Effect of membrane potential on entry of lactoferricin B-derived 6-residue antimicrobial peptide into single Escherichia coli cells and lipid vesicles

Relationship between oligoarginine-induced membrane damage of single Escherichia coli cells and entry of the peptide into the cytoplasm

Cell-penetrating peptides (CPPs) can enter the cytosol of eukaryotic cells without killing them whereas some CPPs exhibit antimicrobial activity against bacterial cells. Here, to elucidate the mode of interaction of the CPP nona-arginine (R9) with bacterial cells, we investigated the interactions of lissamine rhodamine B red-labeled peptide (Rh-R9) with single Escherichia coli cells encapsulating calcein using confocal laser scanning microscopy. After Rh-R9 induced the leakage of a large amount of calcein, the fluorescence intensity of the cytosol due to Rh-R9 greatly increased, indicating that Rh-R9 induces cell membrane damage, thus allowing entry of a significant amount of Rh-R9 into the cytosol. To determine if the lipid bilayer region of the membrane is the main target of Rh-R9, we then investigated the interaction of Rh-R9 with single giant unilamellar vesicles (GUVs) comprising an E. coli polar lipid extract containing small GUVs and AlexaFluor 647 hydrazide (AF647) in the lumen. Rh-R9 entered the GUV lumen without inducing AF647 leakage, but leakage eventually did occur, indicating that GUV membrane damage was induced after the entry of Rh-R9 into the GUV lumen. The Rh-R9 peptide concentration dependence of the fraction of entry of Rh-R9 after a specific interaction time was similar to that of the fraction of leaking GUVs. These results indicate that Rh-R9 can damage the lipid bilayer region of a cell membrane, which may be related to its antimicrobial activity.

Relationship between antimicrobial peptides-induced cell membrane damage and bactericidal activity

Most antimicrobial peptides (AMPs) act by killing bacterial cells. However, there is little information regarding the required interaction time between AMPs and bacterial cells to exert the bactericidal activity. One of the causes of the bactericidal activity is considered to be cell membrane damage, although little direct evidence is available. Here, we investigated the relationship between AMP-induced cell membrane damage in Escherichia coli and AMP-induced cell death at the single-cell level. Magainin 2, lactoferricin B, and PGLa were selected as the AMPs. First, we examined the interaction time (t) of AMPs with cells required to induce cell death using the single-cell analysis. The fraction of microcolonies containing only a single cell, Psingle (t), which indicates the fraction of dead cells, increased with time to reach ∼1 in a short time (≤5 min). Then, we examined the interaction between AMPs and single cells using confocal laser scanning microscopy in the presence of membrane-impermeable SYTOX green. Within a short time interaction, the fluorescence intensity of the cells due to SYTOX green increased, indicating that AMPs induced cell membrane damage through which the dye entered the cytoplasm. The fraction of cells in which SYTOX green entered the cytoplasm among all examined cells after the interaction time (t), Pentry (t), increased with time, reaching ∼1 in a short time (≤5 min). The values of Psingle (t) and Pentry (t) were similar at t ≥ 3 min for all AMPs. The bindings of AMPs to cells were largely reversible, whereas the AMP-induced cell membrane damages were largely irreversible because SYTOX green entered the cells after dilution of AMP concentration. Based on these results, we conclude that the rapid, substantial membrane permeabilization of cytoplasmic contents after a short interaction time with AMPs and the residual damage after dilution induce cell death.

Tunable biomimetic bacterial membranes from binary and ternary lipid mixtures and their application in antimicrobial testing

The reconstruction of accurate yet simplified mimetic models of cell membranes is a very challenging goal of synthetic biology. To date, most of the research focuses on the development of eukaryotic cell membranes, while reconstitution of their prokaryotic counterparts has not been fully addressed, and the proposed models do not reflect well the complexity of bacterial cell envelopes. Here, we describe the reconstitution of biomimetic bacterial membranes with an increasing level of complexity, developed from binary and ternary lipid mixtures. Giant unilamellar vesicles composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE); PC and phosphatidylglycerol (PG); PE and PG; PE, PG and cardiolipin (CA) at varying molar ratios were successfully prepared by the electroformation method. Each of the proposed mimetic models focuses on reproducing specific membrane features such as membrane charge, curvature, leaflets asymmetry, or the presence of phase separation. GUVs were characterized in terms of size distribution, surface charge, and lateral organization. Finally, the developed models were tested against the lipopeptide antibiotic daptomycin. The obtained results showed a clear dependency of daptomycin binding efficiency on the amount of negatively charged lipid species present in the membrane. We anticipate that the models proposed here can be applied not only in antimicrobial testing but also serve as platforms for studying fundamental biological processes in bacteria as well as their interaction with physiologically relevant biomolecules.

