The chemical composition of the membranes of protoplasts and L-forms of Staphylococcus aureus

Lateral membrane organization as target of an antimicrobial peptidomimetic compound

Antimicrobial resistance is one of the leading concerns in medical care. Here we study the mechanism of action of an antimicrobial cationic tripeptide, AMC-109, by combining high speed-atomic force microscopy, molecular dynamics, fluorescence assays, and lipidomic analysis. We show that AMC-109 activity on negatively charged membranes derived from Staphylococcus aureus consists of two crucial steps. First, AMC-109 self-assembles into stable aggregates consisting of a hydrophobic core and a cationic surface, with specificity for negatively charged membranes. Second, upon incorporation into the membrane, individual peptides insert into the outer monolayer, affecting lateral membrane organization and dissolving membrane nanodomains, without forming pores. We propose that membrane domain dissolution triggered by AMC-109 may affect crucial functions such as protein sorting and cell wall synthesis. Our results indicate that the AMC-109 mode of action resembles that of the disinfectant benzalkonium chloride (BAK), but with enhanced selectivity for bacterial membranes.

Is phosphatidylglycerol essential for terrestrial life?

Lipids are of increasing importance in understanding biological systems. Lipids carrying an anionic charge are noted in particular for their electrostatic interactions with both proteins and divalent cations. However, the biological, analytical, chemical and biophysical data of such species are rarely considered together, limiting our ability to assess the true role of such lipids in vivo. In this review, evidence from a range of studies about the lipid phosphatidylglycerol is considered. This evidence supports the conclusions that this lipid is ubiquitous in living systems and generally of low abundance but probably fundamental for terrestrial life. Possible reasons for this are discussed and further questions posed.

Antimicrobial Peptides Targeting Gram-Positive Bacteria

Antimicrobial peptides (AMPs) have remarkably different structures as well as biological activity profiles, whereupon most of these peptides are supposed to kill bacteria via membrane damage. In order to understand their molecular mechanism and target cell specificity for Gram-positive bacteria, it is essential to consider the architecture of their cell envelopes. Before AMPs can interact with the cytoplasmic membrane of Gram-positive bacteria, they have to traverse the cell wall composed of wall- and lipoteichoic acids and peptidoglycan. While interaction of AMPs with peptidoglycan might rather facilitate penetration, interaction with anionic teichoic acids may act as either a trap for AMPs or a ladder for a route to the cytoplasmic membrane. Interaction with the cytoplasmic membrane frequently leads to lipid segregation affecting membrane domain organization, which affects membrane permeability, inhibits cell division processes or leads to delocalization of essential peripheral membrane proteins. Further, precursors of cell wall components, especially the highly conserved lipid II, are directly targeted by AMPs. Thereby, the peptides do not inhibit peptidoglycan synthesis via binding to proteins like common antibiotics, but form a complex with the precursor molecule, which in addition can promote pore formation and membrane disruption. Thus, the multifaceted mode of actions will make AMPs superior to antibiotics that act only on one specific target.

Branched phospholipids render lipid vesicles more susceptible to membrane-active peptides

