Outer membrane of Salmonella typhimurium: Transmembrane diffusion of some hydrophobic substances

The outer membrane, which is composed of lipopolysaccharide, phospholipids, and proteins, is a layer of the cell wall of Gram-negative bacteria, and apparently acts as a penetration barrier for various substances. It had been shown by other workers that “deep rough” mutants of Salmonella typhimurium, whose lipopolysaccharides lack most of the saccharide chains, were much more sensitive than the wild type strain to certain antibiotics and dyes, but not to others. We found that the former group of agents are usually hydrophobic and the latter group mostly hydrophilic. All hydrophilic antibiotics had molecular weights lower than 650, and one of them was shown to diffuse through the outer membrane at 0 °C. In contrast, some hydrophobic antibiotics had molecular weights in excess of 1200, and the rate of diffusion of one of them was shown to be extremely dependent both on temperature and on the structure of lipopolysaccharide present. These data and results presented elsewhere suggest, but do not necessarily prove, that most hydrophilic antibiotics diffuse through aqueous pores, whereas hydrophobic antibiotics and dyes mainly penetrate by dissolving into the hydrocarbon interior of the outer membrane. In contrast to the outer membrane of deep rough mutants, that of the wild type strain and less defective rough mutants was unusual among biological membranes in that it was practically impermeable to hydrophobic agents. It is proposed that the difference in hydrophobic permeability between the two types of strain is due to radical differences in the organization of the outer membrane, more specifically to the presence or absence of exposed phospholipid bilayer regions.     

Architecture of the outer membrane of Escherichia coli K12. II. Freeze fractur morphology of wild type and mutant strains

Freeze fracturing electron microscopy of Escherichia coli K12 cells showed that the outer fracture face of the outer membrane is densily occupied with particles. On the inner fracture face of the outer membrane, pits are visible, which are probably complementary to the particles at opposite fracture face. This observation suggests that the particles are micelle-like. In some mutants which lack one or more major outer membrane proteins the density of particles is reduced. The loss of protein d appeared to a prerequisite for this phenomenon. However, mutants which lack all glucose and heptose-bound phosphate in their lipopolysaccharide also have a reduction in particle density whereas, the amount of protein d is normal. Moreover, loss of lipopolysaccharide by EDTA treatment also caused a reduction in the density of particles. From these results it is hypothesized that the particles consist of lipopolysaccharide aggregates stabilized by divalent cations and probably complexed with protein and/or phospholipid.     

Architecture of the outer membrane of Escherichia coli K12. II. Freeze fracture morphology of wild type and mutant strains.

Freeze fracturing electron microscopy of Escherichia coli K12 cells showed that the outer fracture face of the outer membrane is densily occupied with particles. On the inner fracture face of the outer membrane, pits are visible, which are probably complementary to the particles at opposite fracture face. This observation suggests that the particles are micelle-like. In some mutants which lack one or more major outer membrane proteins the density of particles is reduced. The loss of protein d appeared to a prerequisite for this phenomenon. However, mutants which lack all glucose and heptose-bound phosphate in their lipopolysaccharide also have a reduction in particle density whereas, the amount of protein d is normal. Moreover, loss of lipopolysaccharide by EDTA treatment also caused a reduction in the density of particles. From these results it is hypothesized that the particles consist of lipopolysaccharide aggregates stabilized by divalent cations and probably complexed with protein and/or phospholipid.     

Biosynthesis and structure of lipopolysaccharide in an outer membrane-defective mutant of Escherichia coli J5.

Membrane-defective mutants of Escherichia coli J5 were isolated on the basis of supersensitivity to the antibiotic novobiocin. These mutants display an increased sensitivity to a wide range of antibiotics and to several dyes and detergents. In addition, several mutants leak the periplasmic enzymes, alkyline phosphatase and ribonuclease. This evidence indicates an outer membrane defect in these mutants. The inner and outer membranes of one mutant were separated and subjected to compositional analysis. A deficiency in galactose containing lipopolysaccharide in the outer membrane of the mutant was observed. Two possible causes of this deficiency were examined and discounted: defective galactose uptake into the cell, and defective translocation of lipopolysaccharide from the inner membrane. Extraction and chemical analysis of mutant and wild type lipopolysaccharides suggests that the mutant is defective in the enzyme which transfers glucose to the growing lipopolysaccharide core, UDPglucose transferase. Thus, the mutant's deficiency in galactose-containing lipopolysaccharide can be ascribed to the fact that addition of glucose to the lipopolysaccharide core is a prerequisite for galactose addition. The physiological implications of this alteration are discussed.     

