Cell-surface alterations in Escherichia coli K-12 with chromosmal mutations changing ampicillin resistance

Origins of Yersinia pestis sensitivity to the arylomycin antibiotics and the inhibition of type I signal peptidase

Yersinia pestis is the etiologic agent of the plague. Reports of Y. pestis strains that are resistant to each of the currently approved first-line and prophylactic treatments point to the urgent need to develop novel antibiotics with activity against the pathogen. We previously reported that Y. pestis strain KIM6+, unlike most Enterobacteriaceae, is susceptible to the arylomycins, a novel class of natural-product lipopeptide antibiotics that inhibit signal peptidase I (SPase). In this study, we show that the arylomycin activity is conserved against a broad range of Y. pestis strains and confirm that it results from the inhibition of SPase. We next investigated the origins of this unique arylomycin sensitivity and found that it does not result from an increased affinity of the Y. pestis SPase for the antibiotic and that alterations to each component of the Y. pestis lipopolysaccharide-O antigen, core, and lipid A-make at most only a small contribution. Instead, the origins of the sensitivity can be traced to an increased dependence on SPase activity that results from high levels of protein secretion under physiological conditions. These results highlight the potential of targeting protein secretion in cases where there is a heavy reliance on this process and also have implications for the development of the arylomycins as an antibiotic with activity against Y. pestis and potentially other Gram-negative pathogens.

Magainins and the disruption of membrane-linked free-energy transduction

Magainins, a family of positively charged peptides, are partly if not wholly responsible for antimicrobial activity in skin extracts of Xenopus laevis. We report here that members of the magainin family--i.e., the 21-amino acid peptide PGLa and the 23-amino acid peptide magainin 2 amide (PGSa)--dissipate the electric potential across various energy-transducing membranes and thus uncouple respiration from other free-energy-requiring processes. We propose that this is a likely mechanism for the antimicrobial effects of these compounds.

Interaction between complement subcomponent C1q and bacterial lipopolysaccharides

The heptose-less mutant of Escherichia coli, D31m4, bound complement subcomponent C1q and its collagen-like fragments (C1qCLF) with Ka values of 1.4 x 10(8) and 2.0 x 10(8) M-1 respectively. This binding was suppressed by chemical modification of C1q and C1qCLF using diethyl pyrocarbonate (DEPC). To investigate the role of lipopolysaccharides (LPS) in this binding, biosynthetically labelled [14C]LPS were purified from E. coli D31m4 and incorporated into liposomes prepared from phosphatidylcholine (PC) and phosphatidylethanolamine (PE) [PC/PE/LPS, 2:2:1, by wt.]. Binding of C1q or its collagen-like fragments to the liposomes was estimated via a flotation test. These liposomes bound C1q and C1qCLF with Ka values of 8.0 x 10(7) and 2.0 x 10(7) M-1; this binding was totally inhibited after chemical modification of C1q and C1qCLF by DEPC. Liposomes containing LPS purified from the wild-strain E. coli K-12 S also bound C1q and C1qCLF, whereas direct binding of C1q or C1qCLF to the bacteria was negligible. Diamines at concentrations which dissociate C1 into C1q and (C1r, C1s)2, strongly inhibited the interaction of C1q or C1qCLF with LPS. Removal of 3-deoxy-D-manno-octulosonic acid (2-keto-3-deoxyoctonic acid; KDO) from E. coli D31m4 LPS decreases the binding of C1qCLF to the bacteria by 65%. When this purified and modified LPS was incorporated into liposomes, the C1qCLF binding was completely abolished. These results show: (i) the essential role of the collagen-like moiety and probably its histidine residues in the interaction between C1q and the mutant D31m4; (ii) the contribution of LPS, particularly the anionic charges of KDO, to this interaction.

