Polycations sensitize enteric bacteria to antibiotics

Agents that increase the permeability of the outer membrane

The outer membrane of gram-negative bacteria provides the cell with an effective permeability barrier against external noxious agents, including antibiotics, but is itself a target for antibacterial agents such as polycations and chelators. Both groups of agents weaken the molecular interactions of the lipopolysaccharide constituent of the outer membrane. Various polycations are able, at least under certain conditions, to bind to the anionic sites of lipopolysaccharide. Many of these disorganize and cross the outer membrane and render it permeable to drugs which permeate the intact membrane very poorly. These polycations include polymyxins and their derivatives, protamine, polymers of basic amino acids, compound 48/80, insect cecropins, reptilian magainins, various cationic leukocyte peptides (defensins, bactenecins, bactericidal/permeability-increasing protein, and others), aminoglycosides, and many more. However, the cationic character is not the sole determinant required for the permeabilizing activity, and therefore some of the agents are much more effective permeabilizers than others. They are useful tools in studies in which the poor permeability of the outer membrane poses problems. Some of them undoubtedly have a role as natural antibiotic substances, and they or their derivatives might have some potential as pharmaceutical agents in antibacterial therapy as well. Also, chelators (such as EDTA, nitrilotriacetic acid, and sodium hexametaphosphate), which disintegrate the outer membrane by removing Mg2+ and Ca2+, are effective and valuable permeabilizers.

Agents that increase the permeability of the outer membrane

The outer membrane of gram-negative bacteria provides the cell with an effective permeability barrier against external noxious agents, including antibiotics, but is itself a target for antibacterial agents such as polycations and chelators. Both groups of agents weaken the molecular interactions of the lipopolysaccharide constituent of the outer membrane. Various polycations are able, at least under certain conditions, to bind to the anionic sites of lipopolysaccharide. Many of these disorganize and cross the outer membrane and render it permeable to drugs which permeate the intact membrane very poorly. These polycations include polymyxins and their derivatives, protamine, polymers of basic amino acids, compound 48/80, insect cecropins, reptilian magainins, various cationic leukocyte peptides (defensins, bactenecins, bactericidal/permeability-increasing protein, and others), aminoglycosides, and many more. However, the cationic character is not the sole determinant required for the permeabilizing activity, and therefore some of the agents are much more effective permeabilizers than others. They are useful tools in studies in which the poor permeability of the outer membrane poses problems. Some of them undoubtedly have a role as natural antibiotic substances, and they or their derivatives might have some potential as pharmaceutical agents in antibacterial therapy as well. Also, chelators (such as EDTA, nitrilotriacetic acid, and sodium hexametaphosphate), which disintegrate the outer membrane by removing Mg2+ and Ca2+, are effective and valuable permeabilizers.

Mechanisms of antibacterial action of tachyplesins and polyphemusins, a group of antimicrobial peptides isolated from horseshoe crab hemocytes

Tachyplesins I and II and polyphemusins I and II, cationic peptides isolated from the hemocytes of horseshoe crabs, show bactericidal activities with similar efficiencies for both gram-negative and gram-positive bacteria. Tachyplesin I inhibited bacterial growth irreversibly within 40 min. A subinhibitory concentration of tachyplesin I sensitized gram-negative bacteria to the bactericidal actions of novobiocin and nalidixic acid, although polymyxin B-resistant strains which have altered lipopolysaccharides were susceptible to tachyplesin I. This implies that tachyplesin permeabilizes the outer membrane and that the likely target of its action is outer membrane constituents other than lipopolysaccharides. On the other hand, a defensin-susceptible phoP strain of Salmonella typhimurium was also susceptible to tachyplesin I. Tachyplesin I rapidly depolarized the inverted inner-membrane vesicles of Escherichia coli. These results suggest that depolarization of the cytoplasmic membrane, preceded by the permeabilization of the outer membrane for gram-negative bacteria, is associated with tachyplesin-mediated bactericidal activity. The similarity between the actions of tachyplesin and those of defensin was discussed.

Outer membranes of gram-negative bacteria are permeable to steroid probes

The permeability of bacterial outer membranes was assayed by coupling the influx of highly hydrophobic probes, 3-oxosteroids, with their subsequent oxidation catalysed by 3-oxosteroid delta 1-dehydrogenase, expressed from a gene cloned from Pseudomonas testosteroni. In Salmonella typhimurium producing wild-type lipopolysaccharide, the permeability coefficients for uncharged steroids were 0.45 to 1 x 10(-5) cm s-1, and the diffusion appeared to occur mainly through the lipid bilayer domains of the outer membrane. These rates are one or two magnitudes lower than that expected for their diffusion through the usual biological membranes. The permeation rates were markedly increased (up to 100 times) when the lipopolysaccharide leaflet was perturbed either by adding deacylpolymyxin or by introducing mutations leading to the production of deep rough lipopolysaccharides. An amphiphilic, negatively charged probe, testosterone hemisuccinate, penetrated much more slowly than the uncharged steroids. Study of various Gram-negative species revealed that P. testosteroni, Pseudomonas acidovorans, and Acinetobacter calcoaceticus showed higher outer membrane permeability to steroid probes and higher susceptibility to hydrophobic agents such as fusidic acid, novobiocin and crystal violet relative to S. typhimurium and Escherichia coli.