Biosynthesis and structure of lipopolysaccharide in an outer membrane-defective mutant of Escherichia coli J5

Membrane-defective mutants of Escherichia coli J5 were isolated on the basis of supersensitivity to the antibiotic novobiocin. These mutants display an increased sensitivity to a wide range of antibiotics and to several dyes and detergents. In addition, several mutants leak the periplasmic enzymes, alkaline phosphatase and ribonuclease. This evidence indicates an outer membrane defect in these mutants. The inner and outer membranes of one mutant were separated and subjected to compositional analysis. A deficiency in galactose-containing lipopolysaccharide in the outer membrane of the mutant was observed. Two possible causes of this deficiency were examined and discounted: defective galactose uptake into the cell, and defective translocation of lipopolysaccharide from the inner membrane. Extraction and chemical analysis of mutant and wild type lipopolysaccharides suggests that the mutant is defective in the enzyme which transfers glucose to the growing lipopolysaccharide core, UDPglucose transferase. Thus, the mutant's deficiency in galactose-containing lipopolysaccharide can be ascribed to the fact that addition of glucose to the lipopolysaccharide core is a prerequisite for galactose addition. The physiological implications of this alteration are discussed.     

Mutational change of membrane architecture. Mutants of Escherichia coli K12 missing major proteins of the outer cell envelope membrane

Mutant of Escherichia coli have been analyzed which miss two of the major proteins of the outer cell envelope membrane. The two proteins I and II1, normally are present at high concentrations (about 10⁵ copies per cell).In such mutants, as compared with wild type, the phospholipid-to-protein ratio in the outer membrane has increased by a factor of 2.3 causing a considerable difference in density between wild type and mutant membranes. The concentrations of two other major components of the outer membrane, lipopolysaccharide and Braun's lipoprotein, did not change.The protein-deficient mutants do not exhibit gross functional defects in vitro. An increased sensitivity to EDTA and a slight such increase to dodecyl sulfate (but not to deoxycholate or Triton X-100) was observed, loss of so-called periplasmic enzymes was not found, and other differences to wild type are marginal. The mutants can grow with normal morphology. It is not possible, however, to prepare “ghosts” (particles of size and shape of the cell without murein, surrounded by a derivative of the outer membrane, and posssessing the major proteins of this membrane) from them. This fact confirms our earlier suggestion that the proteins in question are required for the shape maintenance phenomenon in ghosts, and the mutants reject the speculation that these proteins are involved in the expression of the genetic information specifying cellular shape.Freeze-fracturing showed that in mutant cells, and in sharp contrast to wild type, the far predominant fracture plane is within the outer membrane. The concentration of the well known densely packed particles at the outer, concave leaflet of this fracture plane is greatly reduced. It was not possible, however, to clearly establish that one or the other protein is part of these particles because these ultrastructural differences were not apparent in mutants missing either one of the proteins only. The biochemical and ultrastructural data allow the conclusion that the loss of two major proteins and the concomitant increase of phospholipid concentration has changed the architecture of the outer membrane from a highly oriented structure. with a large fraction of protein-protein interaction, to one predominantly exhibiting planar lipid bilayer characteristics. E. coli thus can assemble rather different outer membranes, afact excluding that outer membrane formatin constitutes a highly ordered or strictly sequential assembly-line process.     

Interaction of lipopolysaccharide with detergents and its possible role in the detergent resistance of the outer membrane of Gram-negative bacteria.

