Enterobacteriaceae. Non-cross-reacting enteric bacilli occurred only among bacteria of the genera Salmonella, Proteus, and Providentia. The results revealed that Proteus and Providentia strains differed from other enteric bacilli with regard to BLp synthesis or specificity. A panel of 30 non-enteric Gram-negative bacteria did not cross-react. Testing of MAb binding to bacteria showed that a part of the BLp I, PALp I, and PALp II sites was immunoaccessible in intact homologous bacteria, and that the Hm I and Hm II epitopes were inaccessible. The MAbs should facilitate studies of structure and immunobiological function of enterobacterial OM proteins and should have a potential as immunodiagnostic reagents.

Distribution of newly synthesized lipoprotein over the outer membrane and the peptidoglycan sacculus of an Escherichia coli lac-lpp strain

The insertion of newly synthesized lipoprotein molecules into the cell wall of Escherichia coli was studied topographically by immunoelectron microscopy. Lipoprotein was briefly induced with isopropyl-beta-D-thiogalactopyranoside in cells carrying lac-lpp on a low-copy-number plasmid in an E. coli lpp host. Specific antibodies bound to the newly inserted lipoprotein molecules, which were exposed at the cell surface after treatment of the cells with Tris-EDTA, were detected with a protein A-gold probe. The average distribution of the gold particles over the cell surface of noninduced cells was determined for cells induced for 5 and 10 min. Analysis of 250 to 350 cells showed that the distribution of newly synthesized lipoprotein over the cell surface was homogeneous in both cases. The binding of lipoprotein to the peptidoglycan layer was studied by the same technique, and visual inspection again revealed a homogeneous distribution of bound lipoprotein over the entire sacculus surface. It is therefore concluded that free lipoprotein is inserted equally over the entire cell wall of E. coli, while binding to peptidoglycan also occurs over the entire cell surface. The rate of lipoprotein synthesis increased with cell length in nondividing cells, whereas it was constant in cells which had initiated constriction. Analysis of cells having different amounts of lipoprotein in their cell wall revealed that the cell shape depended on the total lipoprotein content of the cell. Cells having no or only a small amount of lipoprotein grew as spheres, whereas cells with increasing numbers of lipoprotein molecules gradually changed their shape to short rods.

Serum antibodies to outer membrane proteins of Escherichia coli in healthy persons and patients with bacteremia

Antibodies to Escherichia coli outer membrane proteins in sera from healthy persons and from patients bacteremic with various enteric or nonenteric bacteria were measured by an enzyme-linked immunosorbent assay (ELISA). Outer membranes were prepared from E. coli O55. Serum was absorbed with E. coli O55 lipopolysaccharide and diluted 1:100 for immunoglobulin A (IgA) or IgM and 1:1,000 for IgG antibodies. Paired serum specimens were obtained from the 56 patients included in the study (the first specimen on the day of positive blood culture and the second specimen 8 to 12 days later) and compared with sera from blood donors (n = 50) as controls. On an average, the patients bacteremic with enterobacteria (n = 40) showed increased levels of antibodies of all three immunoglobulin classes in the first serum specimens and significantly higher levels in the second specimens compared with the controls, although with considerable case-to-case variation. Increased levels of IgG antibodies showed the best combination of diagnostic specificity (100%) and sensitivity (53%) for bacteremia caused by enteric bacilli. Mostly, the antibody response was directed against the major E. coli O55 outer membrane proteins at 81,000, 38,500, 33,500, and 7,500 molecular weights as shown by Western blot (immunoblot) analysis. Some of the patients bacteremic with nonenteric bacteria showed increased levels of IgA antibodies, but not of IgG or IgM antibodies. Cross-reactivity of the nonenteric blood culture isolates with the E. coli outer membrane preparation was not demonstrated. The cross-reactivity of the E. coli O55 outer membrane proteins with those of enteric bacilli of other genera was examined by absorption experiments. Western blots with serum absorbed with live E. coli O55 provided evidence that the epitopes of the outer membrane protein at 7,500 molecular weight were available for antibody binding at the bacterial surface, and that at least some of the epitopes of the 38,500- and 33,500-molecular -weight proteins were accessible to antibodies. The results suggest that an ELISA for the measurement of antibodies against cross-reactive outer membrane proteins from enteric bacilli may be useful in the diagnosis of serious infections caused by members of the family Enterobacteriaceae, and that antibodies to the major outer membrane proteins may have an immunobiological function.