The lipid A biosynthesis mutation lpxA2 of Escherichia coli results in drastic antibiotic supersusceptibility

The conditionally lethal lpxA2 mutant of Escherichia coli, which lacks detectable UDP-N-acetylglucosamine acyltransferase activity and which produces greatly reduced amounts of lipid A after a shift to 42 degrees C (S. Galloway and C. R. H. Raetz, J. Biol. Chem. 265:6394-6402, 1990), was found to be, at conditions which promote normal growth, remarkably susceptible to a number of antibiotics. The MICs of hydrophobic antibiotics, such as rifampin, erythromycin, clindamycin, and fusidic acid, were 32- to greater than 128-fold lower for the lpxA2 strain than for the parent type strain, and those of the peptide antibiotics vancomycin and bacitracin were 32- and 256-fold lower, respectively. Futhermore, the lpxA2 strain was found to be sensitive to hypoosmotic conditions. Comparisons with the other characterized outer membrane permeability mutants, such as the heptose-deficient strains of E. coli and Salmonella typhimurium, the acrA and abs mutants of E. coli, and the ssc-1 and class SS-B mutants of S. typhimurium, indicated that the lpxA2 mutant had characteristically the most antibiotic-supersusceptible phenotype. These findings advocate the possible use of the lpxA2 strain as a tool in various fields of basic and applied bacterial research in which the impermeability of the outer membrane currently poses problems.

Inactivation of gram-negative bacteria by photosensitized porphyrins

Photosensitization of Escherichia coli and Pseudomonas aeruginosa cells by deuteroporphyrin (DP) is shown to be possible in the presence of the polycationic agent polymyxin nonapeptide (PMNP). Previous studies established complete resistance of Gram-negative bacteria to the photodynamic effects of porphyrins. The present results show that combined treatment of E. coli or P. aeruginosa cultures with DP and PMNP inhibit cell growth and viability. No antibacterial activity of PMNP alone could be demonstrated and cell viability remained unchanged. Spectroscopically, PMNP was found to bind DP, a mechanism which probably assists its penetration into the cell's membranes. Insertion of DP into the cells was monitored by the characteristic fluorescence band of bound DP at 622 nm. Binding times were 5-40 min and the extent of binding increased with decreasing the pH from 8.5 to 6.5. DP binding constants, as well as the concentrations of PMNP which were required for maximal effect on the various Gram-negative bacteria, were determined fluorometrically. By the treatment of DP, PMNP and light the growth of E. coli and P. aeruginosa cultures was stopped and the viability of the culture was dramatically reduced. Within 60 min of treatment the survival fraction of E. coli culture was 9 x 10(-6) and that of P. aeruginosa was 5.2 x 10(-4). Electron microscopy depicted ultrastructural alterations in the Gram-negative cells treated by DP and PMNP. The completion of cell division was inhibited and the chromosomal domain was altered markedly.

In vivo effects of porphyrins on bacterial DNA

The DNA damage in intact Staphylococcus aureus and E. coli cells induced by photosensitized deuteroporphyrin or hemin is described. Treatment of S. aureus cultures with hemin or photosensitized deuteroporphyrin (Dp) caused time-dependent changes in the plasmidial DNA profiles. The major observation was the disappearance of the plasmid supercoiled fraction. The chromosomal DNA was also affected by hemin and by photosensitized Dp, since its degradation products were detected after exposing the bacterial cells to the porphyrin drugs. Photosensitization of E. coli cells, pretreated with Dp and polymyxin B nonapeptide (PMBNP), also resulted in plasmidial damage. No such damage occurred when E. coli cultures were treated with hemin and PMBNP. The above results can be tightly correlated with the antimicrobial action of porphyrins. Their damage to the bacterial DNA seems to reflect one of the in vivo effects of these porphyrins.

Killing of gram-negative bacteria by lactoferrin and lysozyme

Although lactoferrin has antimicrobial activity, its mechanism of action is not full defined. Recently we have shown that the protein alters the Gram-negative outer membrane. As this membrane protects Gram-negative cells from lysozyme, we have studied whether lactoferrin's membrane effect could enhance the antibacterial activity of lysozyme. We have found that while each protein alone is bacteriostatic, together they can be bactericidal for strains of V. cholerae, S. typhimurium, and E. coli. The bactericidal effect is dose dependent, blocked by iron saturation of lactoferrin, and inhibited by high calcium levels, although lactoferrin does not chelate calcium. Using differing media, the effect of lactoferrin and lysozyme can be partially or completely inhibited; the degree of inhibition correlating with media osmolarity. Transmission electron microscopy shows that E. coli cells exposed to lactoferrin and lysozyme at 40 mOsm become enlarged and hypodense, suggesting killing through osmotic damage. Dialysis chamber studies indicate that bacterial killing requires direct contact with lactoferrin, and work with purified LPS suggests that this relates to direct LPS-binding by the protein. As lactoferrin and lysozyme are present together in high levels in mucosal secretions and neutrophil granules, it is probable that their interaction contributes to host defense.

