Selectivity for and destruction of Salmonella typhimurium via a membrane damage mechanism of a cell-penetrating peptide ppTG20 analogue

Controlling soft rot of green pepper by bacteriocin paracin wx3 and its effect on storage quality of green pepper

A bacteriocin paracin wx3 was investigated as a candidate of natural preservative to control green pepper soft rot. Firstly, paracin wx3 was heterologously expressed in Pichia pastoris X33 with an improved yield of 0.537 g/L. Its size and amino acid sequence were confirmed by Tricine-SDS-PAGE and LC-MS/MS. Then, result of antibacterial activity showed that its MIC value against Pectobacterium carotovorum was 16 μg/mL. In vitro, paracin wx3 completely killed the pathogen at high concentrations ≥8 × MIC. In vivo, disease incidence of green pepper soft rot was decreased from 90% (control) to <2% (8 × MIC). Subsequently, results of action mode showed that paracin wx3 inhibited the growth of pathogen by pore-formation on cell membrane. Last, paracin wx3 treatment reduced losses of weight, firmness, total soluble solid, Vc of green pepper during storage. It also inhibited the production of soft rot volatile p-xylene, 1-butanol, 2-methyl-2-propanol, 3-hydroxybutan-2-one-D, 2-pentyl furan, butanal, etc.

Hybrid 2D/3D-quantitative structure-activity relationship studies on the bioactivities and molecular mechanism of antibacterial peptides

Antimicrobial peptide (AMP) is the polypeptide, which protects the organism avoiding attack from pathogenic bacteria. Studies have shown that there were some antimicrobial peptides with molecular action mechanism involved in crossing the cell membrane without inducing severe membrane collapse, then interacting with cytoplasmic target-nucleic acid, and exerting antibacterial activity by interfacing the transmission of genetic information of pathogenic microorganisms. However, the relationship between the antibacterial activities and peptide structures was still unclear. Therefore, in the present work, a series of AMPs with a sequence of 20 amino acids was extracted from DBAASP database, then, quantitative structure-activity relationship (QSAR) methods were conducted on these peptides. In addition, novel antimicrobial peptides with  stronger antimicrobial activities were designed according to the information originated from the constructed models. Hence, the outcome of this study would lay a solid foundation for the in-silico design and exploration of novel antibacterial peptides with improved activity activities.

Isoquinoline Antimicrobial Agent: Activity against Intracellular Bacteria and Effect on Global Bacterial Proteome

A new class of alkynyl isoquinoline antibacterial compounds, synthesized via Sonogashira coupling, with strong bactericidal activity against a plethora of Gram-positive bacteria including methicillin- and vancomycin-resistant Staphylococcus aureus (S. aureus) strains is presented. HSN584 and HSN739, representative compounds in this class, reduce methicillin-resistant S. aureus (MRSA) load in macrophages, whilst vancomycin, a drug of choice for MRSA infections, was unable to clear intracellular MRSA. Additionally, both HSN584 and HSN739 exhibited a low propensity to develop resistance. We utilized comparative global proteomics and macromolecule biosynthesis assays to gain insight into the alkynyl isoquinoline mechanism of action. Our preliminary data show that HSN584 perturb S. aureus cell wall and nucleic acid biosynthesis. The alkynyl isoquinoline moiety is a new scaffold for the development of potent antibacterial agents against fatal multidrug-resistant Gram-positive bacteria.

Antibacterial efficacy of in-house designed cell-penetrating peptide against multi-drug resistant strains of Salmonella Enteritidis and Salmonella Typhimurium