The porin protein of the outer membrane of Escherichia coli: reactivity in immunoblotting, antibody-binding by the native protein, and cross-reactivity with other enteric bacteria

The experimental conditions for antibody-binding by the 38.5 kD porin protein of an E. coli 055 strain in immunoblotting were investigated. A non-ionic detergent in the buffer which contained the primary antibody was required for antibody-binding by electroblots of the SDS-denatured protein. Immunoblotting, using antiserum absorbed with bacteria or the outer membrane (OM) of the E. coli 055 strain, showed results concordant with inaccessibility to antibodies of the 38.5 kD porin protein in its native configuration in the bacterial cells, but immunoreactivity when contained in the OM. OM from strains of different genera of the Enterobacteriaceae and antisera against these strains when used in immunoblot analyses showed that the E. coli 055 porin protein harboured antigenic determinants which are common to the various genera of the enteric bacilli. Cross-reactivity with non-enteric Gram-negative bacteria was not observed.

Immunoadsorbent-purified antibodies in the study of antigenic relatedness of outer membrane proteins of enteric bacilli

Immunoadsorbent chromatography was used for purification of antibodies to E. coli 055 outer membrane proteins. Antibodies to the 33.5 kD and 7.5 kD proteins were eluted when rabbit antisera were applied to an epoxy-activated Sepharose 6B column to which the outer membrane was coupled in the presence of dioxane. ELISA coats prepared with sonicated bacteria showed binding of the eluted antibodies with strains of all of seven different species of the enteric bacilli, but not with other Gram-negative bacilli or cocci, or with Gram-positive cocci; immunoblot analysis of transblots of SDS-PAGE-separated bacteria showed that antibodies to both of the 33.5 kD and 7.5 kD E. coli outer membrane proteins cross-reacted with the enteric bacilli of different species. Both of the anti-33.5 kD and -7.5 kD antibodies were bound by intact E. coli 055 cells, but more efficiently by sonically disrupted or heat-treated bacteria. The results show that affinity-purified anti-OM antibodies were useful for the study of the antigenic relatedness of E. coli OM proteins with proteins of other bacteria.

Identification, immunochemical characterization, and purification of a major lipoprotein antigen associated with the inner (cytoplasmic) membrane of Escherichia coli

A major antigenic constituent of the inner membrane of Escherichia coli ML308-225 was identified as a 28.5-kilodalton lipoprotein containing covalently bound glycerol and palmitate. This lipoprotein corresponded to antigen 47 in the crossed immunoelectrophoresis profile of membrane vesicles (P. Owen and H.R. Kaback, Proc. Natl. Acad. Sci. USA 75:3148-3152, 1978) and to new lipoprotein 4 described for E. coli B by Ichihara et al. (S. Ichihara, H. Hussain, and S. Mizushima, J. Biol. Chem. 256:3125-3129, 1980). Experiments involving isopycnic centrifugation of spheroplast envelopes indicated that antigen 47 was enriched in cytoplasmic membrane subfractions of low density. The protein did not manifest an obvious association with peptidoglycan of the types displayed by the bound form of the Braun (Lpp) lipoprotein, the 21-kilodalton peptidoglycan-associated lipoprotein, or the ompF/C gene products. Antibodies specific for antigen 47 were used to demonstrate that the molecule was immunologically distinct from both the Braun lipoprotein and the peptidoglycan-associated lipoprotein of E. coli. Antigens of similar molecular mass to and cross-reacting with antigen 47 were present in the envelopes of eight type species of the Enterobacteriaceae. A protocol for the purification of antigen 47, based upon its solubility in a chloroform-methanol-water mixture, was developed.

NMR studies of the trp repressor from Escherichia coli. Characterisation and assignments of residue types

High-resolution proton nuclear magnetic resonance spectra of the trp repressor of Escherichia coli under various conditions are reported and analysed. The spectrum of the denatured state agrees with that predicted from the amino acid composition, with the exception of the two histidine residues, which have different chemical shifts although they titrate normally. The spectrum of the native protein shows the presence of extensive secondary and tertiary structure. Using information from chemical shifts, numbers of protons, titration behaviour, homonuclear chemical-shift-correlated spectroscopy and nuclear Overhauser enhancement correlated spectroscopy, most of the aromatic protons have been assigned to residue type. Further, about 30% of the aliphatic protons have been assigned to residue type by two-dimensional spectroscopy. Nuclear Overhauser enhancements establish that high-field methyl groups belonging to a valine residue lie directly over an aromatic ring.