Polyamines as constituents of the outer membranes of Escherichia coli and Salmonella typhimurium

Extraction of whole cells of Salmonella typhimurium and Escherichia coli with 1 M NaCl released 8 to 13% of their total cellular polyamines (putrescine, cadaverine, and spermidine). This extraction did not cause significant cell lysis, release of outer membrane (OM) constituents, or leakage of periplasmic beta-lactamase. The extraction released nearly equal amounts of polyamines from mdo (membrane-derived oligosaccharide) mutants and wild type. These findings suggest that the released polyamines are apparently bound to the cell envelope. NaCl (1 M) was as effective as trichloroacetic acid in releasing polyamines from isolated OM and lipopolysaccharide (LPS). Isolated OM contained four times more polyamines than the cytoplasmic membrane. The increased binding to the OM is apparently due to the association of polyamines with the polyanionic LPS. Nearly identical amounts of polyamines were found in the OM and LPS preparations (as quantified per milligram of LPS). These amounts are equal to those released from the intact cells by 1 M NaCl (quantitation as above). However, redistribution of polyamines took place after cell disruption, because the relative proportions of different polyamines varied in the OM and LPS preparations. These results indicate that polyamines released from intact cells during 1 M NaCl extraction are preferentially derived from the OM.

Delta pH-dependent accumulation of tetracycline in Escherichia coli

The effects of ionophores on tetracycline accumulation in Escherichia coli cells were investigated in the presence of polymyxin B nonapeptide. Accumulation was inhibited by nigericin but not by valinomycin. Tetracycline accumulation was stimulated by decreasing the pH of the medium and inhibited by the addition of magnesium ions. These results indicated that tetracycline enters cells through diffusion as a protonated form (TH2) and is accumulated as a membrane-impermeable magnesium-tetracycline chelate complex (THMg+). This noncarrier diffusion hypothesis was confirmed by the fact that tetracycline accumulated in protein-free liposomes through an artificially imposed pH difference.

Effect of polyelectrolytes on entry of Escherichia coli HB101 (pRI203) into HeLa cells

The role of charged molecules in the entry mechanism of enteroinvasive bacteria was studied using Escherichia coli HB101 harboring a plasmid (pRI203) containing the Yersinia pseudotuberculosis invasion region as an experimental model. We investigated the effect of several anionic and cationic polyelectrolytes on the initial steps of infection of HeLa S3 cells by E. coli HB101 (pRI203). Experiments in which the polyions were added to cell monolayers together with bacteria showed that invasion was only slightly influenced by anions whereas cations strongly enhanced bacterial entry. DEAE-dextran, histone and poly-L-lysine were the most effective enhancers producing an up to five-fold increase in the number of both infected cells and internalized bacteria. Moreover, addition of the active polycations at different stages of infection demonstrated that their action took place during the attachment step, whereas internalization was not affected.

Partitioning of hydrophobic probes into lipopolysaccharide bilayers

Lipophilic solutes permeate rapidly through lipid bilayer membranes. However, the outer membrane of enteric bacteria, which is composed of a lipopolysaccharide monolayer outer leaflet and the glycerophospholipid inner leaflet, shows extremely low permeability to hydrophobic solutes. In order to examine the cause of this exceptionally low permeability, the lipid/water partition behavior of various lipophilic probes was determined by using lipopolysaccharides of various chemotypes and glycerophospholipids. With all probes, under many different conditions, the lipopolysaccharide/water partition coefficients were generally about an order of magnitude smaller than the phospholipid/water partition coefficients, and this result is consistent with the low permeability of the lipopolysaccharide monolayer, and hence the asymmetric bilayer found in the outer membrane. Furthermore, organic polycations significantly increased the partition of N-phenylnaphthylamine into lipopolysaccharides, a result again consistent with the permeability-increasing effect of such cations on intact outer membrane. Very defective, 'deep rough' lipopolysaccharides of chemotypes Rd2, Rd1 and Re, had only slightly (20-75%) higher partition coefficients in comparison with the more complete lipopolysaccharides, and this difference is probably not enough to explain the approximately 100-fold increase in lipophile permeability seen in deep rough strains.

Antimicrobial susceptibility of Salmonella typhimurium carrying the outer membrane permeability mutation SS-B

The antibiotic susceptibility profile of Salmonella typhimurium SS-B, a mutant susceptible to some antimicrobial agents, was studied in detail. Twenty-eight agents were tested, and eleven of these had MICs significantly lower (32- to greater than 250-fold) for the SS-B strain than for its parent. The drugs were generally hydrophobic or amphiphilic. Polymyxin B nonapeptide, which has a known outer membrane permeabilizing action, further reduced the MIC of several of these agents for the SS-B strain by a factor of approximately 10 to 30. In most cases, the resulting MICs were lower than the corresponding MICs for the parent strain grown in the presence of polymyxin B nonapeptide. In addition, the hydrophobic fluorescent probe N-phenyl naphthylamine was rapidly embedded in the membranes of the SS-B strain but was poorly embedded in those of the parent strain.

Sodium hexametaphosphate sensitizes Pseudomonas aeruginosa, several other species of Pseudomonas, and Escherichia coli to hydrophobic drugs

Many gram-negative bacteria are known to be remarkably resistant to hydrophobic noxious agents by virtue of their outer membranes (OM). We investigated, by using four different assay methods, the ability of sodium hexametaphosphate (HMP) to disrupt this OM barrier. (i) In the growth inhibition assay, HMP was found to sensitize strains of Pseudomonas aeruginosa to all the hydrophobic probes tested (rifampin, fusidic acid, dactinomycin, sodium dodecyl sulfate, and Triton X-100). A concentration of 0.3% HMP decreased the MICs of the probes by a factor of approximately 10, and maximally even a 30-fold sensitization was found with 1% HMP. (ii) In the bactericidal assay, 0.3% HMP decreased the MBC of the hydrophobic probe rifampin by a factor of approximately 30. (iii) In the bacteriolytic assay, 0.1% HMP sensitized the target bacteria to lysis by sodium dodecyl sulfate and Triton X-100. (iv) In the fluorescent-probe binding assay, HMP drastically enhanced the binding of fluorescent N-phenyl naphthylamine to the membranes of the target cells. In addition to P. aeruginosa, P. fluorescens, P. putida, P. fragi, and Escherichia coli were susceptible to the OM permeability-increasing action of HMP, while P. cepacia was resistant.