The in vitro antibacterial efficacy of an in-house designed cell-penetrating peptide (CPP) variant of Cecropin A (1-7)-Melittin (CAMA) (CAMA-CPP) against the characterized multi-drug resistant (MDR) field strains of Salmonella Enteritidis and Salmonella Typhimurium were evaluated and compared with two identified CPPs namely, P7 and APP, keeping CAMA as control. Initially, the minimum inhibitory concentration (MIC) (μg ml^-1 ) of in-house designed CAMA-CPP, APP and CAMA was determined to be 3.91, whereas that of P7 was 7.81; however, the minimum bactericidal concentration (MBC) of all the peptides were twice the MIC. CAMA-CPP and CAMA were found to be stable under different conditions (high-end temperatures, proteinase-K, cationic salts, pH and serum) when compared to the other CPPs. Moreover, CAMA-CPP exhibited negligible cytotoxicity in HEp-2 and RAW 264.7 cell lines as well as haemolysis in the sheep and human erythrocytes with no adverse effects against the commensal gut lactobacilli. In vitro time-kill assay revealed that the MBC levels of CAMA-CPP and APP could eliminate the intracellular MDR-Salmonella infections from mammalian cell lines; however, CAMA and P7 peptides were ineffective. CAMA-CPP appears to be a promising antimicrobial candidate and opens up further avenues for its in vivo clinical translation.

Comparison of measurement methods at determining the target sites injured by antimicrobials in Escherichia coli O157:H7 using metabolic inhibitors

Acquiring an understanding of the mechanisms underlying antimicrobial action is important for overcoming bacterial resistance to antimicrobials. This study evaluated three different methods (antimicrobial fixed broth dilution method, metabolic inhibitors fixed broth dilution method, and metabolic inhibitor fixed agar recovery method) for determining the target site of Escherichia coli O157:H7 by treatments with various antimicrobials (ethanol, ethylenediaminetetraacetic acid, polymyxin B, thymol, acetic acid, and citrus fruit extract). However, the results indicated only weak relationships between MIC values and mechanisms of antimicrobials known to cause damage or injury. In addition, the results of three measurement methods using metabolic inhibitors were not correlated. These results suggest that measurement methods using metabolic inhibitors alone may not be suitable for determining the target site injured by antimicrobials. Therefore, various measurement methods should be compared and analyzed to determine the damage or injury sites targeted by antimicrobials in pathogenic bacteria. Further studies are needed to compare and analyze the various measurement methods for determining the target site injured by antimicrobials in pathogenic bacteria.

The determination of antibacterial mode for cationic lipopeptides brevibacillins against Salmonella typhimurium by quantum chemistry calculation

Brevibacillins are broad-spectrum cationic antimicrobial lipopeptides produced by Brevibacillus laterosporus fmb70 CGMCC 18426. The antibacterial mode of brevibacillins against Salmonella typhimurium CICC 21493 was investigated by quantum chemistry calculation in this study. The addition of LPS, Mg²⁺, and Ca²⁺ partially reduced the antimicrobial activity of brevibacillin and brevibacillin V against S. typhimurium, which indicated that the two cationic lipopeptides could bind to LPS and displaced the divalent cations on the LPS network. Release of LPS from S. typhimurium by brevibacillin and brevibacillin V resulted in destroying the dense LPS network and increasing the permeability of the outer membrane. Quantum chemistry calculation analysis revealed that Lys7 is the most critical amino acid residue to destroy the outer membrane. The total average N-H charge difference of the three protonated amino groups (Orn3-NH₃, Lys7-NH₃, and Lys10-NH₃) determined the ability of brevibacillin V to bind LPS stronger than brevibacillin. Calcein complete leakage from liposomes and release of DiSC₃-5 from the cytoplasmic membrane (CM) indicated that brevibacillin and brevibacillin V may destroy the CM. Brevibacillin and brevibacillin V exhibited their antimicrobial activities through membrane damages, where the OM permeability with high concentration of 64-256 µg/mL and membrane damage of CM with a low concentration of 4 μg/mL. Our finding might be helpful to understand the broad-spectrum antimicrobial mechanism of cationic lipopeptide and to design the novel antimicrobial peptide. KEY POINTS: • Brevibacillin V had stronger affinity for LPS than brevibacillin. • The N-H charge difference was the key of the difference in the affinity to LPS. • Brevibacillins inhibited Salmonella by displacing the divalent cations on the LPS.