In the presence of MgCl2, amounts of detergents which disrupted phospholipid vesicles caused lipopolysaccharide I from Proteus mirabilis to aggregate and form vesicular, membrane-like structures. Vesicle formation with P. mirabilis lipopolysaccharide II containing longer O-polysaccharide chains was extremely poor. Lipopolysaccharides of Salmonella minnesota R mutants (chemotypes Ra, Rc and Re) displayed a growing tendency for vesicle formation with increasing deficiency of the R core polysaccharide. Lipopolysaccharides of chemotypes Rc and Re produced vesicles even in the absence of MgCl2 and detergent. Spherical aggregates consisting of P. mirabilis lipopolysaccharide I MgCl2 and detergent were unable to either entrap or retain [14C]-sucrose, [3H=inulin or [3H]dextran. On the other hand, S. minnesota R mutant lipopolysaccharides of chemotypes Rc and Re could entrap all three saccharides and retain them for at least short periods of time. Leakage of [3H]-inulin out of re-lipopolysaccharide vesicles was greatly retarded by addition of MgCl2 to the vesicle system. Incorporation of P. mirabilis lipopolysaccharide I or S. minnesota Rc lipopolysaccharide into phospholipid vesicles protected these model membranes from disruption by detergent. This suggested a similar protective function of lipopolysaccharide in the outer membrane of enteric bacteria against the action of surfactants occurring in their normal intestinal habitat.     

Architecture of the outer membrane of Escherichia coli K12. I. Action of phospholipases A2 and C on wild type strains and outer membrane mutants.

Phospholipids in whole cells of wild type Escherichia coli K12 are not degraded by exogenous phospholipases, whereas those of isolated outer membranes are completely degraded. It is concluded that the resistance of phospholipids in whole cells is due to shielding by one or more other outer membrane components. The nature of the shielding component(s) was investigated by testing the sensitivity of whole cells of a number of outer membrane mutants. Mutants lacking both major outer membrane proteins b and d or the heptose-bound glucose of their lipopolysaccharide, are sensitive to exogenous exogenous phospholipases. Moreover, cells of a mutant which lacks protein d can be sensitized by pretreatment of the cells with EDTA. From these results and from data on the chemical composition of the outer membranes, it is concluded that proteins b and d, the heptose-bound glucose of lipopolysaccharide and divalent cations are responsible for the inaccessibility of phospholipids to to exogenous phospholipases.     

Architecture of the outer membrane of Escherichia coli K12. I. Action of phospholipases A2 and C on wild type strains and outer membrane mutants

Phospholipids in whole cells of wild type Escherichia coli K12 are not degraded by exogenous phospholipases, whereas those of isolated outer membranes are completely degraded. It is concluded that the resistance of phospholipids in whole cells is due to shielding by one or more other outer membrane components. The nature of the shielding component(s) was investigated by testing the sensitivity of whole cells of a number of outer membrane mutants. Mutants lacking both major outer membrane proteins b and d or the heptose-bound glucose of their lipopolysaccharide, are sensitive to exogenous phospholipases. Moreover, cells of a mutant which lacks protein d can be sensitized by pretreatment of the cells with EDTA. From these results and from data on the chemical composition of the outer membranes, it is concluded that proteins b and d, the heptose-bound glucose of lipopolysaccharide and divalent cations are responsible for the inaccessibility of phospholipids to exogenous phospholipases.     

