Interaction of the cyclic antimicrobial cationic peptide bactenecin with the outer and cytoplasmic membrane

Dodecapeptides derived from human cathelicidin with potent activity against carbapenem-resistant Acinetobacter baumannii

The increasing infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) poses a serious threat to global public health. Antimicrobial peptides (AMPs) are alternatives to conventional antibiotics in combating superbugs. However, discovering AMPs with low synthesis costs and strong antibacterial effects against CRAB is challenging. In this study, we synthesized 28 dodecapeptides for bactericidal assessment by site mutation and all-hydrocarbon stapling on the basis of the antibacterial core of human cathelicidin. The linear derivative d12 (Q5RD9I-KR12) and the i, i + 4 stapled peptide d24, which was generated by substituting Val4 and Lys8 of d12 to staples, stood out among the candidates. These short AMPs efficiently bound to bacterial membrane and penetrated it in a lipid A-dependent manner, resulting in low minimal inhibitory concentrations to inactivate CRAB clinical isolates (2.5-20 μg/mL). The CRAB infection mouse models of irradiation-assisted local pulmonary infection and intra-abdominal sepsis revealed that treatment with d12 and d24 significantly eliminated CRAB in vivo and thereby increased mouse survival. Owing to its improved proteolytic resistance, d24 outperformed d12 in suppressing intra-abdominal CRAB infection. The excellent antibacterial effects, good biocompatibility, and facile synthesis make d12 and d24 promising candidates to curb CRAB infections in different application scenarios.

Exploring the repository of de novo-designed bifunctional antimicrobial peptides through deep learning

Antimicrobial peptides (AMPs) are attractive candidates to combat antibiotic resistance for their capability to target biomembranes and restrict a wide range of pathogens. It is a daunting challenge to discover novel AMPs due to their sparse distributions in a vast peptide universe, especially for peptides that demonstrate potencies for both bacterial membranes and viral envelopes. Here, we establish a de novo AMP design framework by bridging a deep generative module and a graph-encoding activity regressor. The generative module learns hidden 'grammars' of AMP features and produces candidates sequentially pass antimicrobial predictor and antiviral classifiers. We discovered 16 bifunctional AMPs and experimentally validated their abilities to inhibit a spectrum of pathogens in vitro and in animal models. Notably, P076 is a highly potent bactericide with the minimal inhibitory concentration of 0.21 μM against multidrug-resistant Acinetobacter baumannii, while P002 broadly inhibits five enveloped viruses. Our study provides feasible means to uncover the sequences that simultaneously encode antimicrobial and antiviral activities, thus bolstering the function spectra of AMPs to combat a wide range of drug-resistant infections.

Mitigating gut microbial degradation of levodopa and enhancing brain dopamine: Implications in Parkinson's disease

Parkinson's disease is managed using levodopa; however, as Parkinson's disease progresses, patients require increased doses of levodopa, which can cause undesirable side effects. Additionally, the oral bioavailability of levodopa decreases in Parkinson's disease patients due to the increased metabolism of levodopa to dopamine by gut bacteria, Enterococcus faecalis, resulting in decreased neuronal uptake and dopamine formation. Parkinson's disease patients have varying levels of these bacteria. Thus, decreasing bacterial metabolism is a promising therapeutic approach to enhance the bioavailability of levodopa in the brain. In this work, we show that Mito-ortho-HNK, formed by modification of a naturally occurring molecule, honokiol, conjugated to a triphenylphosphonium moiety, mitigates the metabolism of levodopa-alone or combined with carbidopa-to dopamine. Mito-ortho-HNK suppresses the growth of E. faecalis, decreases dopamine levels in the gut, and increases dopamine levels in the brain. Mitigating the gut bacterial metabolism of levodopa as shown here could enhance its efficacy.

Host antimicrobial peptide S100A12 disrupts the fungal membrane by direct binding and inhibits growth and biofilm formation of Fusarium species

Fungal keratitis is the foremost cause of corneal infections worldwide, of which Fusariumspp. is the common etiological agent that causes loss of vision and warrants surgical intervention. An increase in resistance to the available drugs along with severe side effects of the existing antifungals demands for new effective antimycotics. Here, we demonstrate that antimicrobial peptide S100A12 directly binds to the phospholipids of the fungal membrane, disrupts the structural integrity, and induces generation of reactive oxygen species in fungus. In addition, it inhibits biofilm formation by Fusariumspp. and exhibits antifungal property against Fusariumspp. both in vitro and in vivo. Taken together, our results delve into specific effect of S100A12 against Fusariumspp. with an aim to investigate new antifungal compounds to combat fungal keratitis.