Antibody-independent interaction between the first component of human complement, C1, and the outer membrane of Escherichia coli D31 m4

The heptoseless mutant of Escherichia coli, E. coli D31 m4, binds C1q and C1 at 0 degrees C and at low ionic strength (I0.07). Under these conditions, the maximum C1q binding averages 3.0 X 10(5) molecules per bacterium, with a Ka of 1.4 X 10(8) M-1. Binding involves the collagen-like region of C1q, as shown by the capacity of C1q pepsin-digest fragments to bind to E. coli D31 m4, and to compete with native C1q. Proenzyme and activated forms of C1 subcomponents C1r and C1s and their Ca2+-dependent association (C1r-C1s)2 do not bind to E. coli D31 m4. In contrast, the C1 complex binds very effectively, with an average fixation of 3.5 X 10(5) molecules per bacterium, and a Ka of 0.25 X 10(8) M-1, both comparable with the values obtained for C1q binding. C1 bound to E. coli D31 m4 undergoes rapid activation at 0 degrees C. The activation process is not affected by C1-inhibitor, and only slightly inhibited by p-nitrophenyl p'-guanidinobenzoate. No turnover of the (C1r-C1s)2 subunit is observed. Once activated, C1 is only partially dissociated by C1-inhibitor. Our observations are in favour of a strong association between C1 and the outer membrane of E. coli D31 m4, involving mainly the collagen-like moiety of C1.

Cell-wall lipopolysaccharides of ampicillin-resistant mutants of Escherichia coli K-12

The lipopolysaccharides of ampicillin-resistant cell-wall-defective mutants of Escherichia coli K-12 were analyzed. From their lipopolysaccharides the respective core oligosaccharides were obtained. Following dephosphorylation,the core oligosaccharides were methylated and analyzed by gas chromatography/mass spectrometry. From core-defective mutants substructures of the K-12 core were obtained. Analysis of the lipopolysaccharide preparations from wild-type K-12 indicated the presence of several core structures with different degrees of completion. The lipopolysaccharide preparation was degraded and the oligosaccharide mixture was partially resolved by gel filtration chromatography. Methylation, gas chromatography and mass spectrometry of the oligosaccharides permitted the tentative formulation of the K-12 core structure. Alternative interpretations for this heterogeneity are discussed.

Immunochemical studies on lipopolysaccharides from wild-type and mutants of Escherichia coli K-12

Lipopolysaccharides from a number of mutants of Escherichia coli K-12 were investigated by means of chemical and serological methods. Inhibition of passive hemagglutination and inhibition of precipitation show that L-rhamnose is the immunodominant sugar in the lipopolysaccharide from wild-type E. coli K-12. The disaccharide rhamnosyl-KDO (where KDO is 3-deoxy-D-manno-octulosonic acid) was isolated and characterized after mild acid hydrolysis of the lipopolysaccharide. It is concluded that rhamnose is present in the innermost part of the core as a side-chain substituent on KDO. From crosses between an E. coli K-12 donor and E. coli O8, hybrids were obtained which contained either one or both of the donor rfa and rfb clusters. Serum absorption studies with lipopolysaccharides from these hybrids indicated that the histidine-linked rfb cluster is responsible for the presence of rhamnose in the K-12 core oligosaccharide. Using paper chromatography of 32P-labelled lipopolysaccharides we have found heterogeneous lipopolysaccharide in two strains as well as some differences between two wild-type strains. The latter difference is believed to be due to varying contents of KDO-linked ethanolamine phosphate. The overall results presented together with those described in the companion paper clearly show that the core oligosaccharide in E. coli K-12 has a structure different from the types previously described for other strains of E. coli (designed coli R1 to coli R4).

Cell envelope of Neisseria gonorrhoeae. A comparative study with Escherichia coli

The cell envelope of Neisseria gonorrhoeae was studied and compared to that of Escherichia coli. Outer membranes (OM) from both species were isolated by an identical method, and subjected to biochemical analysis. Differences in OM structure were sought that might explain the dissimilarity in OM permeability of these two species. The most pronounced difference appeared to reside in the OM proteins as judged by gel electrophoresis. Moreover, gonococcal OM proteins appeared to be more hydrophilic than those of E. coli.