Outer membrane of Salmonella typhimurium. Electron spin resonance studies

The supramolecular structure of the outer membrane of Salmonella typhimurium that produces an Rc-type lipopolysaccharide was studied by adding spin-labeled fatty acid probes to membranes as well as model bilayers. Lipopolysaccharide of this organism apparently formed a bilayer structure in 0.2 M NaCl/0.01 M MgCl₂, and the electron spin resonance spectra suggested that the motion of the segments of hydrocarbon chains near the carboxyl end was quite restricted even at high temperature; this is presumably due to the anchoring of more than a dozen fatty acid residues to a single backbone structure. In the presence of Mg²⁺, we could produce lipopolysaccharide-phospholipid mixed bilayers containing up to 50% (by weight) lipopolysaccharide. Their spectra showed no sign of major heterogeneity, and the maximum hypertine splitting values were considerably larger than in phospholipid-only liposomes; these results suggest that the two components are finely interspersed and that the mobility of phospholipid hydrocarbons in severely restricted by the hydrocarbon chains of lipopolysaccharide. In spite of the presence of lipopolysaccharide in an amount equal to or exceeding that of phospholipids, the outer membrane produced spectra remarkably similar to those of the inner membrane, which does not contain lipopolysaccharide, and there was little sign of immobilization by lipopolysaccharides. Signals corresponding to the pure lipopolysaccharide phase were not detected, either. These results suggest that the phospholipids and lipopolysaccharides are segregated into separate domains in the outer membrane, and the fatty acid probes enter almost exclusively into the phospholipid domains. This conclusion was fully corroborated by determining, through the exchange broadening of line width, the total area of the domains that accommodated the spin label probes.     

Analysis of the host ranges of transposon bacteriophages Mu, MuhP1, and D108 by use of lipopolysaccharide mutants of Salmonella typhimurium LT2.

The lipopolysaccharide receptors for the mutator bacteriophages Mu, MuhP1, and D108 were investigated with lipopolysaccharide mutants of Salmonella typhimurium LT2. Mu adsorbed only to mutants lacking the terminal O antigen but retaining the main chain sugars of the core; the side chain N-acetylglucosamine was not required. MuhP1 and D108 adsorbed partially to cells with the same receptors but adsorbed well only to cells with shorter lipopolysaccharides of the Rc and Rd1 chemotypes.     

Antimicrobial action of nisin against Salmonella typhimurium lipopolysaccharide mutants.

The antimicrobial activity of nisin against outer membrane lipopolysaccharide mutants of Salmonella typhimurium LT2 was investigated. Nisin sensitivity was associated with the extent of saccharide deletions from the outer membrane core oligosaccharide. The results indicated that the core oligosaccharide in lipopolysaccharide plays a role in nisin sensitivity.     

Antimicrobial action of nisin against Salmonella typhimurium lipopolysaccharide mutants.

The antimicrobial activity of nisin against outer membrane lipopolysaccharide mutants of Salmonella typhimurium LT2 was investigated. Nisin sensitivity was associated with the extent of saccharide deletions from the outer membrane core oligosaccharide. The results indicated that the core oligosaccharide in lipopolysaccharide plays a role in nisin sensitivity.     

Supramolecular structure of lipopolysaccharide and free lipid A under physiological conditions as determined by synchrotron small-angle X-ray diffraction

Lipopolysaccharides, the major amphiphilic components of the outer leaflet of the outer membrane of Gram-negative bacteria, may assume various three-dimensional supramolecular structures depending on molecular properties (e.g. chemical structure) and on ambient conditions (e.g. temperature, concentration of divalent cations). We applied synchrotron small-angle X-ray diffraction to investigate the supramolecular structures of natural and synthetic Escherichia-coli-type lipid A, of lipid A from Salmonella minnesota, and of rough mutant lipopolysaccharides of E. coli and S. minnesota under physiological water content (greater than 90%) at different temperatures (20, 37, and 55 degrees C) and at different lipid/divalent cation molar ratios (20:1 to 1:1). We found that in the absence of divalent cations rough mutant lipopolysaccharide and free lipid A form unilamellar structures with the main reflections centered around 4.50 nm for free lipid A, 4.80 nm for Re lipopolysaccharide, and 5.90 nm for Rd1 lipopolysaccharide at 20 degrees C, i.e. below the beta----alpha acyl-chain-melting transition temperature. Above this temperature, the reflections are shifted to 4.30 nm for free lipid A (at 55 degrees C), 4.60 nm for Re lipopolysaccharide (at 37 degrees C), and to 5.50 nm for Rd1 lipopolysaccharide (at 37 degrees C). The addition of divalent cations leads (at lower concentrations, i.e. lipid/cation molar ratios 20:1 to 5:1) to sharper reflections expressing a higher state of order and to a shift of the center of the main reflections lying now at 5.10 nm for free lipid A, 6.40 nm for Re and 7.20 nm for Rd1 lipopolysaccharide at 20 degrees C. At higher concentrations of divalent cations (e.g. lipid/cation molar ratio 1:1), an increasing tendency to form nonlamellar, inverted cubic structures is observed which is indicated by the occurrence of another main periodicity and/or of reflections with spacing ratios 1: square root of 2, 1: square root of 3 of the main periodicity. The tendency to assume inverted cubic structures is only weakly pronounced for rough mutant lipopolysaccharides but dominant for free lipid A even at physiological temperature and divalent cation concentration.     