SlyB encapsulates outer membrane proteins in stress-induced lipid nanodomains

The outer membrane in Gram-negative bacteria consists of an asymmetric phospholipid-lipopolysaccharide bilayer that is densely packed with outer-membrane β-barrel proteins (OMPs) and lipoproteins1. The architecture and composition of this bilayer is closely monitored and is essential to cell integrity and survival2-4. Here we find that SlyB, a lipoprotein in the PhoPQ stress regulon, forms stable stress-induced complexes with the outer-membrane proteome. SlyB comprises a 10 kDa periplasmic β-sandwich domain and a glycine zipper domain that forms a transmembrane α-helical hairpin with discrete phospholipid- and lipopolysaccharide-binding sites. After loss in lipid asymmetry, SlyB oligomerizes into ring-shaped transmembrane complexes that encapsulate β-barrel proteins into lipid nanodomains of variable size. We find that the formation of SlyB nanodomains is essential during lipopolysaccharide destabilization by antimicrobial peptides or acute cation shortage, conditions that result in a loss of OMPs and compromised outer-membrane barrier function in the absence of a functional SlyB. Our data reveal that SlyB is a compartmentalizing transmembrane guard protein that is involved in cell-envelope proteostasis and integrity, and suggest that SlyB represents a larger family of broadly conserved lipoproteins with 2TM glycine zipper domains with the ability to form lipid nanodomains.

Recent Advances on Antimicrobial Peptides from Milk: Molecular Properties, Mechanisms, and Applications

Traditional antibiotics are facing a tremendous challenge due to increased antimicrobial resistance; hence, there is an urgent need to find novel antibiotic alternatives. Milk protein-derived antimicrobial peptides (AMPs) are currently attracting substantial attention considering that they showcase an extensive spectrum of antimicrobial activities, with slower development of antimicrobial resistance and safety of raw materials. This review summarizes the molecular properties, and activity mechanisms and highlights the applications and limitations of AMPs derived from milk proteins comprehensively. Also the analytical technologies, especially bioinformatics methodologies, applied in the process of screening, identification, and mechanism illustration of AMPs were underlined. This review will give some ideas for further research and broadening of the applications of milk protein-derived AMPs in the food field.

GATR-3, a Peptide That Eradicates Preformed Biofilms of Multidrug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a gram-negative bacterium that causes hospital-acquired and opportunistic infections, resulting in pneumonia, sepsis, and severe wound infections that can be difficult to treat due to antimicrobial resistance and the formation of biofilms. There is an urgent need to develop novel antimicrobials to tackle the rapid increase in antimicrobial resistance, and antimicrobial peptides (AMPs) represent an additional class of potential agents with direct antimicrobial and/or host-defense activating activities. In this study, we present GATR-3, a synthetic, designed AMP that was modified from a cryptic peptide discovered in American alligator, as our lead peptide to target multidrug-resistant (MDR) A. baumannii. Antimicrobial susceptibility testing and antibiofilm assays were performed to assess GATR-3 against a panel of 8 MDR A. baumannii strains, including AB5075 and some clinical strains. The GATR-3 mechanism of action was determined to be via loss of membrane integrity as measured by DiSC3(5) and ethidium bromide assays. GATR-3 exhibited potent antimicrobial activity against all tested multidrug-resistant A. baumannii strains with rapid killing. Biofilms are difficult to treat and eradicate. Excitingly, GATR-3 inhibited biofilm formation and, more importantly, eradicated preformed biofilms of MDR A. baumannii AB5075, as evidenced by MBEC assays and scanning electron micrographs. GATR3 did not induce resistance in MDR A. baumannii, unlike colistin. Additionally, the toxicity of GATR-3 was evaluated using human red blood cells, HepG2 cells, and waxworms using hemolysis and MTT assays. GATR-3 demonstrated little to no cytotoxicity against HepG2 and red blood cells, even at 100 μg/mL. GATR-3 injection showed little toxicity in the waxworm model, resulting in a 90% survival rate. The therapeutic index of GATR-3 was estimated (based on the HC50/MIC against human RBCs) to be 1250. Overall, GATR-3 is a promising candidate to advance to preclinical testing to potentially treat MDR A. baumannii infections.