Purification, toxicity, and antiendotoxin activity of polymyxin B nonapeptide

Polymyxin B, a relatively toxic antibiotic, has potent endotoxin-neutralizing properties that may be beneficial as adjunctive therapy in gram-negative sepsis. Polymyxin B nonapeptide (deacylated polymyxin B) is devoid of antibiotic activity but retains the capacity to disorganize the outer membrane of gram-negative bacteria. To evaluate the potential therapeutic usefulness of this derivative, we produced purified polymyxin B nonapeptide, tested its in vivo toxicity in animals, and evaluated its in vitro antiendotoxin activity. Effectiveness as an antiendotoxin agent was assessed by examining the ability of polymyxin B nonapeptide to block the enhanced release of toxic oxygen radicals induced by lipopolysaccharide in human neutrophils (priming). In vivo, at doses of 1.5 and 3.0 mg/kg, polymyxin B nonapeptide did not exhibit the neuromuscular blocking, neurotoxic, or nephrotoxic effects that were observed with polymyxin B sulfate. Both polymyxin B and polymyxin B nonapeptide inhibited lipopolysaccharide-induced neutrophil priming in a concentration-dependent manner, but the parent compound, polymyxin B, was 63 times more effective on a weight basis. The inhibitory activity of both compounds, however, diminished rapidly when they were added after the start of the lipopolysaccharide-neutrophil incubation. We conclude that polymyxin B nonapeptide is less toxic than polymyxin B and, at the doses tested, lacks the neurotoxicity and nephrotoxicity of the parent compound. Polymyxin B nonapeptide retains the antiendotoxin activity of polymyxin B but is much less potent. The findings suggest that these compounds block an early step in the neutrophil priming process, possibly lipopolysaccharide attachment to or insertion into the neutrophil membrane.

Resistance of Escherichia coli to nourseothricin (streptothricin): reduced penetrability of the cell wall as an additional, possibly unspecific mechanism

The resistance of E. coli strains to the antibiotic nourseothricin is known to be caused by an acetyltransferase acetylating the beta-lysine chain of the antibiotic. In addition, most of the resistant strains exhibit reduced penetrability of the outer membrane, presumably caused by a reduced amount of available negative charges. This was shown using crystal violet, Congo red, or the hydrophobic antibiotic novobiocin as indicators.

Effect of small cationic leukocyte peptides (defensins) on the permeability barrier of the outer membrane

Defensins are small cationic antibacterial peptides that are abundant in polymorphonuclear leukocytes from human and other sources (T. Ganz, M. Selsted, D. Szklarek, S. Harwig, K. Daher, D. F. Bainton, and R. J. Lehrer, J. Clin. Invest. 76:1427-1435, 1985). We studied whether subinhibitory concentrations of defensins increase the outer membrane (OM) permeability of Escherichia coli, Salmonella typhimurium, and Pseudomonas aeruginosa to hydrophobic probes, as do many other polycations that have been studied previously. Throughout the study, we used polymyxin B nonapeptide (PMBN) as a reference peptide. PMBN has a known potent OM permeability-increasing action. As a sharp contrast to PMBN, subinhibitory concentrations of defensins did not permeabilize (or, under some test conditions, permeabilized very slightly) the OM to the probes that were used (rifampin and Triton X-100). At bacteriostatic or bactericidal defensin concentrations, some degree of synergism with rifampin was seen.

Mechanism of action of DuP 721: inhibition of an early event during initiation of protein synthesis

The mode of action of DuP 721 was investigated. This compound was active primarily against gram-positive bacteria, including multiply resistant strains of staphylococci. Although inactive against wild-type Escherichia coli, DuP 721 did inhibit E. coli when the outer membrane was perturbed by genetic or chemical means. Pulse-labeling studies with E. coli PLB-3252, a membrane-defective strain, showed that DuP 721 inhibited amino acid incorporation into proteins. The 50% inhibitory concentration of DuP 721 for protein synthesis was 3.8 micrograms/ml, but it was greater than 64 micrograms/ml for RNA and DNA syntheses. The direct addition of DuP 721 to cell-free systems did not inhibit any of the reactions of protein synthesis from chain initiation through chain elongation with either synthetic or natural mRNA as template. However, cell extracts prepared from DuP 721 growth-arrested cells were defective in initiation-dependent polypeptide synthesis directed by MS2 bacteriophage RNA. These cell-free extracts were not defective in polypeptide elongation or in fMet-tRNA(fMet)-dependent polypeptide synthesis stimulated by poly(G.U). We conclude, therefore, that DuP 721 exerts its primary action at a step preceding the interaction of fMet-tRNA(fMet) and 30S ribosomal subunits with the initiator codon.