Antimicrobials from Seaweeds for Food Applications

The exponential growth of emerging multidrug-resistant microorganisms, including foodborne pathogens affecting the shelf-life and quality of foods, has recently increased the needs of the food industry to search for novel, natural and eco-friendly antimicrobial agents. Macroalgae are a bio-diverse group distributed worldwide, known to produce multiple compounds of diverse chemical nature, different to those produced by terrestrial plants. These novel compounds have shown promising health benefits when incorporated into foods, including antimicrobial properties. This review aims to provide an overview of the general methods and novel compounds with antimicrobial properties recently isolated and characterized from macroalgae, emphasizing the molecular pathways of their antimicrobial mechanisms of action. The current scientific evidence on the use of macroalgae or macroalgal extracts to increase the shelf-life of foods and prevent the development of foodborne pathogens in real food products and their influence on the sensory attributes of multiple foods (i.e., meat, dairy, beverages, fish and bakery products) will also be discussed, together with the main challenges and future trends of the use of marine natural products as antimicrobials.

Mechanisms underlying the antimicrobial actions of the antimicrobial peptides Asp-Tyr-Asp-Asp and Asp-Asp-Asp-Tyr

The peptides Asp-Tyr-Asp-Asp (DYDD) and Asp-Asp-Asp-Tyr (DDDY) extracted from Dendrobium aphyllum have antimicrobial effects on Escherichia coli, Pseudomonas aeruginosa, and Monilia albicans, but no effects on Bacillus subtilis and Staphylococcus aureus. The effects of a hydrophobic environment on the secondary structures of these molecules were determined using circular dichroism and atomic force microscopy. Although scanning electron microscopy revealed that DDDY was more destructive to membranes than DYDD, both peptides showed antimicrobial effects against three pathogens. The minimum inhibitory concentration (MIC) of DYDD (18.075 mg/mL) against E. coli was higher than that of DDDY (4.519 mg/mL), and the influence of DYDD on the cell surface potential energy of E. coli was also greater (a decrease of 6.4 ± 0.66 mV) than that of DDDY (a decrease of 4.37 ± 0.77 mV). Moreover, the cell membrane damage and content leakage of DYDD-treated E. coli cells were more severe than those observed in the DDDY-treated cells. However, DDDY showed stronger antibacterial activity against P. aeruginosa and M. albicans than DYDD. A molecular dynamic simulation revealed that the mechanisms underlying the interaction between these two peptides and lipid bilayers were remarkably different. Therefore, two separate models were proposed to describe their antimicrobial activities.

An Enhanced Variant Designed From DLP4 Cationic Peptide Against Staphylococcus aureus CVCC 546

Insect defensins are promising candidates for the development of potent antimicrobials against antibiotic-resistant Staphylococcus aureus (S. aureus). An insect defensin, DLP4, isolated from the hemolymph of Hermetia illucens larvae, showed low antimicrobial activity against Gram-positive (G+) pathogens and high cytotoxicity, which limited its effective therapeutic application. To obtain more potent and low cytotoxicity molecules, a series of peptides was designed based on the DLP4 template by changing the conservative site, secondary structure, charge, or hydrophobicity. Among them, a variant designated as ID13 exhibited strong antibacterial activity at low MIC values of 4-8 μg/mL to G+ pathogens (S. aureus: 4 μg/mL; Staphylococcus epidermidis: 8 μg/mL; Streptococcus pneumoniae: 4 μg/mL; Streptococcus suis: 4 μg/mL), which were lower than those of DLP4 (S. aureus: 16 μg/mL; S. epidermidis: 64 μg/mL; S. pneumoniae: 32 μg/mL; S. suis: 16 μg/mL), and cytotoxicity of ID13 (71.4% viability) was less than that of DLP4 (63.8% viability). ID13 could penetrate and destroy the cell membrane of S. aureus CVCC 546, resulting in an increase in potassium ion leakage; it bound to genomic DNA (gDNA) and led to the change of gDNA conformation. After treatment with ID13, perforated, wrinkled, and collapsed S. aureus CVCC 546 cells were observed in electron microscopy. Additionally, ID13 killed over 99.99% of S. aureus within 1 h, 2 × MIC of ID13 induced a post-antibiotic effect (PAE) of 12.78 ± 0.28 h, and 10 mg/kg ID13 caused a 1.8 log10 (CFU/g) (CFU: colony-forming units) reduction of S. aureus in infected mouse thigh muscles and a downregulation of TNF-α, IL-6, and IL-10 levels, which were superior to those of DLP4 or vancomycin. These findings indicate that ID13 may be a promising peptide antimicrobial agent for therapeutic application.