An Update on the Therapeutic Potential of Antimicrobial Peptides against Acinetobacter baumannii Infections

The rise in antibiotic-resistant strains of clinically important pathogens is a major threat to global health. The World Health Organization (WHO) has recognized the urgent need to develop alternative treatments to address the growing list of priority pathogens. Antimicrobial peptides (AMPs) rank among the suggested options with proven activity and high potential to be developed into effective drugs. Many AMPs are naturally produced by living organisms protecting the host against pathogens as a part of their innate immunity. Mechanisms associated with AMP actions include cell membrane disruption, cell wall weakening, protein synthesis inhibition, and interference in nucleic acid dynamics, inducing apoptosis and necrosis. Acinetobacter baumannii is a critical pathogen, as severe clinical implications have developed from isolates resistant to current antibiotic treatments and conventional control procedures, such as UV light, disinfectants, and drying. Here, we review the natural AMPs representing primary candidates for new anti-A. baumannii drugs in post-antibiotic-era and present computational tools to develop the next generation of AMPs with greater microbicidal activity and reduced toxicity.

Antimicrobial Peptide Synergies for Fighting Infectious Diseases

Antimicrobial peptides (AMPs) are essential elements of thehost defense system. Characterized by heterogenous structures and broad-spectrumaction, they are promising candidates for combating multidrug resistance. Thecombined use of AMPs with other antimicrobial agents provides a new arsenal ofdrugs with synergistic action, thereby overcoming the drawback of monotherapiesduring infections. AMPs kill microbes via pore formation, thus inhibitingintracellular functions. This mechanism of action by AMPs is an advantage overantibiotics as it hinders the development of drug resistance. The synergisticeffect of AMPs will allow the repurposing of conventional antimicrobials andenhance their clinical outcomes, reduce toxicity, and, most significantly,prevent the development of resistance. In this review, various synergies ofAMPs with antimicrobials and miscellaneous agents are discussed. The effect ofstructural diversity and chemical modification on AMP properties is firstaddressed and then different combinations that can lead to synergistic action,whether this combination is between AMPs and antimicrobials, or AMPs andmiscellaneous compounds, are attended. This review can serve as guidance whenredesigning and repurposing the use of AMPs in combination with other antimicrobialagents for enhanced clinical outcomes.

Antibiofilm assay for antimicrobial peptides combating the sulfate-reducing bacteria Desulfovibrio vulgaris

In medical, environmental, and industrial processes, the accumulation of bacteria in biofilms can disrupt many processes. Antimicrobial peptides (AMPs) are receiving increasing attention in the development of new substances to avoid or reduce biofilm formation. There is a lack of parallel testing of the effect against biofilms in this area, as well as in the testing of other antibiofilm agents. In this paper, a high-throughput screening was developed for the analysis of the antibiofilm activity of AMPs, differentiated into inhibition and removal of a biofilm. The sulfate-reducing bacterium Desulfovibrio vulgaris was used as a model organism. D. vulgaris represents an undesirable bacterium, which is considered one of the major triggers of microbiologically influenced corrosion. The application of a 96-well plate and steel rivets as a growth surface realizes real-life conditions and at the same time establishes a flexible, simple, fast, and cost-effective assay. All peptides tested in this study demonstrated antibiofilm activity, although these peptides should be individually selected depending on the addressed aim. For biofilm inhibition, the peptide DASamP1 is the most suitable, with a sustained effect for up to 21 days. The preferred peptides for biofilm removal are S6L3-33, in regard to bacteria reduction, and Bactenecin, regarding total biomass reduction.