Effects of membrane potentials on the electroporation of giant unilamellar vesicles

Living organisms maintain a resting membrane potential, which plays an important role in various biophysical and biological processes. In the context of medical applications, irreversible electroporation (IRE) is a non-thermal and minimally invasive technique that utilizes precisely controlled electric field pulses of micro- to millisecond durations to effectively ablate cancer and tumor cells. Previous studies on IRE-induced rupture of cell-mimetic giant unilamellar vesicles (GUVs) have primarily been conducted in the absence of membrane potentials. In this study, we investigated the electroporation of GUVs, including parameters such as the rate constant of rupture and the probability of rupture, in the presence of various negative membrane potentials. The membranes of GUVs were prepared using lipids and channel forming proteins. As the membrane potential increased from 0 to -90 mV, the rate constant of rupture showed a significant increase from (7.5 ± 1.6)×10-3 to (35.6 ± 5.5)×10-3 s-1. The corresponding probability of rupture also exhibited a notable increase from 0.40 ± 0.05 to 0.68 ± 0.05. To estimate the pore edge tension, the electric tension-dependent logarithm of the rate constant was fitted with the Arrhenius equation for different membrane potentials. The presence of membrane potential did not lead to any significant changes in the pore edge tension. The increase in electroporation is reasonably explained by the decrease in the prepore free energy barrier. The choice of buffer used in GUVs can significantly influence the kinetics of electroporation. This study provides valuable insights that can contribute to the application of electroporation techniques in the biomedical field.

Increase in anionic Fe3O4 nanoparticle-induced membrane poration and vesicle deformation due to membrane potential - an experimental study

The membrane potential plays a significant role in various cellular processes while interacting with membrane active agents. So far, all the investigations of the interaction of nanoparticles (NPs) with lipid vesicles have been performed in the absence of membrane potential. In this study, the anionic magnetite NP-induced poration along with deformation of cell-mimetic giant unilamellar vesicles (GUVs) has been studied in the presence of various membrane potentials. Lipids 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and channel forming protein gramicidin A (GrA) are used to synthesize the DOPG/DOPC/GrA-GUVs. The static and dynamic nature of GUVs is investigated using phase contrast fluorescent microscopy. The presence of GrA in the membrane decreases the leakage constant of the encapsulating fluorescent probe (calcein) in the absence of membrane potential. With the increase of negative membrane potential, the leakage shifts from a single exponential to two exponential functions, obtaining two leakage constants. The leakage became faster at the initial stage, and at the final stage, it became slower with the increase in negative membrane potential. Both the fraction of poration and deformation increase with the increase of negative membrane potential. These results suggested that the membrane potential enhances the NP-induced poration along with the deformation of DOPG/DOPC/GrA-GUVs. The increase of the binding constant in the NPs with membrane potential is one of the important factors for increasing membrane permeation and vesicle deformation.

Time to Kill and Time to Heal: The Multifaceted Role of Lactoferrin and Lactoferricin in Host Defense

Lactoferrin is an iron-binding glycoprotein present in most human exocrine fluids, particularly breast milk. Lactoferrin is also released from neutrophil granules, and its concentration increases rapidly at the site of inflammation. Immune cells of both the innate and the adaptive immune system express receptors for lactoferrin to modulate their functions in response to it. On the basis of these interactions, lactoferrin plays many roles in host defense, ranging from augmenting or calming inflammatory pathways to direct killing of pathogens. Complex biological activities of lactoferrin are determined by its ability to sequester iron and by its highly basic N-terminus, via which lactoferrin binds to a plethora of negatively charged surfaces of microorganisms and viruses, as well as to mammalian cells, both normal and cancerous. Proteolytic cleavage of lactoferrin in the digestive tract generates smaller peptides, such as N-terminally derived lactoferricin. Lactoferricin shares some of the properties of lactoferrin, but also exhibits unique characteristics and functions. In this review, we discuss the structure, functions, and potential therapeutic uses of lactoferrin, lactoferricin, and other lactoferrin-derived bioactive peptides in treating various infections and inflammatory conditions. Furthermore, we summarize clinical trials examining the effect of lactoferrin supplementation in disease treatment, with a special focus on its potential use in treating COVID-19.