Designing Chimeric Molecules for Drug Discovery by Leveraging Chemical Biology

After the first seed concept introduced in the 18th century, different disciplines have attributed different names to dual-functional molecules depending on their application, including bioconjugates, bifunctional compounds, multitargeting molecules, chimeras, hybrids, engineered compounds. However, these engineered constructs share a general structure: a first component that targets a specific cell and a second component that exerts the pharmacological activity. A stable or cleavable linker connects the two modules of a chimera. Herein, we discuss the recent advances in the rapidly expanding field of chimeric molecules leveraging chemical biology concepts. This Perspective is focused on bifunctional compounds in which one component is a lead compound or a drug. In detail, we discuss chemical features of chimeric molecules and their use for targeted delivery and for target engagement studies.

Mechanisms of Action for Antimicrobial Peptides With Antibacterial and Antibiofilm Functions

The antibiotic crisis has led to a pressing need for alternatives such as antimicrobial peptides (AMPs). Recent work has shown that these molecules have great potential not only as antimicrobials, but also as antibiofilm agents, immune modulators, anti-cancer agents and anti-inflammatories. A better understanding of the mechanism of action (MOA) of AMPs is an important part of the discovery of more potent and less toxic AMPs. Many models and techniques have been utilized to describe the MOA. This review will examine how biological assays and biophysical methods can be utilized in the context of the specific antibacterial and antibiofilm functions of AMPs.

Structure and mode of action of a novel antibacterial peptide from the blood of Andrias davidianus

Andrias davidianus is widely recognized in traditional medicine as a cure-all to treat a plethora of ailments. In a previous study, a novel antibacterial peptide named andricin B was isolated from A. davidianus blood. In this study, we investigated andricin B structure and its mode of action. Circular dichroism spectra suggested that andricin B adopts a random coil state in aqueous solution and a more rigid conformation in the presence of bacteria. Moreover propidium iodide/fluorescein diacetate double staining indicated that bacteria treated with andricin B were not immediately eliminated. Rather, there is a gradual bacterial death, followed by a sublethal stage. Scanning electronic microscope imaging indicates that andricin B might form pores on cell membranes, leading to the release of cytoplasmic contents. These results were consistent with flow cytometry analysis. Furthermore, Fourier transform infrared spectroscopy suggests that andricin B induces changes in the chemical properties in the areas surrounding these "pores" on the cell membranes. SIGNIFICANCE AND IMPACT OF THE STUDY: The results of this study suggested the new perspectives about the mode of action of antimicrobial peptide (AMP) active against sensitive bacteria. The AMP was able to be in a random coiled state in aqueous solution but to change to a more rigid one in the presence of sensitive bacteria. Exposure to AMP might not lead to immediate death of treated bacteria, rather bacteria concentration decreased gradually flattening at a sublethal stage. These findings will help people to understand better how the AMPs activate against sensitive bacteria.

Mode of action of the antimicrobial peptide Mel4 is independent of Staphylococcus aureus cell membrane permeability