The diversification of the antimicrobial peptides from marine worms is driven by environmental conditions

Antimicrobial peptides (AMPs) play a key role in the external immunity of animals, offering an interesting model for studying the influence of the environment on the diversification and evolution of immune effectors. Alvinellacin (ALV), arenicin (ARE) and polaricin (POL, a novel AMP identified here), characterized from three marine worms inhabiting contrasted habitats ('hot' vents, temperate and polar respectively), possess a well conserved BRICHOS domain in their precursor molecule despite a profound amino acid and structural diversification of the C-terminal part containing the core peptide. Data not only showed that ARE, ALV and POL display an optimal bactericidal activity against the bacteria typical of the habitat where each worm species lives but also that this killing efficacy is optimal under the thermochemical conditions encountered by their producers in their environment. Moreover, the correlation between species habitat and the cysteine contents of POL, ARE and ALV led us to investigate the importance of disulfide bridges in their biological efficacy as a function of abiotic pressures (pH and temperature). The construction of variants using non-proteinogenic residues instead of cysteines (α-aminobutyric acid variants) leading to AMPs devoid of disulfide bridges, provided evidence that the disulfide pattern of the three AMPs allows for a better bactericidal activity and suggests an adaptive way to sustain the fluctuations of the worm's environment. This work shows that the external immune effectors exemplified here by BRICHOS AMPs are evolving under strong diversifying environmental pressures to be structurally shaped and more efficient/specific under the ecological niche of their producer.

Antimicrobial activity of α-mangostin against Staphylococcus species from companion animals in vitro and therapeutic potential of α-mangostin in skin diseases caused by S. pseudintermedius

Antimicrobial resistance in Staphylococcus species from companion animals is becoming increasingly prevalent worldwide. S. pseudintermedius is a leading cause of skin infections in companion animals. α-mangostin (α-MG) exhibits various pharmacological activities, including antimicrobial activity against G (+) bacteria. This study investigated the antimicrobial activity of α-MG against clinical isolates of Staphylococcus species from companion animals and assessed the therapeutic potential of α-MG in skin diseases induced by S. pseudintermedius in a murine model. Furthermore, the action mechanisms of α-MG against S. pseudintermedius were investigated. α-MG exhibited antimicrobial activity against clinical isolates of five different Staphylococcus species from skin diseases of companion animals in vitro, but not G (-) bacteria. α-MG specifically interacted with the major histocompatibility complex II analogous protein (MAP) domain-containing protein located in the cytoplasmic membrane of S. pseudintermedius via hydroxyl groups at C-3 and C-6. Pretreatment of S. pseudintermedius with anti-MAP domain-containing protein polyclonal serum significantly reduced the antimicrobial activity of α-MG. The sub-minimum inhibitory concentration of α-MG differentially regulated 194 genes, especially metabolic pathway and virulence determinants, in S. pseudintermedius. α-MG in pluronic lecithin organogel significantly reduced the bacterial number, partially restored the epidermal barrier, and suppressed the expression of cytokine genes associated with pro-inflammatory, Th1, Th2, and Th17 in skin lesions induced by S. pseudintermedius in a murine model. Thus, α-MG is a potential therapeutic candidate for treating skin diseases caused by Staphylococcus species in companion animals.

Diastereoselective synthesis of cyclic tetrapeptide pseudoxylallemycin A illuminates the impact of base during macrolactamization

Therapeutic agents with unique molecular structures and new mechanisms of action are needed to confront the phenomenon of multidrug resistance among bacteria. Pseudoxylallemycins, cyclic tetrapeptide (CTP) natural products, have exhibited modest antibiotic activity, but their synthesis has proven challenging. Inherent ring strain in CTPs decreases the rate of cyclization in lieu of polymerization and racemization pathways, which has resulted in previous syntheses describing mixtures of diastereomers containing predominantly an undesired epimer. We have optimized the cyclization step of pseudoxylallemycin A to favor production of the natural diastereomer; notably, variation of the base, temperature, and solvent with peptide coupling reagent propylphosphonic anhydride (T3P) afforded exquisite selectivity for the natural product in as high as 97 : 3 DR, and our conditions can provide the natural product in up to 32% overall yield through 8 steps. Employing weaker bases than those typically used in peptide coupling reactions led to the greatest improvement in diastereoselectivity, and these studies demonstrated that the identity of the amine base has enormous impact on the rate of C-terminal epimerization when T3P is used, a variable usually considered of lesser consequence when combined with typical amide coupling reagents. Toward fully characterizing pseudoxylallemycin stereoisomers, variable temperature NMR was described as a tool to more clearly analyze CTPs that exhibit multiple conformational states. These synthetic and spectroscopic insights were applied toward synthesizing several natural product analogues, and their antibacterial activity was examined using microdilution assays.