Iso- and anteiso-branched lipids are abundant in the cytoplasmic membranes of bacteria. Their function is assumed to be similar to that of unsaturated lipids in other organisms - to maintain the membrane in a fluid state. However, the presence of terminally branched membrane lipids is likely to impact other membrane properties as well. For instance, lipid acyl chain structure has been shown to influence the activity of antimicrobial peptides. Moreover, the development of resistance to antimicrobial agents in Staphylococcus aureus is accompanied by a shift in the fatty acid composition toward a higher fraction of anteiso-branched lipids. Little is known about how branched lipids and the location of the branch point affect the activity of membrane-active peptides. We hypothesized that bilayers containing lipids with low phase transition temperatures would tend to exclude peptides and be less susceptible to peptide-induced perturbation than those made from higher temperature melting lipids. To test this hypothesis, we synthesized a series of asymmetric phospholipids that only differ in the type of fatty acid esterified at the sn-2 position of the lipid glycerol backbone. We tested the influence of acyl chain structure on peptide activity by measuring the kinetics of release from dye-encapsulated lipid vesicles made from these synthetic lipids. The results were compared to those obtained using vesicles made from S. aureus and Staphylococcus sciuri membrane lipid extracts. Anteiso-branched phospholipids, which melt at very low temperatures, produced lipid vesicles that were only slightly less susceptible to peptide-induced dye release than those made from the iso-branched isomer. However, liposomes made from bacterial phospholipid extracts were generally much more resistant to peptide-induced perturbation than those made from any of the synthetic lipids. The results suggest that the increase in the fraction of anteiso-branched fatty acids in antibiotic-resistant strains of S. aureus is unlikely to be the sole factor responsible for the observed increased antibiotic resistance. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Multiple peptide resistance factor (MprF)-mediated Resistance of Staphylococcus aureus against antimicrobial peptides coincides with a modulated peptide interaction with artificial membranes comprising lysyl-phosphatidylglycerol

Modification of the membrane lipid phosphatidylglycerol (PG) of Staphylococcus aureus by enzymatic transfer of a l-lysine residue leading to lysyl-PG converts the net charge of PG from -1 to +1 and is thought to confer resistance to cationic antimicrobial peptides (AMPs). Lysyl-PG synthesis and translocation to the outer leaflet of the bacterial membrane are achieved by the membrane protein MprF. Consequently, mutants lacking a functional mprF gene are in particular vulnerable to the action of AMPs. Hence, we aim at elucidating whether and to which extent lysyl-PG modulates membrane binding, insertion, and permeabilization by various AMPs. Lysyl-PG was incorporated into artificial lipid bilayers, mimicking the cytoplasmic membrane of S. aureus. Moreover, we determined the activity of the peptides against a clinical isolate of S. aureus strain SA113 and two mutants lacking a functional mprF gene and visualized peptide-induced ultrastructural changes of bacteria by transmission electron microscopy. The studied peptides were: (i) NK-2, an α-helical fragment of mammalian NK-lysin, (ii) arenicin-1, a lugworm β-sheet peptide, and (iii) bee venom melittin. Biophysical data obtained by FRET spectroscopy, Fourier transform infrared spectroscopy, and electrical measurements with planar lipid bilayers were correlated with the biological activities of the peptides. They strongly support the hypothesis that peptide-membrane interactions are a prerequisite for eradication of S. aureus. However, degree and mode of modulation of membrane properties such as fluidity, capacitance, and conductivity were unique for each of the peptides. Altogether, our data support and underline the significance of lysyl-PG for S. aureus resistance to AMPs.

Insights into in vivo activities of lantibiotics from gallidermin and epidermin mode-of-action studies

The activity of lanthionine-containing peptide antibiotics (lantibiotics) is based on different killing mechanisms which may be combined in one molecule. The prototype lantibiotic nisin inhibits peptidoglycan synthesis and forms pores through specific interaction with the cell wall precursor lipid II. Gallidermin and epidermin possess the same putative lipid II binding motif as nisin; however, both peptides are considerably shorter (22 amino acids, compared to 34 in nisin). We demonstrate that in model membranes, lipid II-mediated pore formation by gallidermin depends on membrane thickness. With intact cells, pore formation was less pronounced than for nisin and occurred only in some strains. In Lactococcus lactis subsp. cremoris HP, gallidermin was not able to release K+, and a mutant peptide, [A12L]gallidermin, in which the ability to form pores was disrupted, was as potent as wild-type gallidermin, indicating that pore formation does not contribute to killing. In contrast, nisin rapidly formed pores in the L. lactis strain; however, it was approximately 10-fold less effective in killing. The superior activity of gallidermin in a cell wall biosynthesis assay may help to explain this high potency. Generally, it appears that the multiple activities of lantibiotics combine differently for individual target strains.