Biochemical and immunological characterization of the cell surface of the fish pathogenic bacterium Aeromonas salmonicida

The identification of lipopolysaccharide as periodic acid-Schiff positive material, present in the membrane fraction of the fish pathogenic Gram-negative bacterium Aeromonas salmonicida, analyzed by SDS-polyacrylamide gel electrophoresis, is shown. Such analysis has revealed several periodic acid-Schiff positive bands and many membrane proteins among which a pathogenicity-related Mr 54000 protein as a constituent of an additional surface layer outside the outer membrane (Evenberg et al., (1982) Biochim. Biophys. Acta 684, 241-248). The latter protein, designated as additional cell envelope protein or ACE protein, has been purified and characterized in our laboratory (Evenberg and Lugtenberg, (1982) Biochim. Biophys. Acta 684, 249-254). Most strains produce both high and low molecular weight lipopolysaccharide species, presumably corresponding with the presence and (virtual) absence, respectively, of an O-antigenic chain. The property to produce high molecular weight lipopolysaccharide can be lost upon subculturing in laboratory growth media and such is greatly enhanced by the prior loss of the ability to produce ACE protein. Lipopolysaccharide and ACE protein were identified as the major antigens. A new polysaccharide-like antigen, designated as PS-antigen, was detected. Moreover, immunological indications for the presence of a lipoprotein in A. salmonicida are described. The surface localization of the antigens was determined by testing whether preadsorption of antisera by intact cells decreased the binding of IgG to these antigens, or decreased the ability of the sera to agglutinate cells. According to these criteria lipopolysaccharide, ACE protein and PS-antigen are the major surface-located antigens. Material cross-reactive with lipopolysaccharide, ACE protein and PS-antigen has been found in a large number of strains. Several lines of evidence indicate the presence of interactions between ACE protein and lipopolysaccharide. Based on these results a molecular model of the cell envelope of virulent A. salmonicida is presented.

Antigenic and immunogenic characteristics of subcellular fractions and whole cells of a rough E. coli 0111 (J5) mutant

Little information is available on the antigenic and immunogenic properties of an E. coli rough mutant: J5 derived from E. coli 0111:B4, and the relationship of J5 to other cross-reacting antigens of Enterobacteriaceae. Subcellular fractions of J5 and various antigen preparations were tested against antisera to Enterobacterial Common Antigen (ECA-Kunin) and to the Re mutant of S. minnesota (R595). ECA-Kunin was demonstrated in all subcellular fractions of the J5 mutant by means of indirect hemagglutination and by hemagglutination inhibition tests. This common antigen was separable from J5 LPS by ethanol fractionation and by phenol-chloroform-petroleum ether extraction. Treatment with alkali destroyed the hemagglutinating reactivity of ECA-Kunin and revealed the complex nature of J5 surface antigens; an alkalinized 20p30 fraction (containing cell wall components) contained both specific J5 antigen and an antigen which cross-reacted with S. minnesota and S. typhimurium. This antigen was shown by absorption studies to be a new common antigen other than Re LPS or ECA-Kunin. Studies of J5 LPS by ELISA demonstrated that there was a shared cross-reactive immunodeterminant between the glycolipids of J5 and Re. Accordingly, heat-killed J5 preparations were complex vaccines which were able to elicit an antibody response with at least two specificities: ECA-Kunin and the specific J5 antigen.

Transformation of Salmonella typhimurium with plasmid DNA: differences between rough and smooth strains

Lipopolysaccharide-defective mutants of Salmonella typhimurium were transformed by plasmid DNA with a Ca2+ treatment method. Only those mutants with an Rc or Rd2 chemotype, due to galE or rfaF mutations, respectively, gave efficiencies greater than 10(5) transformants per microgram of DNA, frequencies 8- to 630-fold higher than with smooth strains or other rough mutants.

Filamentous phage pre-coat is an integral membrane protein: analysis by a new method of membrane preparation

We show, using a simple, rapid fractionation method, that the precursor to the filamentous phage major coat protein is an integral membrane protein. The method, which consists of treatment of Escherichia coli with 0.1 N NaOH followed by centrifugation, leaves a subset of inner and outer membrane proteins in the NaOH pellet. Most proteins partition into the NaOH pellet (membrane) or supernatant (cytoplasm and periplasm) in a manner consistent with their subcellular location as determined by more conventional techniques. We find no evidence for cytoplasmic filamentous phage pre-coat protein in either wild-type of mutant-infected cells. Our evidence suggests that a protein identified as "soluble procoat" by K. Ito, G. Mandell and W. Wickner may be the amber fragment of a different phage protein.