Characteristics of antimicrobial peptide OaBac5mini and its bactericidal mechanism against Escherichia coli

Introduction

Antimicrobial peptides (AMPs) play an important role in defending against the attack of pathogenic microorganisms. Among them, the proline-rich antibacterial peptides (PrAMPs) have been attracting close attention due to their simple structure, strong antibacterial activity, and low cell toxicity. OaBac5mini is an active fragment of the sheep-derived OaBac5 belonging to the PrAMPs family.

Methods

In this study, the antibacterial activity of OaBac5mini was investigated by testing the MICs against different stains of E. coli and S. aureus as well as the time-kill curve. The bactericidal mechanism was explored by determining the effect of OaBac5mini on the cell membrane. The stability and biosafety were also evaluated.

Results

The susceptibility test demonstrated that OaBac5mini showed potent antibacterial activity against the multidrug-resistant (MDR) E. coli isolates. It is noticeable that the absence of inner membrane protein SbmA in E. coli ATCC 25922 caused the MIC of OaBac5mini to increase 4-fold, implying OaBac5mini can enter into the cytoplasm via SbmA and plays its antibacterial activity. Moreover, the antibacterial activity of OaBac5mini against E. coli ATCC 25922 was not remarkably affected by the serum salts except for CaCl₂ at a physiological concentration, pH, temperature, repeated freeze-thawing and proteases (trypsin < 20 μg/mL, pepsin or proteinase K). Time-kill curve analysis showed OaBac5mini at the concentration of 200 μg/mL (8 × MICs) could effectively kill E. coli ATCC 25922 after co-incubation for 12 h. In addition, OaBac5mini was not hemolytic against rabbit red blood cells and also was not cytotoxic to porcine small intestinal epithelial cells (IPEC-J2). Bioinformatic analysis indicated that OaBac5mini is a linear peptide with 8 net positive charges. Furthermore, OaBac5mini significantly increased the outer membrane permeability and impaired the inner membrane integrity and ultrastructure of E. coli ATCC25922.

Conclusion

OaBac5mini is a stable and potent PrAMP that kills E. coli by two different modes of action - inhibiting intracellular target(s) and damaging cell membrane.

γ-Core Guided Antibiotic Design Based on Human Enteric Defensin 5

An increase in the number of infections caused by resistant bacteria worldwide necessitates the development of alternatives to antibiotics. Human defensin (HD) 5 is an innate immune peptide with broad-spectrum antibacterial activity, but its complicated structure makes its preparation difficult. Herein, we truncated the HD5 structure by extracting the highly conserved γ-core motif. A structure-activity study showed that this motif was ineffective in killing bacteria in the absence of specific spatial conformation. Notably, after the introduction of two intramolecular disulfide bonds, its antibacterial activity was markedly improved. Glu and Ser residues were then replaced with Arg to create the derivative RC18, which exhibited stronger potency than HD5, particularly against methicillin-resistant S. aureus (MRSA). Mechanistically, RC18 bound to lipid A and lipoteichoic acid at higher affinities than HD5. Furthermore, RC18 was more efficient than HD5 in penetrating the bacterial membranes. Molecular dynamics simulation revealed that five Arg residues, Arg1, Arg7, Arg9, Arg15, and Arg18, mediated most of the polar interactions of RC18 with the phospholipid head groups during membrane penetration. In vivo experiments indicated that RC18 decreased MRSA colonization and dramatically improved the survival of infected mice, thus demonstrating that RC18 is a promising drug candidate to treat MRSA infections.

Antimicrobial activity of the recombinant peptide Melittin-Thanatin with three glycine to tryptophan mutations

The antimicrobial peptide was considered an important target for developing novel antibacterial drugs. However, the unstable biological activity and the low antibacterial activity are challenges for the application of recombinant proteins. In this study, the fusion peptide of Melittin-Thanatin (MT) was designed and produced, and its derivative sequence (MT-W) was obtained by replacing three glycines (Gly, G) with tryptophan (Trp, W). The MT-W peptide were synthesized in Bacillus subtilis WB700 by EDDIE self-cleavage protein fusion. Compared with MT, MT-W exhibited 2-4 times higher antibacterial rate against Escherichia coli K88. In addition, MT-W showed lower cytotoxicity (IC50 > 300 mg·L^-1) to the red blood cell, and more stable biological activities under the conditions of different temperatures (20, 30, 40, 50, 60, 70, 80, and 90 °C), pH values (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0) and different proteases. Especially, MT-W showed a broader antibacterial effect on three drug-resistant strains than florfenicol and oxytetracycline calcium. In conclusion, compared with MT, the MT-W showed increased antibacterial activity, stability, lower cytotoxicity, and broader antimicrobial effect. Therefore, it would become a promising alternative to conventional antibiotics.