Mel4 is a novel cationic peptide with potent activity against Gram-positive bacteria. The current study examined the anti-staphylococcal mechanism of action of Mel4 and its precursor peptide melimine. The interaction of peptides with lipoteichoic acid (LTA) and with the cytoplasmic membrane using DiSC(3)-5, Sytox green, Syto-9 and PI dyes were studied. Release of ATP and DNA/RNA from cells exposed to the peptides were determined. Bacteriolysis and autolysin-activated cell death were determined by measuring decreases in OD620nm and killing of Micrococcus lysodeikticus cells by cell-free media. Both peptides bound to LTA and rapidly dissipated the membrane potential (within 30 seconds) without affecting bacterial viability. Disturbance of the membrane potential was followed by the release of ATP (50% of total cellular ATP) by melimine and by Mel4 (20%) after 2 minutes exposure (p<0.001). Mel4 resulted in staphylococcal cells taking up PI with 3.9% cells predominantly stained after 150 min exposure, whereas melimine showed 34% staining. Unlike melimine, Mel4 did not release DNA/RNA. Cell-free media from Mel4 treated cells hydrolysed peptidoglycan and produced greater zones of inhibition against M. lysodeikticus lawn than melimine treated samples. These findings suggest that pore formation is unlikely to be involved in Mel4-mediated membrane destabilization for staphylococci, since there was no significant Mel4-induced PI staining and DNA/RNA leakage. It is likely that the S. aureus killing mechanism of Mel4 involves the release of autolysins followed by cell death. Whereas, membrane interaction is the primary bactericidal activity of melimine, which includes membrane depolarization, pore formation, release of cellular contents leading to cell death.

Comparative mode of action of the antimicrobial peptide melimine and its derivative Mel4 against Pseudomonas aeruginosa

Melimine and Mel4 are chimeric cationic peptides with broad-spectrum antimicrobial activity. They have been shown to be highly biocompatible in animal models and human clinical trials. The current study examined the mechanism of action of these two antimicrobial peptides against P. aeruginosa. The effect of the peptides of endotoxin neutralization, and their interactions with cytoplasmic membranes using DiSC(3)-5 and Sytox green, Syto-9 and PI dyes were analysed. Release of ATP and DNA/RNA were determined using ATP luminescence and increase in OD_260 nm. The bacteriolytic ability of the peptides was determined by measuring decreases in OD_620 nm. Both the peptides neutralized LPS suggesting their interaction with lipid A. Cytoplasmic membrane was disrupted within 30 seconds, which correlated with reductions in cellular viability. At 2 minutes melimine or Mel4, released 75% and 36% cellular ATP respectively (P < 0.001). Membrane permeabilization started 5 minutes with simultaneous release of DNA/RNA. Flow cytometry demonstrated 52% and 18% bacteria were stained with PI after 30 minutes. Overall, melimine showed higher capacity for membrane disruption compared to Mel4 (P < 0.001). The findings of this study have been summarized as a timeline of bactericidal activity, suggesting that the peptides permeabilized P. aeruginosa within 5 minutes, started lysis within 2 hours of exposure.

Safety Aspect of Enterococcus faecium FL31 Strain and Antibacterial Mechanism of Its Hydroxylated Bacteriocin BacFL31 against Listeria monocytogenes

In previous work we have isolated and identified a new strain called Enterococcus faecium FL31. The active compound secreted by this strain, "BacFL31", has been purified and characterized. In the present study, safety aspect, assessed by microbiological and molecular tests, demonstrated that Enterococcus faecium FL31 was susceptible to relevant antibiotics, free of hemolytic, gelatinase, DNase, and lipase activities. In addition, it did not harbor virulence and antibiotic resistance genes. Combined SYTOX Green dye and UV-absorbing experiments, along with released extracellular potassium and transmembrane electrical potential measurements, showed that pure BacFL31 at a concentration of 1×MIC (50 μg/mL) could damage cytoplasmic membrane of the pathogen Listeria monocytogenes ATCC19117. The same concentration causes the leakage of its intracellular constituents and leads to the destruction of this pathogenic microorganism. In summary, this work reflected characteristics of Enterococcus faecium FL31 strain and its bacteriocin in terms of functional and safety perspectives.

The antibacterial activity and modes of LI-F type antimicrobial peptides against Bacillus cereus in vitro

AIMS

LI-Fs are a family of highly potent cyclic lipodepsipeptide antibiotics with a broad antimicrobial spectrum (Gram-positive bacteria and fungi). In this study, LI-F-type antimicrobial peptides (AMP-jsa9) composing of LI-F03a, LI-F03b, LI-F04a, LI-F04b and LI-F05b were isolated from Paenibacillus polymyxa JSA-9. To better understand the antimicrobial mechanism of AMP-jsa9, the potency and action(s) of AMP-jsa9 against Bacillus cereus were examined.