Enhanced binding of polycationic antibiotics to lipopolysaccharide from an aminoglycoside-supersusceptible, tolA mutant strain of Pseudomonas aeruginosa

The lipopolysaccharide (LPS) of the aminoglycoside-supersusceptible Pseudomonas aeruginosa tolA mutant PAO1715 was compared with its parent strain PAO1670 and tol+ revertant PAO1716. Electrophoretic separation of purified LPSs from the three isolates showed similar LPS banding patterns. Analysis of the Western blots of these LPSs from the three isolates with O-antigen-specific monoclonal antibody indicated that the ladder pattern consisted of doublet bands, which presumably reflected a modification of core or lipid A; the level of one of the bands in the doublet was in much lower amounts in the isolate from the tolA mutant than in that from the parent or revertant. Results of competitive displacement experiments, in which the cationic spin probe 4-dodecyldimethylammonium-1-oxyl-2,2,6,6-tetramethylpiperidine bromide was displaced from its LPS-binding site by polycations, revealed that the tolA mutant had a much higher affinity for gentamicin, polymyxin, Ca2+, and Mg2+ than did the parent or revertant. The order of affinity for all samples was polymyxin B much greater than gentamicin C much greater than Ca2+ greater than Mg2+. Both gentamicin and polymyxin induced rigidification of all of the LPS samples, but for the sample from the tolA mutant, rigidification occurred at substantially lower concentrations. Dansyl polymyxin titration experiments with intact cells demonstrated that the increased affinity of the LPS from the tolA mutant for polycations was reflected in an increase in the affinity of binding to the cell. Together these data suggest that the tolA mutant is supersusceptible to aminoglycosides by virtue of an LPS change which increases the binding affinity of the LPS for polycations, including gentamicin.

Phenylmethylsulfonyl fluoride inhibits chemotactic peptide-induced actin polymerization and oxidative burst activity in human neutrophils by an effect unrelated to its anti-proteinase activity

Stimulation of polymorphonuclear leukocytes with the chemotactic peptide N-formylmethionylleucylphenylalanine (fMet-Leu-Phe) causes conversion of monomeric actin to polymeric actin. We studied the role of proteinase inhibitors phenylmethylsulfonyl fluoride PMSF) and diisopropyl fluorophosphate in fMet-Leu-Phe-induced actin polymerization in polymorphonuclear leukocytes. Pre-incubation of cells with PMSF (2 mM) for 1 min caused inhibition of fMet-Leu-Phe-induced actin polymerization, as studied by 7-nitrobenz-2-oxa-1,3-diazole (NBD) -phallacidin labeling and flow cytometry. PMSF also inhibited fMet-Leu-Phe-induced hydrogen peroxide release, superoxide anion generation and chemiluminescence. In contrast, diisopropyl fluorophosphate (5 mM) was unable to inhibit fMet-Leu-Phe-induced actin polymerization and superoxide generation, but was effective in inhibiting hydrogen peroxide production and chemiluminescence. PMSF did not cause any change in membrane potential by itself and failed to inhibit the membrane potential changes induced by fMet-Leu-Phe, indicating that PMSF does not affect the binding of fMet-Leu-Phe to the receptors. The high concentration of PMSF required coupled with the fact that diisopropyl fluorophosphate was unable to inhibit fMet-Leu-Phe-induced actin polymerization suggested that this activity of PMSF might be unrelated to proteinase inhibitory activity. Polymyxin B, a membrane-active antibiotic, had an effect similar to PMSF on fMet-Leu-Phe-induced actin polymerization. This suggests that PMSF may also be acting via its membrane effect rather than its anti-proteinase effect.

The interaction of Escherichia coli with normal human serum: factors affecting the capacity of serum to mediate lipopolysaccharide release

We previously demonstrated that incubation of E. coli in normal human serum (NHS) resulted in the release of a finite fraction (approximately 30%) of LPS from the bacterial outer membrane. In experiments reported here, we examined factors which may enhance or diminish the capacity of NHS to mediate this limited LPS release. Both the susceptibility to serum killing and LPS release were dependent on growth phase. Optimal killing and release coincided with the midlogarithmic growth phase. The composition of LPS subunits in the outer membrane appeared to influence serum-mediated LPS release. Serum treated E. coli enriched for Rc-chemotype LPS released less LPS from their outer membrane than the wildtype 'smooth' bacteria during exponential growth. LPS fractions released by NHS or EDTA appeared to a large degree to overlap, suggesting that NHS-mediated LPS release may involve the action of a serum chelator. A serum-resistant mutant failed to release LPS in either NHS or EDTA. This latter observation suggests that LPS release may be a relevant event in serum killing. We did not detect any modulation of LPS release when E. coli were pre-incubated with a series of antibiotics prior to treatment with NHS.

Effects of polymyxin B nonapeptide on Aeromonas salmonicida

In contrast to polymyxin B-susceptible gram-negative bacteria of human origin, the fish pathogen Aeromonas salmonicida was resistant to sensitization by polymyxin B nonapeptide (PMBN) to hydrophobic antibiotics. Similarly, sensitization of A. salmonicida strains by PMBN to the bactericidal action of brook trout (Salvelinus fontinalis) serum complement was less pronounced than the similar effect of PMBN against other gram-negative bacteria in certain mammalian sera. The surface array protein (A layer), overlying the outer membrane of virulent A. salmonicida strains, did not appear to cause resistance to PMBN sensitization since an A-layer-deficient mutant showed similar responses to PMBN as its parent strain. Electron micrographs of PMBN-treated A. salmonicida cells revealed very little visible outer membrane disruption when compared with the extensive blebbing on the outer membrane surface caused by PMBN in certain enteric bacteria, including Escherichia coli K-37. However, small extracellular vesiclelike components, which may have been derived from the outer membrane, were numerous around PMBN-treated A. salmonicida cells. In this connection, PMBN caused disruption of the A layer in the form of bulges, breaks, and detached fragments which appeared to be associated with the accumulation of these vesicles underneath the A layer of wild-type A. salmonicida strains.