Ultrastructural study of polymyxin-resistant isolates of Pseudomonas aeruginosa

Upon exposure to 6,000 U of polymyxin B sulfate per ml, cells of the polymyxin-sensitive PAO 1 strain of Pseudomonas aeruginosa displayed in thin sections long projections arising from the outer membrane of the cell wall and extensive cytoplasmic degradation with accumulation of cytoplasmic membrane infoldings. Polymyxin-resistant isolates derived from the PAO 1 strain, however, grew well in the presence of 6,000 U of polymyxin per ml and exhibited none of these effects, having instead the appearance of a typically healthy cell. Freeze-etching of cells of the sensitive strain grown in basal medium without polymyxin revealed a concave cell wall layer studded with numerous particles. Freeze-etching of cells of the resistant isolates grown in basal medium containing 6,000 U of polymyxin per ml revealed a concave cell wall layer (i.e., the outer half of the outer membrane) in which most of these particles were absent. Thus, acquisition of resistance to polymyxin was correlated with an alteration in the architecture of the outer membrane. When the resistant isolates were grown in the basal medium lacking polymyxin and then freeze-etched, the particle distribution in the concave cell wall layer resembled that of the sensitive parent strain. The cells had regained sensitivity to polymyxin upon suspension in medium containing 6,000 U/ml as determined by their failure to grow and by internal damages seen in thin sections. These cells also had acquired increased sensitivity to ethylenediaminetetraacetate, whereas the polymyxin-resistant cells grown in the presence of polymyxin were resistant to lysis by ethylenediaminetetraacetate. The polymyxin-resistant isolates were not stable mutants but instead represented an adaptive response to the presence of polymyxin in the medium.

Alterations in envelope structure of heptose-deficient mutants of Escherichia coli as revealed by freeze-etching

The surface of freeze-etched E. coli strain GR467, a heptose-deficient ("deep rough") mutant derived from CR34, was studied by electron microscopy. The outer membrane of GR467 has an increased ratio of phospholipid to protein, mainly due to a decreased protein content. Freeze-etched CR34 showed structural features indistinguishable for wild-type E. coli, i.e., the primary cleavage occurring in the inner membrane with only minor appearance of cleavage within the outer membrane. In contrast to this, in mutant GR467 most of the freeze-cleavages had taken place along a new plane, presumably in a hydrophobic region of the outer membrane. In this cleavage plane numerous particles were seen. Often the cleavage extended over the entire exposed cell surface; occasionally only a few large plateaus were visible, around which the next deeper cleavage plane, that of the protoplasmic or inner membrane, was discernible. Two spontaneous revertants (R11 and R16) with protein and lipid A levels similar to wild-type cells showed mostly freeze fractures with wild-type characteristics, and only a few cells had retained fracturing properties of GR467. A partial revertant revealed intermediate characteristics. Thus, there appears to be a morphological correlation with the chemical data relating the amount of outer membrane protein with the heptose content of the lipopolysaccharide.

Genetic analysis of drug resistance in Neisseria gonorrhoeae: production of increased resistance by the combination of two antibiotic resistance loci

The studies reported here demonstrate that increased resistance of Neisseria gonorrhoeae to penicillin, tetracycline, and chloramphenicol results from the combined effect of two resistance loci. As shown by experiments with deoxyribonucleic acid from transformants carrying only a single resistance locus, transformants with an incresed level of resistance to penicillin result from the combination of a penicillin-specific locus, pen, and a multiple resistance locus, mtr. Similarly, transformants with an increased level of resistance to tetracycline result from the combination of mtr and a tetracycline-specific locus, tet. Transformants with an increased level of resistance to chloramphenicol result from the combination of mtr and a chloramphenicol-specific locus, cml. Deoxyribonucleic acid dilution experiments established that only a single dose of each of the two required resistance loci is necessary to give higher-level resistance. Higher-level-resistant transformants were not obtained when a double dose of one resistance locus or a combination of loci pairs other than mtr and pen, mtr and tet, or mtr and cml was introduced into a recipient. Combinations of the mtr and tet genes resulted in increased resistance to semisynthetic tetracyclines. The presence of the mtr and pen genes resulted in increased resistance to penicillinase-stable penicillins.