Outer membrane of Salmonella typhimurium. Electron spin resonance studies.

The supramolecular structure of the outer membrane of Salmonella typhimurium that produces an Rc-type lipopolysaccharide was studied by adding spin-labeled fatty acid probes to membranes as well as model bilayers. Lipopolysaccharide of this organism apparently formed a bilayer structure in 0.2 M NaCl/0.01 M MgCl2, and the electron spin resonance spectra suggested that the motion of the segments of hydrocarbon chains near the carboxyl end was quite restricted even at high temperature; this is presumably due to the anchoring of more than a dozen fatty acid residues to a single backbone structure. In the presence of Mg2+, we could produce lipoplysaccharide-phospholipid mixed bilayers contining up to 50% (by weight) lipoplysaccharide. Their spectra showed no sign of major heterogeneity, and the maximum hyperfine splitting values were considerably larger than in phospholipid-only liposomes; these results suggest that the two components are finely interspersed and that the mobility of phospholipid hydrocarbons is severely restricted by the hydrocarbon chains of lipopolysaccharide. In spite of the presence of lipoplysaccharide in an amount equal to or exceeding that of phospholipids, the outer membrane produced spectra remarkably similar to those of the inner membrane, which does not contain lipoplysaccharide, and there was little sign of immobilization by lipopolysaccharides. Signals corresponding to the pure lipoplysaccharide phase were not detected, either. These results suggest that the phospholipids and lipopolysaccharides are segregated into separate domains in the outer membrane, and the fatty acid probes enter almost exclusively into the phospholipid domains. This conclusion was fully corroborated by determining, through the exchange broadening of line width, the total area of the domains that accommodated the spin label probes.     

rfaP mutants of Salmonella typhimurium

Salmonella typhimurium rfaP mutants were isolated and characterised with respect to their sensitivity towards hydrophobic antibiotics and detergents, and their lipopolysaccharides were chemically analysed. The rfaP mutants were selected after diethylsulfate mutagenesis or as spontaneous mutants. The mutation in two independent mutants SH7770 (line LT2) and SH8551 (line TML) was mapped by cotransduction with cysE to the rfa locus. The mutants were sensitive to hydrophobic antibiotics (clindamycin, erythromycin and novobiocin) and detergents (benzalkoniumchloride and sodium dodecyl sulfate). Analysis of their lipopolysaccharides by chemical methods and by sodium dodecyl sulfate/polyacrylamide gel electrophoresis revealed that their saccharide portion was, to a large extent, of chemotype Rc with small proportions of material containing a more complete core oligosaccharide and O-specific chains. Only 2.5 mol phosphate/mol lipopolysaccharide was found whereas the phosphate content of the lipopolysaccharide of a galE mutant strain was 4.8 mol. Thus the rfaP mutant lipopolysaccharides lacked more than two phosphate residues. Assessment of the location of phosphate groups in rfaP lipopolysaccharides revealed the presence of at least 2 mol phosphate in lipid A, indicating that the core oligosaccharide was almost devoid of phosphate. The chemical, physiological and genetic data obtained for these mutants are in full agreement with those reported earlier for rfaP mutants of Salmonella minnesota.     