Membrane permeability of Pseudomonas aeruginosa to 4-quinolones

The outer membrane-disorganizing agent polymyxin B nonapeptide sensitized Pseudomonas aeruginosa strains 2-40fold to ciprofloxacin, norfloxacin, and ofloxacin, and 80-200fold to nalidixic acid. Sensitized P. aeruginosa strains were nearly as susceptible to nalidixic acid as Escherichia coli strains. The data suggest that the outer membrane permeability is of critical importance for the antipseudomonal activity of 4-quinolones. The low penetration of the outer membrane is the major cause of the nalidixic acid resistance of P. aeruginosa.

Effects of antibiotics and other inhibitors on ATP-dependent protein translocation into membrane vesicles

The effects of several membrane antibiotics and other agents on ATP-dependent protein translocation were examined in membrane vesicles under conditions where no significant proton motive force was present. The membrane perturbants ethanol and procaine abolished ATP-dependent protein translocation. Phenethyl alcohol at low concentrations abolished translocation, whereas at high concentrations it allowed precursors to be translocated but inhibited their processing. Translocation of precursors promoted by phenethyl alcohol was temperature dependent and occurred without an added energy source but was enhanced by ATP. However, such precursors could not be further processed to mature forms upon removal of the alcohol. The membrane-active antibiotics polymyxin B and gramicidin S were strong inhibitors of translocation, whereas gramicidin D, cerulenin, and mycobacillin had no effect even at higher concentrations, indicating some specificity in interference with protein translocation. Duramycin, an antibiotic previously shown to affect protein-lipid interaction, severely impaired protein translocation. These results showed that membrane structures play important roles, either directly or indirectly, in protein translocation. Chelating agents 1,10-phenanthroline and EDTA, but not EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid], also abolished protein translocation.

The effect of polymyxin B nonapeptide (PMBN) on transformation

The effect of the outer membrane permeabilizing polycation, polymyxin B nonapeptide (PMBN) on the transformation of E. coli HB101 with pBR322 plasmid DNA was investigated. Pretreatment of cells with PMBN (followed by suspending the cells in PMBN-free medium) did not stimulate the development of competence induced by the calcium heat pulse. In the absence of calcium-ions, a high PMBN concentration (1 mM) was able to induce a low transformation frequency provided that PMBN was not removed before the addition of DNA.

Reversible binding of Salmonella typhimurium lipopolysaccharides by immobilized protamine

The ability of agarose-linked protamine to bind Salmonella typhimurium lipopolysaccharides was investigated. Radioactively labelled lipopolysaccharides were isolated both from a smooth strain (SH6749, labelled with [14C]galactose) and from a rough strain (SH5014, lipopolysaccharide chemotype Rb2, labelled with [3H]acetate). From 50-micrograms samples of the lipopolysaccharides, protamine-agarose columns bound 99.5-99.9% of the input radioactivity. The binding efficacy was not affected by pH in the range from 3.7 to 10.5. Maximal binding capacity of protamine-agarose for highly soluble (triethylamine form) lipopolysaccharide of SH5014 was estimated to be 13.5 mg/ml packed adsorbent. The bound lipopolysaccharides could be totally released from the columns and recovered by elution with the anionic detergent sodium deoxycholate, or with 0.5 M NaCl in the presence of the uncharged detergent Triton X-100. By analysis in sodium dodecyl sulfate/polyacrylamide gels, the macromolecular quality of the recovered lipopolysaccharide was shown to be identical to that of the original. Protamine-agarose chromatography can thus be applied to purify lipopolysaccharide preparations, and to separate as well as concentrate lipopolysaccharides from dilute solutions without altering their composition. This application was challenged with water as well as insulin solution experimentally contaminated with radiolabelled lipopolysaccharide. While the insulin protein did not bind to the protamine-agarose, the contaminating lipopolysaccharide was effectively trapped.

Decreased binding of antibiotics to lipopolysaccharides from polymyxin-resistant strains of Escherichia coli and Salmonella typhimurium

The interactions of polycationic antibiotics with lipopolysaccharide (LPS) isolated from parental and polymyxin-resistant strains of Salmonella typhimurium and Escherichia coli were measured by using a cationic spin probe. Electron spin resonance spectra indicated that increasing concentrations of cations competitively displaced probe from LPS aggregates. Polymyxin B and other cations displaced less probe from LPS of polymyxin-resistant strains than from LPS of the parental strains, whereas the same amount or more probe was displaced from isolates of the mutants by the structurally similar antibiotic, EM 49 (octapeptin). In general, the differential affinities of these antibiotics for LPS correlated with their antibiotic activity in vivo, suggesting that resistance results from a decrease in antibiotic permeability across the outer membrane due to alterations in the LPS which affect antibiotic binding. The alterations in the structure of LPS from the polymyxin-resistant mutants of E. coli were characterized using 31P nuclear magnetic resonance spectroscopy. The results suggested that esterification of the core-lipid A phosphates is responsible for increased resistance to polymyxin B and that this alteration is different from that previously proposed for the S. typhimurium strains. In both cases, however, resistance was the result of modifications that result in a less acidic lipid A.