In silico genome-scale reconstruction and validation of the Staphylococcus aureus metabolic network

A genome-scale metabolic model of the Gram-positive, facultative anaerobic opportunistic pathogen Staphylococcus aureus N315 was constructed based on current genomic data, literature, and physiological information. The model comprises 774 metabolic processes representing approximately 23% of all protein-coding regions. The model was extensively validated against experimental observations and it correctly predicted main physiological properties of the wild-type strain, such as aerobic and anaerobic respiration and fermentation. Due to the frequent involvement of S. aureus in hospital-acquired bacterial infections combined with its increasing antibiotic resistance, we also investigated the clinically relevant phenotype of small colony variants and found that the model predictions agreed with recent findings of proteome analyses. This indicates that the model is useful in assisting future experiments to elucidate the interrelationship of bacterial metabolism and resistance. To help directing future studies for novel chemotherapeutic targets, we conducted a large-scale in silico gene deletion study that identified 158 essential intracellular reactions. A more detailed analysis showed that the biosynthesis of glycans and lipids is rather rigid with respect to circumventing gene deletions, which should make these areas particularly interesting for antibiotic development. The combination of this stoichiometric model with transcriptomic and proteomic data should allow a new quality in the analysis of clinically relevant organisms and a more rationalized system-level search for novel drug targets.

Lipid and fatty acid composition of cytoplasmic membranes from Streptomyces hygroscopicus and its stable protoplast-type L form

The cells of an L-form strain of Streptomyces hygroscopicus have been grown for 20 years without a cell wall. Their cytoplasmic membranes have high stability and an unusual structural polymorphism. To clarify the importance of the lipid components for these membrane properties, a comparative analysis has been carried out with purified membranes of L-form cells, of parent vegetative hyphal cells (N-form cells), and of protoplasts derived from the latter. The phospholipid classes and fatty acids were determined by thin-layer chromatography (TLC), two-dimensional TLC, high-performance liquid chromatography, gas chromatography, and mass spectrometry. The qualitative compositions of cardiolipin (CL), lyso-cardiolipin (LCL), phosphatidylethanolamine (PE1 and PE2), lyso-phosphatidylethanolamine (LPE), phosphatidylinositolmannoside (PIM), phosphatidic acid (PA), dilyso-cardiolipin-phosphatidylinositol (DLCL-PI), and the 13 main fatty acids were the same in the three membrane types. However, significant quantitative differences were observed in the L-form membrane. They consist of a three- to fourfold-higher content of total, extractable lipids, 20% more phospholipids, an increased content of CL and PIM, and a reduced amount of the component DLCL-PI. Furthermore, the L-form membrane is characterized by a higher content of branched anteiso 15:0 and anteiso 17:0 fatty acids compared to that of the membranes of the walled vegetative cells. These fatty acids have lower melting points than their straight and iso-branched counterparts and make the membrane more fluid. The phospholipid composition of the protoplast membrane differs quantitatively from that of the N form and the L form. Whereas the phospholipid classes are mostly similar to that of the N form, the fatty acid pattern tends to be closer to that of the L-form membrane. The membranes of both the L-form cells and the protoplasts need to be more fluid because of their spherical cell shape and higher degree of curvature compared with N-form membranes.

Studies on osmotic stability of liposomes prepared with bacterial membrane lipids by carboxyfluorescein release

The authors measured the osmotic stability of liposomes prepared with membrane lipids of bacteria, using the osmotic-shock release of entrapped carboxyfluorescein as an indicator. The sub-second physical changes of liposomes suspended in a solution of low osmotic pressure were examined by stopped flow spectrophotometry. The entrapped carboxyfluorescein was released when the liposomes burst on inflow of excess water. Liposomes prepared with the lipids of a stable Staphylococcus aureus L-form strain were more resistant to low osmotic pressure than those prepared from the wild strain of S. aureus, and liposomes prepared from Mycoplasma orale were even more resistant. Cardiolipin enhanced the lipid membrane stability in S. aureus and cholesterol in M. orale. The stability of lipid membranes to low osmotic pressure could be precisely determined by the present method.