Selected Antimicrobial Peptides Inhibit In Vitro Growth of Campylobacter spp

Campylobacter is a major cause of acute human diarrheal illness. Broiler chickens constitute a primary reservoir for C. jejuni leading to human infection. Consequently, there is a need for developing novel intervention methods. Antimicrobial peptides (AMP) are small proteins which have evolved in most lifeforms to provide defense against microbial infections. To date, over 3000 AMP have been discovered; however, few of them have been analyzed specifically for ability to kill campylobacters. We selected and evaluated a set of 11 unique chemically synthesized AMP for ability to inhibit growth of C. jejuni. Six of the AMP we tested produced zones of inhibition on lawns of C. jejuni. These AMP included: NRC-13, RL-37, Temporin L, Cecropin–Magainin, Dermaseptin, and C12K-2β12. In addition, MIC were determined for Cecropin–Magainin, RL-37 and C12K-2β12 against 15 isolates of Campylobacter representing the three most common pathogenic strains. MIC for campylobacters were approximately 3.1 µg/mL for AMP RL-37 and C12K-2β12. MIC were slightly higher for the Cecropin–Magainin AMP in the range of 12.5 to 100 µg/mL. These AMP are attractive subjects for future study and potential in vivo delivery to poultry to reduce Campylobacter spp. populations.

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.

Molecular Aspects of the Interaction with Gram-Negative and Gram-Positive Bacteria of Hydrothermal Carbon Nanoparticles Associated with Bac8c2,5Leu Antimicrobial Peptide

Antimicrobial peptides (AMPs) are widely studied as therapeutic agents due to their broad-spectrum efficacy against infections. However, their clinical use is hampered by the low in vivo bioavailability and systemic toxicity. Such limitations might be overcome by using appropriate drug delivery systems. Here, the preparation of a drug delivery system (DDS) by physical conjugation of an arginine-rich peptide and hydrothermal carbon nanoparticles (CNPs) has been explored, and its antimicrobial efficacy against Eschericia coli (E. coli) and Staphylococcus aureus investigated in comparison with the unloaded carrier and the free peptide. The mechanism of interaction between CNPs and the bacteria was investigated by scanning electron microscopy and a combined dielectrophoresis-Raman spectroscopy method for real-time analysis. In view of a possible systemic administration, the effect of proteins on the stability of the DDS was investigated by using albumin as a model protein. The peptide was bounded electrostatically to the CNPs surface, establishing an equilibrium modulated by pH and albumin. The DDS exhibited antimicrobial activity toward the two bacterial strains, albeit lower as compared to the free peptide. The decrease in effectiveness toward E. coli was likely due to the rapid formation of a particle-induced extracellular matrix. The present results are relevant for the future development of hydrothermal CNPs as drug delivery agents of AMPs.

The Role of Antimicrobial Peptides as Antimicrobial and Antibiofilm Agents in Tackling the Silent Pandemic of Antimicrobial Resistance

Just over a million people died globally in 2019 due to antibiotic resistance caused by ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species). The World Health Organization (WHO) also lists antibiotic-resistant Campylobacter and Helicobacter as bacteria that pose the greatest threat to human health. As it is becoming increasingly difficult to discover new antibiotics, new alternatives are needed to solve the crisis of antimicrobial resistance (AMR). Bacteria commonly found in complex communities enclosed within self-produced matrices called biofilms are difficult to eradicate and develop increased stress and antimicrobial tolerance. This review summarises the role of antimicrobial peptides (AMPs) in combating the silent pandemic of AMR and their application in clinical medicine, focusing on both the advantages and disadvantages of AMPs as antibiofilm agents. It is known that many AMPs display broad-spectrum antimicrobial activities, but in a variety of organisms AMPs are not stable (short half-life) or have some toxic side effects. Hence, it is also important to develop new AMP analogues for their potential use as drug candidates. The use of one health approach along with developing novel therapies using phages and breakthroughs in novel antimicrobial peptide synthesis can help us in tackling the problem of AMR.