Involvement of outer membrane of Pseudomonas cepacia in aminoglycoside and polymyxin resistance

Pseudomonas cepacia was found to be resistant to the outer membrane-permeabilizing effects of aminoglycoside antibiotics, polymyxin B, and EDTA. Permeabilization of P. cepacia to the fluorescent probe 1-N-phenylnaphthylamine was not achieved at concentrations 100- to 1,000-fold above those required to permeabilize Pseudomonas aeruginosa. Furthermore, in contrast to P. aeruginosa cells, intact cells of P. cepacia did not bind the fluorescent probe dansyl-polymyxin. However, purified lipopolysaccharide (LPS) from P. cepacia bound dansyl-polymyxin with approximately the same affinity as did LPS from P. aeruginosa. Also, binding of dansyl-polymyxin to P. cepacia (and P. aeruginosa) LPS was inhibited by polymyxin B, streptomycin, gentamicin, and Mg2+. These data suggest that P. cepacia does not utilize the self-promoted pathway for aminoglycoside uptake and that the outer membrane is arranged in a way that conceals or protects cation-binding sites on LPS which are capable of binding polycations such as aminoglycosides or polymxyin.

Mode of action of gramicidin S on Escherichia coli membrane

The action of a cationic antibiotic gramicidin S on the outer and cytoplasmic membranes of Escherichia coli was studied. It was found that gramicidin S disrupted the permeability barrier of the outer membrane, permitting the permeation of an antibiotic ionophore, this being similar to the action of the dimer in compound 48/80 (Katsu, T., Shibata, M. and Fujita, Y. (1985) Biochim. Biophys. Acta 818, 61-66). However, differently from the dimer, gramicidin S further stimulated the efflux of K+ through the cytoplasmic membrane of E. coli. The time course of K+ permeability change accorded well with that of change in the viability of E. coli cells. These changes occurred at temperatures above the phase transition of the cytoplasmic membrane. This temperature range differed greatly from the case of polymyxin B, a polycationic antibiotic acting at temperatures above the phase transition of the outer membrane. We discuss the mode of gramicidin S action on the cytoplasmic membrane of E. coli, in comparison with the results on red blood cells and liposomes.

Leakage of periplasmic proteins from Escherichia coli mediated by polymyxin B nonapeptide

The effects of polymyxin B and polymyxin B nonapeptide (PMBN) on cell envelope integrity in Escherichia coli were compared. Both compounds caused loss of proteins from E. coli K-12 3300(pBR322), although PMBN released less protein than did polymyxin B. The origin of the released protein was determined both by polyacrylamide gel electrophoresis and by using specific enzyme markers (beta-lactamase in periplasm, beta-galactosidase in cytoplasm). The proteins released by both compounds were derived principally from the periplasm, accompanied in the case of polymyxin B by a low level of cytoplasmic proteins. Although polymyxin B and PMBN both caused release of periplasmic proteins, the individual proteins released by the compounds differed. The periplasmic fraction contained six principal polypeptides with molecular weights between 62,000 (polypeptide 1) and 29,000 (polypeptide 6). Polypeptide 6 was identified as the pBR322-encoded beta-lactamase, but the other proteins were not specifically identified. Polymyxin B caused considerable release of polypeptides 1, 2, and 5 with some release of polypeptides 4 and 6. PMBN released polypeptide 1 (trace), 3, 4, and 6 (trace). Scanning electron microscopy showed that polymyxin B and PMBN both caused surface damage in E. coli. However, polymyxin B produced greater morphological changes than PMBN.

Interaction of polycationic antibiotics with Pseudomonas aeruginosa lipopolysaccharide and lipid A studied by using dansyl-polymyxin

A fluorescent derivative of polymyxin B (dansyl-polymyxin) was used to study the interaction of polycations with lipopolysaccharide (LPS) and lipid A from Pseudomonas aeruginosa. Dansyl-polymyxin became bound to LPS and lipid A sites, including Mg2+-binding sites, resulting in a 20-fold enhancement of fluorescence. A Hill plot of the binding data showed that the binding of dansyl-polymyxin to LPS was cooperative (n = 1.98) and of high affinity (S0.5 = 0.38 microM). The maximal binding capacity of LPS was approximately four molecules of dansyl-polymyxin per mol of LPS. The dansyl-polymyxin interaction with lipid A displayed similar kinetics (n = 2.26; S0.5 = 0.38 microM), and the maximal binding capacity was approximately 2 mol of dansyl-polymyxin per mol of lipid A. A variety of polycationic compounds, including gentamicin, streptomycin, and polymyxin B, as well as Mg2+, were able to displace dansyl-polymyxin bound to LPS or to lipid A. Marked differences both in terms of the degree of displacement and in terms of the amount of competing polycation required to displace a given amount of dansyl-polymyxin were observed. Addition of excess polymyxin B resulted in displacement of all of the dansyl-polymyxin, demonstrating that only polymyxin-binding sites were being probed. Our data demonstrate that polymyxin B binds to multiple sites on LPS, including sites which bind aminoglycoside antibiotics and other polycationic compounds.

Polymyxin B nonapeptide inhibits mating in Saccharomyces cerevisiae

Polymyxin B nonapeptide enhanced susceptibility of yeast cells to various hydrophobic antibiotics and to mating pheromones. At much lower concentrations, the nonapeptide severely inhibited mating. The inhibition was caused by interference with sexual agglutination.