METHODS AND RESULTS

Flow cytometry, confocal laser microscopy, scanning electron microscopy, transmission electron microscopy (TEM) and atomic force microscopy observation, as well as determination of peptidoglycan and cell wall-associated protein and other methods were used. The results indicate that AMP-jsa9 exhibits strong, broad-spectrum antimicrobial activity. Moreover, AMP-jsa9 targets the cell wall and membrane of B. cereus to impair membrane integrity, increase membrane permeability and enhance cytoplasm leakage (e.g. K⁺ , protein, nucleic acid). This leads to bacterial cells with irregular, withered and coarse surfaces. In addition, AMP-jsa9 is also able to bind to DNA and break down B. cereus biofilms.

CONCLUSIONS

In this study, the action mechanism of LI-Fs against B. cereus was clarified in details.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study provide a theoretical basis for utilizing AMP-jsa9 or similar analogues as natural and effective preservatives in the food and feed industries. These efforts could also stimulate research activities interested in understanding the specific effects of other antimicrobial agents.

Killing of Staphylococcus aureus and Salmonella enteritidis and neutralization of lipopolysaccharide by 17-residue bovine lactoferricins: improved activity of Trp/Ala-containing molecules

Bovine lactoferricin (LfcinB) has potent antibacterial, antifungal and antiparasitic activities but is also hemolytic. Our objective was to identify LfcinB17-31 derivatives with reduced hemolysis and improved antimicrobial activity via substituting Cys3, Arg4, Gln7, Met10, and Gly14 with more hydrophobic residues. Two peptides, Lfcin4 and Lfcin5, showed higher activity against Staphylococcus aureus and Salmonella enteritidis and lower hemolytic activity than the parent peptide LfcinB17-31. These peptides permeabilized the outer and inner membranes of S. enteritidis; however, Lfcin5 did not permeabilize the inner membrane of S. aureus. Gel retardation and circular dichroism spectra showed that Lfcin4 and Lfcin5 bound to bacterial genomic DNA. Lfcin4 inhibited DNA, RNA and protein synthesis. Both peptides induced the peeling of membranes and the lysis of S. enteritidis. At doses of 10 and 15 mg/kg, Lfcin4 and Lfcin5 reduced the bacterial counts in infected thigh muscles by 0.03‒0.10 and 0.05‒0.63 log₁₀ CFU/g of tissue, respectively, within 10 h. Lfcin4 and Lfcin5 enhanced the survival rate of endotoxemic mice; reduced serum IL-6, IL-1β and TNF-α levels; and protected mice from lipopolysaccharide-induced lung injury. These data suggest that Lfcin4 and Lfcin5 may be antimicrobial and anti-endotoxin peptides that could serve as the basis for the development of dual-function agents.

Membrane damage as first and DNA as the secondary target for anti-candidal activity of antimicrobial peptide P7 derived from cell-penetrating peptide ppTG20 against Candida albicans

P7, a peptide analogue derived from cell-penetrating peptide ppTG20, possesses antibacterial and antitumor activities without significant hemolytic activity. In this study, we investigated the antifungal effect of P7 and its anti-Candida acting mode in Candida albicans. P7 displayed antifungal activity against the reference C. albicans (MIC = 4 μM), Aspergilla niger (MIC = 32 μM), Aspergillus flavus (MIC = 8 μM), and Trichopyton rubrum (MIC = 16 μM). The effect of P7 on the C. albicans cell membrane was examined by investigating the calcein leakage from fungal membrane models made of egg yolk l-phosphatidylcholine/ergosterol (10 : 1, w/w) liposomes. P7 showed potent leakage effects against fungal liposomes similar to Melittin-treated cells. C. albicans protoplast regeneration assay demonstrated that P7 interacted with the C. albicans plasma membrane. Flow cytometry of the plasma membrane potential and integrity of C. albicans showed that P7 caused 60.9 ± 1.8% depolarization of the membrane potential of intact C. albicans cells and caused 58.1 ± 3.2% C. albicans cell membrane damage. Confocal laser scanning microscopy demonstrated that part of FITC-P7 accumulated in the cytoplasm. DNA retardation analysis was also performed, which showed that P7 interacted with C. albicans genomic DNA after penetrating the cell membrane, completely inhibiting the migration of genomic DNA above the weight ratio (peptide : DNA) of 6. Our results indicated that the plasma membrane was the primary target, and DNA was the secondary intracellular target of the mode of action of P7 against C. albicans. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