Morphological detection of filipin-sterol complexes in the cytoplasmic membrane of staphylococcal L-form

Filipin, a sterol-specific antibiotic, and freeze-fracture electron microscopy were used to study the presence and distribution of sterol in the cytoplasmic membrane of stable staphylococcal L-form cells. Fixed cells were treated with filipin, and then observed by freeze-fracture electron microscopy. Freeze-fractured profiles of the L-form cells treated with filipin demonstrated irregular distribution of protuberances or pits of 25-30 nm, representing filipin-sterol complexes, on the proto-plasmic fracture face (PF) and exoplasmic fracture face (EF) of the cytoplasmic membrane. In contrast, no such structure was detected in the filipin-treated parent cells or protoplasts. The results suggest that some sterol molecules, which are usually not found in staphylococcal or other bacterial cells, emerged on the cytoplasmic membrane after the cells were converted to the stable L-form.

Diglycosyl diacylglycerol of Mycobacterium tuberculosis

A diglycosyl diacylglycerol was isolated from Mycobacterium tuberculosis, and its structure was established by a combination of methylation analysis, 1H nuclear magnetic resonance, and fast atom bombardment-mass spectrometry. It is a 1,2-diacyl-[beta-D-glucopyranosyl(1"----6')-beta-D-glucopyranosyl(1'---- 3)]- sn-glycerol and exists in at least five molecular species differing in fatty acyl substituents. The major constituent fatty acids were identified as iso- and anteisopentadecanoate, iso- and n-hexadecanoate, and iso- and anteisoheptadecanoate. Although glycosyl diacylglycerols are common membrane components of gram-positive bacteria, this report represents the first substantial evidence for the presence of a glycosyl diacylglycerol within a member of the Mycobacterium genus. Although the glycolipid is not a major component of M. tuberculosis, it reacts readily in enzyme-linked immunosorbent assay against rabbit antibodies raised against whole bacteria and thus may be useful for the serodiagnosis of tuberculosis.

Adhesion and phagocytosis of Staphylococcus aureus L-forms

Electron microscopical investigations on in vitro and in vivo interactions of normal Staphylococcus aureus cells with rat peritoneal macrophages showed that these bacteria were rapidly endocytosed and digested even in the absence of specific antibodies. In contrast to the parental strains oxacilin-induced and stable variante lacking a cell wall (L-forms) were ingested without subsequent formation of phagolysomes and digestive vacuoles. The intracytoplasmic L-form bodies retained their characteristic ultrastructure, i.e. no visible alterations occurred. Some morphological aspects of the L-forms and their persistence in macrophages 7 days after intraperitoneal administration of L-form to rats, suggest the possibility of their intracellular survival.

Alteration in phospholipid composition of Staphylococcus aureus during formation of autoplast

The phospholipid composition of autoplasts (protoplasts made by autolysis of Staphylococcus aureus 209P was examined. The autoplasts were prepared by incubation of 209P cells in 1.2 M sucrose--0.33 M acetate buffer (pH 5.8). Cardiolipin comprised nearly half the total phospholipid in these autoplasts. Autoplasts had a lower phosphatidylglycerol content than intact cells but similar lysylphosphatidylglycerol content. The increase in cardiolipin content during release of autoplasts was not affected by pH or temperature. The result indicates that removal of the cell wall caused the increase in cardiolipin content. The total amount of phospholipids increased slightly during autoplast formation, but there was no significant increase in fatty acids or diglycerides. The changes of phospholipid composition during formation of the autoplast was due to de novo synthesis of cardiolipin from phosphatidylglycerol.

Lipid composition of Staphylococcus aureus and its derived L-forms

Two strains of Staphylococcus aureus (Newman and Tazaki) and their derived L-forms were cultured in serum-containing broth and the differences in their lipid compositions were analyzed. Cardiolipin accounted for more than 50% of the total phospholipid phosphorus in L-forms, but for less than 25% in parent bacteria. The cardiolipin content of L-forms was very high through all growth phases, although it increased gradually as growth proceeded. Significant amounts of cholesterol and its esters were present in parent strains and L-forms, all of which incorporated serum cholesterol into the cell membrane. On the other hand, they could be detected in the L-forms but not in the parent strains when they were cultured in serum-free broth. To examine the ability of L-forms to synthesize cholesterol, the cholesterol content of L-forms cultured in serum-free broth was compared with that of the medium. The results indicated that staphylococcal L-forms could synthesize cholesterol and its esters. These differences in lipid composition suggested that modification of membrane lipids may occur as an adaptational change in response to the disappearance of the cell wall.