Cytolytic activity of liposomes containing stearylamine

In order to develop the cytotoxic liposome, the cytolytic effect of polycationic liposome was examined. Upon incubation of the stearylamine-containing liposome (stearylamine-liposome) with rabbit erythrocyte, a significant extent of hemolysis was observed. Hemolytic activity of the liposome depends on the amount of stearylamine in the liposome membrane. The plots of the initial rate of hemolysis versus the concentration of stearylamine-liposome showed a sigmoidal curve, suggesting that stearylamine-liposomes act cooperatively on the erythrocyte membrane. Hemolytic activity of stearylamine-liposome was markedly influenced by the composition of hydrocarbon chains of the phospholipids in the liposome membrane, suggesting that the membrane fluidity of stearylamine-liposome is important to evoke the hemolysis. Since the liposomes containing acidic phospholipids inhibited markedly the stearylamine-liposome-caused hemolysis, it is likely that the primary target of stearylamine-liposome is the negatively charged component(s) such as acidic phospholipids on the erythrocyte membrane. Furthermore, stearylamine-liposome induced the release of the intravesicular contents from the liposome made of acidic phospholipids but not from the liposome made of phosphatidylcholine only. These results suggest that stearylamine-liposome interacted with the negative charges of the erythrocyte membrane and eventually damaged the cell. Erythrocytes from rabbit, horse and guinea pig are highly susceptible to stearylamine-liposome but those from man, sheep, cow and chicken are less so.

Outer-membrane permeability of bacteria

Gram-negative bacteria evolved to survive under the conditions in which a number of hazardous compounds are abundant. The outer membrane which protects the cell interior acts as a barrier against such hazardous agents, yet the cells must incorporate the chemicals that are essential for the cellular activity. The devices that Gram-negative bacteria developed to incorporate such essence are the transmembrane pores. These pores could be subdivided into three categories: (1) pore made of porins has a weak solute selectivity; (2) pore made of lamB protein and tsx proteins hold intermediate solute specificity. and (3) pores for the diffusion of vitamin B12 and ferric ion-chelator complexes have a tight solute specificity. Porins are identified from a number of Gram-negatives and from the outer membrane of mitochondria of various sources. Studies on the diffusion properties of these outer-membrane proteins provided essential information to understand membrane transports.

Antimycobacterial spectrum of colistin (polymixin E)

Minimal inhibitory concentrations and minimal bactericidal concentrations of colistin (polymyxin E) were determined for the type strains of fifteen mycobacterial species. Colistin was found to be active against pathogenic species Mycobacterium xenopi, M. intracellulare, M. tuberculosis, M. fortuitum and also against the rapidly growing, non-pathogenic species M. phlei and M. smegmatis. The discriminatory potential of susceptibility to colistin as a test was investigated on 25 strains of the M. fortuitum/M. chelonei complex, and also on 11 strains of the M. avium/M. intracellulare complex. The experimental data indicated the potential of colistin susceptibility testing for discriminating M. fortuitum from M. chelonei.

Dication and trication which can increase the permeability of Escherichia coli outer membrane

Recent success in the preparation of the monomer, dimer and trimer in compound 48/80 prompted us to investigate the action of these compounds on Escherichia coli cells. It was found that compound 48/80 inhibited growth of E. coli cells, while the monomer, dimer and trimer in 48/80 did not. However, the following experiments showed that the dimer and trimer disrupted the permeability barrier of the outer membrane of E. coli. First, addition of the dimer or trimer in cell suspension stimulated the uptake of tetraphenylphosphonium cation. Second, the synergistic effect of the dimer on the action of gramicidin caused the efflux of K+. In experiments using isolated cytoplasmic membrane vesicles, addition of gramicidin alone caused the efflux of K+. Thus, it was speculated that, with whole cells, the dimer formed some defect structure in the outer membrane, through which gramicidin reached the cytoplasmic membrane and increased the K+ permeability. The temperature dependence of efflux K+ showed that the dimer in 48/80 rendered the outer membrane permeable to gramicidin at temperatures above the phase transition of the outer membrane.

Mechanism of polymyxin B-mediated lysis of lipopolysaccharide-treated erythrocytes

A novel system was used previously to characterize the dynamic interaction of a polysaccharide-deficient, lipid-rich lipopolysaccharide (LPS) with rabbit erythrocytes (RaRBC). Exposure of the RaRBC to the LPS rendered them sensitive to induction of hemolysis by the cationic antibiotic polymyxin B (PB) in a time- and temperature-independent manner. Subsequent decay in the response of LPS-sensitized cells to PB was shown to be critically dependent on both the time and temperature of incubation of RaRBC with LPS and to be independent of a change in LPS binding (Carr and Morrison, Infect. Immun. 43:600-606, 1984). In the present study, we performed experiments designed to define the mechanism by which PB mediates hemolysis of LPS-sensitized RaRBC. Experiments were performed to examine the molecular requirements of the LPS and the PB that were essential for hemolytic activity. The capacity of various cations to mediate hemolysis of LPS-sensitized RaRBC or to block PB-mediated hemolysis and the temperature dependence of the PB lytic reaction were investigated. The results of these experiments suggest that PB-mediated hemolysis of LPS-treated erythrocytes is dependent upon an initial ionic association of PB with erythrocyte membrane-bound LPS, followed by hydrophobic insertion of the PB fatty acid into the erythrocyte membrane lipid bilayer.