Structural and immunochemical homogeneity of Aeromonas salmonicida lipopolysaccharide.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to analyze the lipopolysaccharides of typical and atypical strains of the fish pathogen Aeromonas salmonicida. 32P intrinsically radiolabeled lipopolysaccharide in sarcosinate-extracted outer membrane preparations, lipopolysaccharide stained by silver in proteinase K-digested outer membrane preparations and whole cell lysates, as well as purified lipopolysaccharide, displayed O-polysaccharide chains which were unusually homogeneous with respect to chain length. Chemical analysis further revealed that the sugar composition of the smooth lipopolysaccharide purified from three typical strains was very similar. Immunoblotting and immunofluorescent staining with both polyclonal and monoclonal antibody showed that the O-polysaccharide chains were strongly immunogenic and were antigenically cross-reactive on typical and atypical strains from diverse origins. Immunofluorescence analysis and phage binding studies demonstrated that a number of these O-polysaccharide chains traversed the surface protein array of virulent strains of A. salmonicida and were exposed on the cell surface.     

Freeze etch morphology of outer membrane mutants of Escherichia coli K12

Freeze etching showed that the loss of each of the major outer membrane proteins b, c or d in mutants of Escherichia coli K12 does not influence the morphology of fracture faces of the outer membrane.Mutants that possess a heptose-deficient lipopolysaccharide and which in addition are deficient in one or more major outer membrane proteins exhibit a reduction in the number of intramembranous particles of the outer membrane.Moreover it was shown that lipid phase transitions induce a lateral lipid protein separation in the outer membrane, similar to that found in the cytoplasmic membrane.     

Localization of the terminal steps of O-antigen synthesis in Salmonella typhimurium.

Previous immunoelectron microscopic studies have shown that both the final intermediate in O-antigen synthesis, undecaprenol-linked O polymer, and newly synthesized O-antigenic lipopolysaccharide are localized to the periplasmic face of the inner membrane (C. A. Mulford and M. J. Osborn, Proc. Natl. Acad. Sci. USA 80:1159-1163, 1983). In vivo pulse-chase experiments now provide further evidence that attachment of O antigen to core lipopolysaccharide, as well as polymerization of O-specific polysaccharide chains, takes place at the periplasmic face of the membrane. Mutants doubly conditional in lipopolysaccharide synthesis [kdsA(Ts) pmi] were constructed in which synthesis of core lipopolysaccharide and O antigen are temperature sensitive and mannose dependent, respectively. Periplasmic orientation of O antigen:core lipopolysaccharide ligase was established by experiments showing rapid chase of undecaprenol-linked O polymer, previously accumulated at 42 degrees C in the absence of core synthesis, into lipopolysaccharide following resumption of core formation at 30 degrees C. In addition, chase of the monomeric O-specific tetrasaccharide unit into lipopolysaccharide was found in similar experiments in an O-polymerase-negative [rfc kdsA(Ts) pmi] mutant, suggesting that polymerization of O chains also occurs at the external face of the inner membrane.     

Size heterogeneity of Salmonella typhimurium lipopolysaccharides in outer membranes and culture supernatant membrane fragments.

Enterobacteriaceae cells growing in liquid media shed fragments of their outer membranes. These fragments, which may constitute a biologically important form of gram-negative bacterial endotoxin, have been reported to contain proteins, phospholipids, and lipopolysaccharides (LPS). In this study we compared the sizes of LPS molecules in shed membrane fragments and outer membranes from cells growing in broth cultures. Using conditional mutants of Salmonella typhimurium which incorporate specific sugars into LPS, we analyzed radiolabeled LPS by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This technique revealed that S. typhimurium LPS are more heterogeneous than previously known; molecules possessing from 0 to more than 30 O-chain repeat units were identified in outer membranes, supernatant fragments, and purified LPS. The size distributions of LPS molecules in outer membranes and supernatant fragments were similar; supernatant fragments appeared to be slightly enriched in molecules with long O-polysaccharide chains. Our results indicate the LPS molecules of many sizes are synthesized, translocated to outer membranes, and released into culture supernatants. Since the hydrophilic O-polysaccharides extend from bacterial surfaces into the aqueous environment, our findings suggest that the cell surface topography of this bacterium may be very irregular. We also speculate that heterogeneity in the degree of polymerization of O-antigenic side chains may influence the interactions of the toxic moiety of LPS (lipid A) with host constituents.