Immunological properties of teichoic acids

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The effect of the polar moiety of lipids on the ion permeability of bilayer membranes

Bilayer membranes were prepared with the negatively charged lipids phosphatidylglycerol and diphosphatidylglycerol, the positively charged lipid lysyl phosphatidylglycerol, the zwitterionic lipid phosphatidylethanolamine, and an uncharged glycolipid, diglucosyldiglyceride, all isolated from gram-positive bacteria. Bilayer membranes of all these lipids manifested specific resistances of 10(7) to 10(9) Ω cm(2) and capacitances of 0.3 to 0.4 μF cm(-2). The membrane potentials of these bilayers were measured as a function of the sodium chloride, potassium chloride, and hydrogen chloride transmembrane concentration gradients (0.01 to 0.10M) and were found to be linear with the logarithm of the salt activity gradients. Membranes made from lysyl phosphatidylglycerol (one net positive charge) were almost completely chloride selective, whereas membranes from phosphatidylglycerol and diphosphatidylglycerol (one and two net negative charges, respectively) were highly cation selective. Membranes prepared with either diglucosyldiglyceride or phosphatidylethanolamine showed only slight cation selectivity. These findings indicate that the charge on the polar head group of membrane lipids plays an important role in controlling the ion-selective permeability of the bilayer.

Bacterial glycolipids

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Isolation of spheroplast membranes and stability of spheroplasts of Bacillus stearothermophilus

Spheroplasts were prepared by lysozyme digestion of the cell wall and ruptured by suspension in 0.15 m NaCl, followed by centrifugation at 30,900 x g for 35 min, and by a final suspension in 0.05 m NaCl for 12 to 16 hr at 5 C. The membrane ghosts were washed four times in tris(hydroxylmethyl)aminomethane (Tris) magnesium buffer and once in distilled water. The intact membranes resembled empty sacs with narrow slits in which the cytoplasm was extruded. A 92% recovery of cell membrane was obtained with all membrane preparations. The spheroplasts do not require a stabilizing medium to keep them from rupturing, and they are stable for 2 to 3 hr when exposed to a temperature of 65 C. The membrane content of the cell increases with age of culture (mid-log, 16.5%; late-log, 17.0%; and stationary, 17.6%) and temperature of growth (55 C, 16.5%; and 65 C, 17.8%), and it is unaffected by composition of the growth medium. The ratio of the protein to lipid content of the membrane increases with the complexity of the medium, age of culture (mid-log, 3.65; late-log, 3.91; and stationary, 4.15), and temperature of growth (55 C, 3.65; and 65 C, 5.22). The ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) content of the membranes was 9.0 to 13.7% and 0.3 to 0.8%, respectively. Reducing sugar (determined as glucose) amounts to 0.9 to 1.0% of the membrane weight and did not significantly vary for the different membrane preparations. Medium composition, age of culture, and temperature of growth have no significant effect on the amount of each amino acid in the membrane. Aspartic acid, glutamic acid, alanine, leucine, and lysine are present in the greatest amount and represent 12.9 to 14.1%, 10.4 to 11.3%, 9.6 to 10.3%, 7.7 to 8.8%, and 7.6 to 8.5% of the membrane peptide, respectively. Prior to the rupture of the spheroplasts, 25.0, 15.7, and 50.0% of the protein, RNA, and DNA, respectively, is lost. In potassium phosphate-magnesium buffer without sucrose, 90% of the protein and RNA and 95% of the DNA is lost from the spheroplasts. In the presence of sucrose, the leakage of RNA and DNA is similar to that observed for spheroplasts suspended in Tris magnesium buffer; however, the leakage of protein is 2.4 times greater.