Hydrophobic peptide auxotrophy in Salmonella typhimurium

The growth of a pleiotropic membrane mutant of Salmonella typhimurium with modified lipopolysaccharide composition was found to be strictly dependent on the peptone component of complex media. Nutritional Shiftdown into minimal media allowed growth for three to four generations. Of 20 commercial peptones, only enzymatic digests supported growth to varying degrees. Neither trace cations, amino acids, vitamins, carbohydrates, lipids, glutathione, polyamines, carbodimides, nor synthetic peptides stimulated growth; however, cells still metabolized carbohydrates, and amino acid transport systems were shown to be functional. A tryptic digest of casein was fractionated into four electrophoretically different peptide fractions of 1,000 to 1,200 molecular weight which supported growth to varying degrees. The best of these was further fractionated to two highly hydrophopic peptides. N-terminal modifications eliminated biological activity. Fluorescein-conjugated goat antibody to rabbit immunoglobulin G was used as a probe to detect antipeptide antibody-peptide complexes on membrane preparations. Cells grown on peptone distributed the peptide into both inner and outer membranes. The peptide could be removed with chaotropic agents, and cells had to be pregrown in peptone-containing media to bind the hydrophobic peptide. The gene (hyp) responsible for peptide auxotrophy was mapped at 44 to 45 units by conjugation.

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.

Localization of enterobacterial common antigen:immunogenic and nonimmunogenic enterobacterial common antigen-containing Escherichia coli

In rabbits immunized with intact bacteria, the immune response to the enterobacterial common antigen (ECA) predominantly consists of the production of immunoglobulin M antibodies. This is not dependent on whether the animals are immunized for a short (2 weeks) or a long (3 months) period of time. The highest ECA-specific immunoglobulin G titers were observed after a short immunization with living bacteria. ECA-specific antisera were obtained by absorption with appropriate ECA-negative mutants. The absorbed antisera were then separated on Sephadex G-200. The resulting immunoglobulin G fractions were conjugated to ferritin by glutardialdehyde and used to visualize the distribution of ECA in E. coli. Bacterial strains either possessing the immunogenic form of ECA (F470, 2387) or solely the nonimmunogenic form (F614) or being devoid of both (ECA-negative mutants F1283 and F1327) were labeled with the conjugates. Freezeetchings of ferritin-labeled strains showed a dense labeling of the outer membrane in case of ECA-immunogenic strains, an essentially weaker labeling of the non-immunogenic ECA mutant and, as expected, no labeling of ECA-negative mutants. Comparable results were obtained with the indirect immunofluorescence technique: the whole cell envelope of strain F470 showed a brilliant fluorescence, whereas a much lesser, spotty distribution of fluorescence was noted with strain F614 and none at all was noted with the ECA-negative strains. These data show that ECA is localized in the outer membrane of ECA-containing strains and further demonstrate that there is more in the immunogenic strains than in the nonimmunogenic ones.

A lipopolysaccharide-binding cell-surface protein from Salmonella minnesota. Isolation, partial characterization and occurrence in different Enterobacteriaceae

1. Protein extracts obtained from Salmonella minnesota Re mutant cells by treatment with EDTA/NaC1 solution contain a protein which exhibits high affinity to bacterial lipopolysaccharides. The isolation and partial characterization of this lipopolysaccharide-binding protein is described. 2. The protein was purified from EDTA extracts by a two-step procedure consisting of ion-exchange chromatography on CM-Sephadex and preparative polyacrylamide gel electrophoresis at pH 9.5. The yield of the total purification procedure was around 16%. 3. The resulting protein preparation was homogeneous on the basis of disc gel electrophoresis, dodecylsulfate gel electrophoresis, isoelectric focusing in polyacrylamide gel and immunoelectrophoresis. 4. The isoelectric point of the protein was found to be 10.3 at 4 degrees C. Its molecular weight determined by dodecylsulfate gel electrophoresis is 15000. Its amino acid composition is characterized by the absence of histidine and proline, a low content in tyrosine and high amounts of alanine, lysine, aspartic and glutamic acid residues, or their respective amides. 5. The lipopolysaccharide-protein association was shown to be mainly due to ionic interactions of the basic protein with negatively charged groups (probably phosphate and pyrophosphate groups) of the lipid A moiety. 6. Purified lipopolysaccharide-binding protein is immunogenic in rabbits, thus enabling the preparation of specific antiserum. 7. The protein is located at the surface of Salmonella minnesota Re mutant cells as revealed by antiserum absorption with total bacteria. Ferritin-labelling studies further demonstrated that it is evenly spread over the entire cell surface. 8. Comparative antiserum absorption studies using smooth and rough strains of Salmonella minnesota, Salmonella typhimurium, Escherichia coli, Klebsiella and Shigella revealed the presence of lipopolysaccharide-binding protein (or a serologically cross-reacting antigen) in most of the strains tested. From these results the protein can be considered as a common antigen of Enterobacteriaceae.