Antimicrobial peptide magainin 2-induced rupture of single giant unilamellar vesicles comprising E. coli polar lipids

Most antimicrobial peptides (AMPs) damage the cell membrane of bacterial cells and induce rapid leakage of the internal cell contents, which is a main cause of their bactericidal activity. One of the AMPs, magainin 2 (Mag), forms nanopores in giant unilamellar vesicles (GUVs) comprising phosphatidylcholine (PC) and phosphatidylglycerol (PG), inducing leakage of fluorescent probes. In this study, to elucidate the Mag-induced pore formation in lipid bilayer region in E. coli cell membrane, we examined the interaction of Mag with single GUVs comprising E. coli polar lipids (E. coli-lipid-GUVs). First, we investigated the Mag-induced leakage of a fluorescent probe AF488 from single E. coli-lipid-GUVs, and found that Mag caused rupture of GUVs, inducing rapid AF488 leakage. The rate constant of Mag-induced GUV rupture increased with the Mag concentration. Using fluorescence microscopy with a time resolution of 5 ms, we revealed the GUV rupture process: first, a small micropore was observed in the GUV membrane, then the pore radius increased within 50 ms without changing the GUV diameter, the thickness of the membrane at the pore rim concomitantly increased, and eventually membrane aggregates were formed. Mag bound to only the outer monolayer of the GUV before GUV rupture, which increased the area of the GUV bilayer. We also examined the physical properties of E. coli-lipid-GUVs themselves. We found that the rate constant of the constant tension-induced rupture of E. coli-lipid-GUVs was higher than that of PG/PC-GUVs. Based on these results, we discussed the Mag-induced rupture of E. coli-lipid-GUVs and its mechanism.

Role of interfacial hydrophobicity in antimicrobial peptide magainin 2-induced nanopore formation

Antimicrobial peptide magainin 2 (Mag) forms nanopores in lipid bilayers and induces membrane permeation of the internal contents from vesicles. The binding of Mag to the membrane interface of a giant unilamellar vesicle (GUV) increases its fractional area change, δ, which is one of the main causes of Mag-induced nanopore formation. However, the role of its amino acid composition in the Mag-induced area increase and the following nanopore formation is not well understood. Here, to elucidate it we examined the role of interfacial hydrophobicity of Mag in its nanopore formation activity by investigating de novo-designed Mag mutants-induced nanopore formation in GUVs. Aligned amino acid residues in the α-helix of Mag were replaced to create 3 mutants: F5A-Mag, A9F-Mag, and F5,12,16A-Mag. These mutants have different interfacial hydrophobicity due to the variation of the numbers of Phe and Ala because the interfacial hydrophobicity of Phe is higher than that of Ala. The rate constant of Mag mutant-induced nanopore formation, k_p, increased with increasing numbers of Phe residues at the same peptide concentration. Further, the Mag mutant-induced δ increased with increasing numbers of Phe residues at the same peptide concentration. These results indicate that k_p and δ increase with increasing interfacial hydrophobicity of Mag mutants. The relationship between k_p and δ in the Mag and its mutants clearly indicates that k_p increases with increasing δ, irrespective of the difference in mutants. Based on these results, we can conclude that the interfacial hydrophobicity of Mag plays an important role in its nanopore formation activity.

Single-Cell Analysis of the Antimicrobial and Bactericidal Activities of the Antimicrobial Peptide Magainin 2