Two new 7,20-epoxy-ent-kaurane diterpenoids from the aerial parts of Isodon serra

Two new compounds (1 and 2), belonging to C-20 oxygenated ent-kauranes-type diterpenoids, were identified from the aerial parts of Isodon serra. Their structures were elucidated by extensive analysis of HRESI-MS and NMR spectroscopic data. Both these two compounds possess a common 7,20-epoxy-ent-kauranes skeleton with a hydroxyl group rarely occurring at C-13. Compounds 1 and 2 were evaluated for their cytotoxic activity against Hela-60 and HepG2 as well as the antibacterial activity against Staphylococcus aureus, Bacillus cereus and Escherichia coli.

Targeting Multidrug Resistance With Antimicrobial Peptide-Decorated Nanoparticles and Polymers

As a category of small peptides frequently found in nature, antimicrobial peptides (AMPs) constitute a major part of the innate immune system of various organisms. Antimicrobial peptides feature various inhibitory effects against fungi, bacteria, viruses, and parasites. Due to the increasing concerns of antibiotic resistance among microorganisms, development of antimicrobial peptides is an emerging tool as a favorable applicability prospect in food, medicine, aquaculture, animal husbandry, and agriculture. This review presents the latest research progress made in the field of antimicrobial peptides, such as their mechanism of action, classification, application status, design techniques, and a review on decoration of nanoparticles and polymers with AMPs that are used in treating multidrug resistance. Lastly, we will highlight recent progress in antiviral peptides to treat emerging viral diseases (e.g., anti-coronavirus peptides) and discuss the outlook of AMP applications.

Design of Membrane Active Peptides Considering Multi-Objective Optimization for Biomedical Application

A multitude of membrane active peptides exists that divides into subclasses, such as cell penetrating peptides (CPPs) capable to enter eukaryotic cells or antimicrobial peptides (AMPs) able to interact with prokaryotic cell envelops. Peptide membrane interactions arise from unique sequence motifs of the peptides that account for particular physicochemical properties. Membrane active peptides are mainly cationic, often primary or secondary amphipathic, and they interact with membranes depending on the composition of the bilayer lipids. Sequences of these peptides consist of short 5–30 amino acid sections derived from natural proteins or synthetic sources. Membrane active peptides can be designed using computational methods or can be identified in screenings of combinatorial libraries. This review focuses on strategies that were successfully applied to the design and optimization of membrane active peptides with respect to the fact that diverse features of successful peptide candidates are prerequisites for biomedical application. Not only membrane activity but also degradation stability in biological environments, propensity to induce resistances, and advantageous toxicological properties are crucial parameters that have to be considered in attempts to design useful membrane active peptides. Reliable assay systems to access the different biological characteristics of numerous membrane active peptides are essential tools for multi-objective peptide optimization.

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

Repurposing Approved Drugs as Fluoroquinolone Potentiators to Overcome Efflux Pump Resistance in Staphylococcus aureus

Staphylococcus aureus is a versatile human commensal bacteria and pathogen that causes various community and hospital-acquired infections. The S. aureus efflux pump NorA which belongs to the major facilitator superfamily, confers resistance to a range of substrates. Many efflux pump inhibitors (EPIs) have been discovered, but none is clinically approved due to their undesirable toxicities. In this study, we have screened clinically approved drugs for possible NorA EPI-like activity. We identified six drugs that showed the best efflux pump inhibition in vitro, with a fractional inhibitory concentration index of ≤0.5, indicating synergism with hydrophilic fluoroquinolones. The mechanistic validation of efflux inhibitory potential was demonstrated in ethidium bromide-based accumulation and efflux inhibition assays. We further confirmed the functionality of EPIs by norfloxacin accumulation assay depicting more realistic proof of the conjecture. None of the EPIs disturbed membrane function or depleted the ATP synthesis levels in bacteria. Both raloxifene and pyrvinium displayed an increase in bactericidal activity of ciprofloxacin in time-kill kinetics, prolonged its post-antibiotic effect, and reduced the frequency of spontaneous resistant mutant development. The combination of EPIs with ciprofloxacin caused significant eradication of preformed biofilms. Moreover, in the murine thigh infection model, a single dose of pyrvinium combined with ciprofloxacin reduced the bacterial burden significantly compared to untreated control and ciprofloxacin alone, indicating the efficacy of the combination. Conclusively, this study represents approved drugs that can be repurposed and combined with antibiotics as NorA EPIs, having anti-biofilm properties to treat severe S. aureus infections at clinically relevant concentrations.