Ultrastructural study of Salmonella typhimurium treated with membrane-active agents: specific reaction dansylchloride with cell envelope components

Amino groups of cell envelope proteins, lipids, and lipopolysaccharides cannot be labeled in intact cells of Salmonella typhimurium G 30 by using 5-dimethylaminonaphthalene-1-sulfonylchloride incorporated in lecithin-cholesterol vesicles. However, application of membrane-interacting agents like tris(hydroxymethyl)aminomethane (Tris)-hydrochloride, ethylenediaminetetraacetate (Na salt) (EDTA), divalent cations, and sublethal doses of the cationic antibacterial agents polymyxin B and chlorhexidine induced specific fluorescent labeling of envelope proteins and lipids but not of cytoplasmic compounds, with the exception of a soluble protein with a molecular weight of 46,000 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Treatment with Tris-hydrochloride buffer produced labeling of the heat-modifiable protein B/B(+) and of proteins with molecular weights of 26,000, 22,000, and below 17,000. A combination of Tris-hydrochloride and EDTA induced additional dansylation of the major protein A and of proteins of molecular weights 80,000, 60,000, and 44,000. Polymyxin B and chlorhexidine caused similar labeling patterns. In every case, except with divalent cation treatment, protein B/B(+) was the most prominently labeled species. Phosphatidylethanolamine was dansylated up to 30%. Lipopolysaccharide was not reactive under any condition or treatment. In addition, the peptidoglycan-bound lipoprotein did not react with dansylchloride in either intact or Tris-hydrochloride-treated cells. The results are discussed with regard to a possible localization of labeled and unlabeled compounds of the cell envelope on the basis of a model placing cell envelope amino groups into ion-ion interactions with anionic components of other envelope compounds like phosphate and carboxyl groups.

Immunochemical analysis of inner and outer membranes of Escherichia coli by crossed immunoelectrophoresis

Isolated membrane fractions of Escherichia coli K-12 yielded complex immunoprecipitate patterns when Triton X-100 and sodium dodecyl sulfate extracts were examined by crossed immunoelectrophoresis with antienvelope immunoglobulins. Twelve of the 46 antigens in the immunoprecipitate patterns of inner (plasma) membranes were identified by zymograms and/or by the use of specific antisera. The following enzyme activities were detected in immunoprecipitates: 6-phosphogluconate dehydrogenase (EC 1.1.1.43); adenosine triphosphatase (EC 3.6.1.3); glutamate dehydrogenase (EC 1.4.1.4), two separate components; malate dehydrogenase (EC 1.1.1.37); dihydroorotate dehydrogenase (EC 1.3.3.1); succinate dehydrogenase (EC 1.3.99.1); lactate dehydrogeanse (EC 1.1.1.27); reduced nicotinamide adenine dinucleotide dehydrogenase (EC 1.6.99.3); protease (EC 3.4.21.1); and glycerol 3-phosphate dehydrogenase (EC 1.1.99.5). The corresponding immunoprecipitate pattern for isolated outer membranes consisted of at least 25 discrete antigens and differed strikingly from that obtained with inner membranes. Two major immunogens were identified as lipopolysaccharide and Braun lipoprotein. A protease-active immunoprecipitate was also detected in this fraction, but attempts to identify the Rosenbusch matrix protein in the crossed immunoelectrophoretic profile were unsuccessful.

[Membrane permeability and antibiotic resistance in bacteria]

The resistance of mainly gram-negative bacteria against antibiotics is discussed with respect to new discoveries on the permeability barrier of the outer membrane. Proteins originally characterized as virus receptors have been found to be involved in the translocation of certain substrates across the membrane. Antibiotics can reach their target by the same "pores".