Quantitation of the loss of the bacteriophage lambda receptor protein from the outer membrane of lipopolysaccharide-deficient strains of Escherichia coli

The recpetor for the phage lambda, a protein component of the outer membrane, is present at decreased levels in strains of Escherichia coli that are deficient in lipopolysaccharide. Loss of the protein was quantitated both by an assay of the phage receptor function and by an assay of antiserum-blocking ability to detect inactive protein. The loss of protein was correlated with the loss of sugar residues and phosphage from the core region of the lipopolysaccharide. Implications for the importance of ionic interactions in the stabilization of the outer membrane are discussed.

Cell envelope of Neisseria gonorrhoeae: outer membrane and peptidoglycan composition of penicillin-sensitive and-resistant strains

The cell envelope of Neisseria gonorrhoeae, colony type 4, was studied. Outer membrane was isolated by lysozyme and ethylenediaminetetraacetic acid treatment of plasmolyzed cells according to Wolf-Watz et al. (1973). The degree of purity of the membrane preparations was checked by electron microscopy. The membrane fraction obtained had a density of 1.25 g/cm(3), was rich in phospholipase A and lysophospholipase, and contained only 10% of the total membrane activity of succinate dehydrogenase and d-lactate dehydrogenase. The outer membrane protein profile after sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed at least six major proteins. The predominating protein showed a molecular weight of 35,000. The lipopolysaccharide component was characterized by gas chromatography. The carbohydrates found were galactose, glucose, and glucosamine. d-Glycero-l-manno-heptose was present in very low amounts. Lipid A contained lauric acid, stearic acid, and beta-hydroxy-myristic acid. About 20% of the fatty acids in the outer membrane was derived from lipid A. The phospholipids were characterized as phosphatidylethanolamine, phosphatidylglycerol, and diphosphatidylglycerol. There was no evidence for a lipoprotein anchored to the peptidoglycan. The peptidoglycan of N. gonorrhoeae was of the chemotype I. The cell envelope of N. gonorrhoeae was found to be highly permeable to gentian violet. Cell envelopes of one penicillin-resistant and two penicillin-sensitive strains were compared. Only moderate differences in fatty acid composition were found.

Characterization of lipopolysaccharides from Escherichia coli K-12 mutants

Chemical analyses of the carbohydrate composition of lipopolysaccharides (LPS) from a number of LPS mutants were used to propose a schematic composition for the LPS from Escherichia coli K-12. The formula contains four regions: the first consists of lipid A, ketodeoxyoctonoic acid, and a phosphorous component; the second contains only heptose; the third only glucose; and the fourth additional glucose, galactose, and rhamnose. LPS from E. coli B may have a similar composition but lacks the galactose and rhamnose units. A set of LPS-specific bacteriophages were used for comparing three mutants of Salmonella with a number of LPS mutants of E. coli K-12. The results confirm that there are basic similarities in the first and second regions of the LPS structure; they also support the four region divisions of the LPS formula. Paper chromatography was used for characterization of 32-P-labeled LPS from different strains of E. coli and Salmonella. The Rf values for LPS varied from 0.27 to 0.75 depending on the amounts of carbohydrates in the molecule. LPS from all strains studied was homogenous except for strain D31 which produced two types of LPS. Mild acid hydrolysis of labeled LPS liberated lipid A and two other components with phosphate, one of which was assigned to the first region. It is suggested that paper chromatography can be used in biosynthetic studies concerning regions 2 to 4.

Insect immunity. I. Characteristics of an inducible cell-free antibacterial reaction in hemolymph of Samia cynthia pupae