Antimicrobial peptides (AMPs) inhibit the proliferation of or kill bacterial cells. To measure these activities, several methods have been used, which provide only the average value of many cells. Here, we report the development of a method to examine the antimicrobial and bactericidal activities of AMPs at the single-cell level (i.e., single-cell analysis) and apply this strategy to examine the interaction of an AMP, magainin 2 (Mag), with Escherichia coli cells. Using this method, we monitored the proliferation of single cells on agar in a microchamber and measured the distribution of the number of cells in each microcolony using optical microscopy. For method A, we incubated cells in the presence of various concentrations of AMPs for 3 h. The fraction of microcolonies containing only a single cell, Psingle, increased with the Mag concentration and reached 1 at a specific concentration, which corresponded to the MIC. For method B, after the interaction of a cell suspension with an AMP for a specific time, an aliquot was diluted to stop the interaction, and the proliferation of single cells then was monitored after a 3-h incubation; this method permits the definition of Psingle(t), the fraction of dead cells after the interaction. For the interaction of Mag with E. coli cells, Psingle(t) increased with the interaction time, reaching ~1 at 10 and 20 min for 25 and 13 μM Mag, respectively. Thus, these results indicate that a short interaction time between Mag and E. coli cells is sufficient to induce bacterial cell death. IMPORTANCE To elucidate the activity of antimicrobial peptides (AMPs) against bacterial cells, it is important to estimate the interaction time that is sufficient to induce cell death. We have developed a method to examine the antimicrobial and bactericidal activities of AMPs at the single-cell level (i.e., single-cell analysis). Using this method, we monitored the proliferation of single cells on agar in a microchamber and measured the distribution of the number of cells in each microcolony using optical microscopy. We found that during the interaction of magainin 2 (Mag) with E. coli cells, the fraction of dead cells, Psingle(t), increased with the interaction time, rapidly reaching 1 (e.g., 10 min for 25 μM Mag). This result indicates that Mag induces cell death after a short time of interaction.

Antimicrobial effect and mechanism of bovine lactoferrin against the potato common scab pathogen Streptomyces scabiei

Lactoferrin (LF) is a multifunctional protein with a broad spectrum of antimicrobial activities. In this study, we investigated the antimicrobial activity of LF against the potato common scab pathogen Streptomyces scabiei, which causes severe damage to potato tubers. LF derived from bovine (bLF) had much higher activity against S. scabiei than human LF. The minimal inhibitory concentration of bLF was 3.9 μM. The effects of both apo-bLF (iron-free) and holo-bLF (iron-saturated) on S. scabiei were not different. Bovine lactoferricin (LFcinB), a short peptide with a length of 25 amino acid residues located in the N-terminal region of bLF, showed antimicrobial activity against S. scabiei, similar to that of bLF. These results indicated that the antimicrobial activity of bLF against S. scabiei cannot be attributed to its iron-chelating effect but to the bioactivity of its peptides. When S. scabiei was treated with the fusion protein of mCherry-LFcinB (red fluorescent protein) expressed in Escherichia coli, the pseudohyphal cells instantly glowed, indicating that the peptide electrostatically binds to the surface of S. scabiei. An assay of synthetic peptides, with modified number of arginine (Arg) and tryptophan (Trp) residues based on the antimicrobial center (RRWQWR) of LFcinB showed that Trp residues are implicated in the antimicrobial activity against S. scabiei; however, Arg residues are also necessary to carry Trp residues to the cell surface to fully exert its activity. Although the single amino acid effect of Trp had low activity, Trp derivatives showed much higher activity against S. scabiei, suggesting that the derivatives effectively bind to the cell surface (cell membrane) by themselves without a carrier. Thus, amino acid derivatives might be considered effective and alternative antimicrobial substances.

Translocation of the nonlabeled antimicrobial peptide PGLa across lipid bilayers and its entry into vesicle lumens without pore formation

Fluorescent-probe-labeled peptides are used to study the interactions of peptides with cells and lipid vesicles but labeling peptides with fluorescent probes can significantly change these interactions. We recently developed a new method to detect the entry of nonlabeled peptides into the lumen of single giant unilamellar vesicles (GUVs). Here we applied this method to examine the interaction of the antimicrobial peptide PGLa with single GUVs to elucidate whether PGLa can enter the GUV lumen without pore formation. First, we examined the interaction of nonlabeled PGLa with single GUVs comprising dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (4/6) whose lumens contain the fluorescent probe AF647 and DOPG/DOPC (8/2)-large unilamellar vesicles encapsulating a high concentration of calcein. After a large lag period from starting the interaction with PGLa, the fluorescence intensity of the GUV lumen due to calcein (I_calcein) increased gradually without leakage of AF647, indicating that PGLa enters the GUV lumen without pore formation in the GUV membrane. The fraction of entry of PGLa increased with increasing PGLa concentration. Simultaneous measurement of the fractional area change of the GUV membrane (δ) and PGLa-induced increase in I_calcein showed that the entry of PGLa occurs only during the second increase in δ, indicating that PGLa enters the lumen during its translocation from the outer leaflet to the inner leaflet. The fraction of entry of PGLa without pore formation increased with increasing membrane tension. Based on these results, we discuss the elementary processes and the mechanism of the entry of PGLa into the GUV lumen.