Outer membrane of Salmonella typhimurium. Identification of proteins exposed on cell surface

Proteins exposed on the outer surface of the outer membrane of Salmonella typhimurium were identified by reacting intact cells with a covalent labeling reagent. Since the outer membrane permitted the free diffusion of small hydrophilic molecules, we used a macromolecular reagent, CNBr-activated dextran, as the non-penetrating labeling agent. We also used a mutant producing a lipopolysaccharide with a very short (i.e. hexasaccharide) carbohydrate chain, in order to avoid steric hindrance by the carbohydrates on membrane surface. Results showed that out of the four "major" proteins of molecular weight around 35 000, three were exposed, and that at least six other proteins were also exposed on cell surface. Only two or three outer membrane proteins consistently did not react with the reagent in intact cells.

Conformational studies on murein-lipoprotein from the outer membrane of Escherichia coli

Conformational studies on an isolated integral membrane protein are reported. Lipoprotein of Escherichia coli outer membrane was released from murein by treatment with either lysozyme or trypsin. The isolated lysozyme-released lipoprotein (lipoprotein I) contained 2 or 3 muropeptides covalently linked at the C-terminal end, while the trypsin-released lipoprotein (lipoprotein II) was free of muropeptides and lacked the C-terminal peptide Tyr-Arg-Lys. Circular dichroism spectra of the two preparations were essentially identical, and they show an alpha-helix content of about 80%. According to calculations based on the Chou-Fasman rules for proteins of known sequence, lipoprotein is 64% alpha-helix and 15% beta-structure. Infrared spectroscopy qualitatively supports these values. The conformation was stable in the pH range of 5 - 12. Danaturation of lipoprotein by heat, 8 M urea, or sodium dodecylsulphate was a fully reversible, cooperative process. The thermal denaturation of lipoprotein occurs in two steps with transition points at 79.4 degrees C for lipoprotein I and at 85.1 degrees C for lipoprotein II. Lioprotein markedly changes conformation at dodecylsulphate concentrations where micelle formation sets in. The unusual behaviour of the lipoprotein convormation in sodium dodecylsulphate is discussed in relation to the lipoprotein conformation and aggregation within the membrane.

Functional organization of the outer membrane of escherichia coli: phage and colicin receptors as components of iron uptake systems

The functional interaction of outer membrane proteins of E. coli can be studied using phage and colicin receptors which are essential components of penetration systems. The uptake of ferric iron in the form of the ferrichrome complex requires the ton A and ton B functions in the outer membrane of E. coli. The ton A gene product is the receptor protein for phage T5 and is required together with the ton B function by the phages T1 and ø80 to infect cells and by colicin M and the antibiotic albomycin, a structural analogue of ferrichrome, to kill cells. The ton B function is necessary for the uptake of ferric iron complexed by citrate. Iron complexed by enterochelin is only transported in the presence of the ton B and feu functions. Cells which have lost the feu function are resistant to the colicins B, I or V while ton B mutants are resistant to all 3 colicins. The interaction of the ton A, ton B, and feu functions apparently permits quite different "substrates" to overcome the permeability barrier of the outer membrane. It was shown for ferrichrome dependent iron uptake that the complexing agent was not altered and could be used repeatedly. Only very low amounts of 3H-labeled ferrichrome were found in the cell. It is possible that the iron is mobilized in the membrane and that desferri-ferrichrome is released into the medium without having entered the cytoplasm. Growth on ferrichrome as the sole iron source was used to select revertants of T5 resistant ton A mutants. All revertants exhibited wild-type properties with the exception of partial revertants. In these 4 strains, as in the ton A mutants, the ton A protein was not detectable by SDS polyacrylamide gel electrophoreses of outer membranes. Albomycin resistant mutants were selected and shown to fall into 5 categories: 1) ton A; 2) ton B mutants; 3) mutants with no iron transport defects and normal ton A/ton B functions, which might be target site mutants; 4) mutants which were deficient in ferrichrome-mediated iron uptake but had normal ton A/ton B functions. We tentatively consider that the defect might be located in the active transport system of the cytoplasmic membrane; 5) a variety of mutants with the following general properties: most of them were resistant to colicin M, transported iron poorly, and, like ton B mutants, contained additional proteins in the outer membrane. The outer membrane protein patterns of wild-type and ton B mutant strains were compared by slab gel electrophoresis in an attempt to identify a ton B protein. It was observed that under most growth conditions, ton B mutants overproduced 3 proteins of molecular weights 74,000-83,000. In extracted, iron-deficient medium, both the wild-type and ton B mutant strains had similar large amounts of these proteins in their outer membranes. The appearance of these proteins was suppressed by excess iron in both wild-type and mutant. From this evidence it is apparent that the proteins appear as a response to low intracellular iron rather than being controlled by the ton B gene...