Pupae of the silk moth, Samia cynthia, were found to contain an inducible antibacterial activity in their hemolymph. This immunity response was provoked by primary infections with either Escherichia coli K-12 or Enterobacter cloacae. In both cases the antibacterial activity was directed chiefly towards E. coli. During standard conditions, 1% of hemolymph could kill 10(3) to 10(4) viable E. coli, strain D31, within 5 min. A lower level of antibacterial activity was induced by injections of a sterile salt solution. The killing of strain D31 followed single-hit kinetics, and increasing rate constants were obtained for increasing amounts of hemolymph. The reaction was sensitive to pretreatment with trypsin and it was protected by reducing agents. The activity was inhibited by microgram quantities of lipopolysaccharide (LPS) prepared from certain LPS mutants of E. coli K-12. A comparison of the susceptibility showed that "heptose-less" LPS mutants were more sensitive to killing than other strains. During standard conditions hemolymph will lyse both E. coli and Micrococcus lysodeikticus. Lysis of E. coli followed a multi-hit kinetics and it was inhibited by LPS, whereas lysis of M. lysodeikticus was unaffected by LPS. Hemolymph was fractionated on Sephadex G-200, and the lytic activities were recovered in partly overlapping peaks. Reconstitution with pooled fractions gave synergistic effects with the killing assay.

Alterations in the outer membrane of the cell envelope of heptose-deficient mutants of Escherichia coli

The composition of the cell envelope of a heptose-deficient lipopolysaccharide mutant of Escherichia coli, GR467, was studied after fractionation into its outer and cytoplasmic membrane components by means of sucrose density gradient centrifugation. The outer membrane of GR467 had a lower density than that of its parent strain, CR34. Analysis of the fractionated membranes of GR467 indicated that the phospholipid-to-protein ratio had increased 2.4-fold in the outer membrane. The ratio in the mutant cytoplasmic membrane was also increased, although to a lesser extent. By employing a third parameter, the lipid A content of the outer membrane, it was found that the observed phospholipid-to-protein change in the outer membrane was due predominantly to a decrease in the relative amount of protein. This decrease in protein was particularly significant, since it was concomitant with a 68% decrease in the lipid A recovered in the outer membrane of GR467 relative to the lipid A recovered in the outer membrane of CR34. Similar findings were observed in a second heptose-deficient mutant of E. coli, RC-59. The apparent protein deficiency in GR467 was further studied by subjecting solubilized envelope proteins to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was found that major envelope proteins which were localized in the outer membrane were greatly diminished in GR467. Two revertants of GR467 with the wild-type amounts of heptose had wild-type relative levels of protein in their outer membranes. A partial heptose revertant had a relative level of protein in its outer membrane between those of the mutant and wild type.

Isolation of the bacteriophage lambda receptor from Escherichia coli

A factor which inactivates the phage lambda can be extracted from Escherichia coli. This factor is a protein and is located in the outer membrane of the bacterial envelope. It is found in extracts of strains which are sensitive to phage lambda, but not in extracts of strains specifically resistant to this phage. We conclude that this factor is the lambda receptor, responsible for the specific adsorption of the phage lambda to E. coli cells. A partial purification of the lambda receptor is described. Inactivation of the phage by purified receptor is shown to be accompanied by the release of deoxyribonucleic acid from the phage.

Ampicillin-resistant mutants of Escherichia coli K-12 with lipopolysaccharide alterations affecting mating ability and susceptibility to sex-specific bacteriophages

Mutations from moderate (class I) to high (class III) ampicillin resistance in a male and a female strain of Escherichia coli K-12 have been found to be accompanied by surface alterations, first demonstrated as hindrance in the formation of mating pairs. These changes have now been studied with the ribonucleic acid phage MS2, and especially with the "female-specific" phage phiW. Several class III mutations in male and female strains were found to make the cells susceptible to phage phiW and to reduce their abilities to form mating pairs. Spontaneous phage phiW-resistant mutants isolated from class III strains were found also to have acquired changes in ampicillin resistance and ability to form mating pairs. One mutant had reverted to parental class I type in all three properties. Lipopolysaccharides (LPS) prepared from phiW-sensitive class III strains inactivated the phage in vitro, whereas LPS from phage-resistant strains had no effect. Carbohydrate analyses of LPS preparations showed that two class III mutants, compared to their parental strains, had lost significant parts of the rhamnose, galactose, and glucose from the LPS. One of the phage phiW-resistant mutants showed a partial restoration of its carbohydrate composition. Other phiW-resistant mutants showed, instead, further losses of carbohydrates in their LPS. It is suggested that genes exist which simultaneously mediate a female-specific mating site, ampicillin resistance, and the receptors for phage phiW.