Effect of acyl chain length on therapeutic activity and mode of action of the CX-KYR-NH2 antimicrobial lipopeptide

Peptide lipidation has proven to be an inexpensive and effective strategy for designing next-generation peptide-based drug compounds. In this study, the effect of the acyl chain length of ultrashort LiPs (CX-KYR-NH2; X=10, 12, 14 and 16) on their bacterial killing and membrane disruption kinetics was investigated. The geometric mean of the minimum inhibitory concentration (MIC) values for 4 pathogenic bacterial strains was 25 μM, with a selectivity index of 10.24 for C14-KYR-NH2. LiPs at all concentrations exhibited no cytotoxicity towards human erythrocytes, but towards Vero cells at 80 μM. All the LiPs adopted secondary structure in a membrane mimicking environment. C14-KYR-NH2 aggregated above 256 μM, while C16-KYR-NH2 did above 80 μM. All LiPs showed outer membrane permeabilization within 3 min after treatment, yet the extent and kinetics of inner membrane penetration and depolarization were dependent on the acyl chain length. Cell death subsequently occurred within 10 min, and killing activity appeared to correlate most with depolarization activity but not with outer or inner membrane permeability. AFM imaging of cells treated with C14-KYR-NH2 revealed rupture of the cell surface and cytosolic leakage depending on the length of incubation. This study highlights and follows the progression of events that occur during the membrane disintegration process over time, and determines the optimal amphipathicity of ultrashort LiPs with 12-14 carbon atoms for this membrane disrupting activity. The fast acting bactericidal properties of ultrashort LiPs with optimal chain lengths make them promising candidates for drug lead compounds.

A dual mechanism involved in membrane and nucleic acid disruption of AvBD103b, a new avian defensin from the king penguin, against Salmonella enteritidis CVCC3377

The food-borne bacterial gastrointestinal infection is a serious public health threat. Defensins are evolutionarily conserved innate immune components with broad-spectrum antibacterial activity that do not easily induce resistance. AvBD103b, an avian defensin with potent activity against Salmonella enteritidis, was isolated from the stomach contents of the king penguin (Aptenodytes patagonicus). To elucidate further the antibacterial mechanism of AvBD103b, its effect on the S. enteritidis CVCC3377 cell membrane and intracellular DNA was researched. The cell surface hydrophobicity and a N-phenyl-1-naphthylamine uptake assay demonstrated that AvBD103b treatment increased the cell surface hydrophobicity and outer membrane permeability. Atomic absorption spectrometry, ultraviolet spectrophotometry, flow cytometry, and transmission electron microscopy (TEM) indicated that AvBD103b treatment can lead to the release of the cellular contents and cell death through damage of the membrane. DNA gel retardation and circular dichroism analysis demonstrated that AvBD103b interacted with DNA and intercalated into the DNA base pairs. A cell cycle assay demonstrated that AvBD103b affected cellular functions, such as DNA synthesis. Our results confirmed that AvBD103b exerts its antibacterial activity by damaging the cell membrane and interfering with intracellular DNA, ultimately causing cell death, and suggested that AvBD103b may be a promising candidate as an alternative to antibiotics against S. enteritidis.

PA-1, a novel synthesized pyrrolizidine alkaloid, inhibits the growth of Escherichia coli and Staphylococcus aureus by damaging the cell membrane

In the present study, antimicrobial activity and mode of a novel synthesized pyrrolizidine alkaloid (PA-1) were investigated. PA-1 exhibited predominantly strong antibacterial activity toward six bacteria tested with minimal inhibitory concentration (MIC) values ranging from 0.0039 to 0.025 mg ml(-1). The time-kill assay indicated that PA-1 killed Escherichia coli and Staphylococcus aureus completely at 2MIC (minimum bactericidal concentration) within 8 h. Besides, PA-1-induced death rates of most sensitive strains (E. coli, 97.80% and S. aureus, 96.24%) were analyzed by flow cytometry. A combination of approaches was used to verify the membrane damage of E. coli and S. aureus. Results showed that release of 260 nm absorbing materials quickly increased after PA-1 treatment. PA-1 also rapidly promoted the uptake of crystal violet from 24.52 to 97.12% for E. coli and from 19.68 to 97.63% for S. aureus when the concentrations were changed from MIC to 4MIC. Furthermore, the cellular membrane damages were testified by the significant increase of fluorescence intensity and decrease of membrane potential. Finally, lecithin and phosphate groups were applied to search the possibly targets on the cytoplasmic membrane. Results showed that PA-1 acted on cytoplasmic membrane phospholipids and phosphate groups of S. aureus but not of E. coli. In conclusion, the novel synthesized PA-1 exerted its antibacterial activity by acting on membrane phospholipids and phosphate groups and then damaging the structures of cellular membrane, which finally led to cell death.

Cell penetrating peptides and cationic antibacterial peptides: two sides of the same coin

Cell penetrating peptides (CPP) and cationic antibacterial peptides (CAP) have similar physicochemical properties and yet it is not understood how such similar peptides display different activities. To address this question, we used Iztli peptide 1 (IP-1) because it has both CPP and CAP activities. Combining experimental and computational modeling of the internalization of IP-1, we show it is not internalized by receptor-mediated endocytosis, yet it permeates into many different cell types, including fungi and human cells. We also show that IP-1 makes pores in the presence of high electrical potential at the membrane, such as those found in bacteria and mitochondria. These results provide the basis to understand the functional redundancy of CPPs and CAPs.

Effect of intracellular expression of antimicrobial peptide LL-37 on growth of escherichia coli strain TOP10 under aerobic and anaerobic conditions

Antimicrobial peptides (AMPs) can cause lysis of target bacteria by directly inserting themselves into the lipid bilayer. This killing mechanism confounds the identification of the intracellular targets of AMPs. To circumvent this, we used a shuttle vector containing the inducible expression of a human cathelicidin-related AMP, LL-37, to examine its effect on Escherichia coli TOP10 under aerobic and anaerobic growth conditions. Induction of LL-37 caused growth inhibition and alteration in cell morphology to a filamentous phenotype. Further examination of the E. coli cell division protein FtsZ revealed that LL-37 did not interact with FtsZ. Moreover, intracellular expression of LL-37 results in the enhanced production of reactive oxygen species (ROS), causing lethal membrane depolarization under aerobic conditions. Additionally, the membrane permeability was increased after intracellular expression of LL37 under both aerobic and anaerobic conditions. Transcriptomic analysis revealed that intracellular LL-37 mainly affected the expression of genes related to energy production and carbohydrate metabolism. More specifically, genes related to oxidative phosphorylation under both aerobic and anaerobic growth conditions were affected. Collectively, our current study demonstrates that intracellular expression of LL-37 in E. coli can inhibit growth under aerobic and anaerobic conditions. While we confirmed that the generation of ROS is a bactericidal mechanism for LL-37 under aerobic growth conditions, we also found that the intracellular accumulation of cationic LL-37 influences the redox and ion status of the cells under both growth conditions. These data suggest that there is a new AMP-mediated bacterial killing mechanism that targets energy metabolism.

Therapeutic potential of cell-penetrating peptides

The ability of cell-penetrating peptides to cross plasma membranes has been used for various applications, including the delivery of bioactive molecules to inhibit disease-producing cellular mechanisms. Selective drug delivery into target cells improves drug distribution and decreases dosing and toxicity. In this review, the authors outline the main challenges in the field, namely clarification of mechanisms of entry into cells, as well as current and future perspectives regarding cell-penetrating peptides application for human therapeutics. Here, the authors discuss some of the factors that influence efficacy of delivery and review the current status of preclinical studies and clinical trials involving the use of cell-penetrating peptide-mediated delivery of therapeutics.