IMPORTANCEStaphylococcus aureus is a frequent pathogen bacterium and the predominant cause of worsened nosocomial infections. Efflux pumps contribute to drug efflux and are reportedly associated with biofilm formation, thereby promoting difficult-to-treat biofilm-associated S. aureus infections. One strategy to combat these bacteria is to reduce active efflux and increase pathogen sensitivity to existing antibiotics. Repurposing approved drugs may solve the classical toxicity issues with previous efflux pump inhibitors and help reach sufficient plasma concentrations. We describe the in silico-based screening of FDA-approved drugs that identified six different molecules able to inhibit NorA pump (Major Facilitator Superfamily). Our study highlights that these compounds bind to and block the activity of the NorA pump and increase the sensitivity of S. aureus and methicillin-resistant S. aureus to fluoroquinolones. These drugs combined with fluoroquinolones significantly reduced the preformed biofilms and displayed significant efficacy in the murine thigh infection model when compared to untreated control and ciprofloxacin alone.

Expression, Purification and Characterization of a Novel Hybrid Peptide CLP with Excellent Antibacterial Activity

CLP is a novel hybrid peptide derived from CM4, LL37 and TP5, with significantly reduced hemolytic activity and increased antibacterial activity than parental antimicrobial peptides. To avoid host toxicity and obtain high-level bio-production of CLP, we established a His-tagged SUMO fusion expression system in Escherichia coli. The fusion protein can be purified using a Nickel column, cleaved by TEV protease, and further purified in flow-through of the Nickel column. As a result, the recombinant CLP with a yield of 27.56 mg/L and a purity of 93.6% was obtained. The purified CLP exhibits potent antimicrobial activity against gram+ and gram- bacteria. Furthermore, the result of propidium iodide staining and scanning electron microscopy (SEM) showed that CLP can induce the membrane permeabilization and cell death of Enterotoxigenic Escherichia coli (ETEC) K88. The analysis of thermal stability results showed that the antibacterial activity of CLP decreases slightly below 70 °C for 30 min. However, when the temperature was above 70 °C, the antibacterial activity was significantly decreased. In addition, the antibacterial activity of CLP was stable in the pH range from 4.0 to 9.0; however, when pH was below 4.0 and over 9.0, the activity of CLP decreased significantly. In the presence of various proteases, such as pepsin, papain, trypsin and proteinase K, the antibacterial activity of CLP remained above 46.2%. In summary, this study not only provides an effective strategy for high-level production of antimicrobial peptides and evaluates the interference factors that affect the biological activity of hybrid peptide CLP, but also paves the way for further exploration of the treatment of multidrug-resistant bacterial infections.

Antimicrobial Peptides: A Potent Alternative to Antibiotics

Antimicrobial peptides constitute one of the most promising alternatives to antibiotics since they could be used to treat bacterial infections, especially those caused by multidrug-resistant pathogens. Many antimicrobial peptides, with various activity spectra and mechanisms of actions, have been described. This review focuses on their use against ESKAPE bacteria, especially in biofilm treatments, their synergistic activity, and their application as prophylactic agents. Limitations and challenges restricting therapeutic applications are highlighted, and solutions for each challenge are evaluated to analyze whether antimicrobial peptides could replace antibiotics in the near future.

Antifungal Azoles as Tetracycline Resistance Modifiers in Staphylococcus aureus

Staphylococcus aureus has developed resistance to antimicrobials since their first use. The S. aureus major facilitator superfamily (MFS) efflux pump Tet(K) contributes to resistance to tetracyclines. The efflux pump diminishes antibiotic accumulation, and biofilm hampers the diffusion of antibiotics. None of the currently known compounds have been approved as efflux pump inhibitors (EPIs) for clinical use. In the current study, we screened clinically approved drugs for possible Tet(K) efflux pump inhibition. By performing in silico docking followed by in vitro checkerboard assays, we identified five azoles (the fungal ergosterol synthesis inhibitors) showing putative EPI-like potential with a fractional inhibitory concentration index of ≤0.5, indicating synergism. The functionality of the azoles was confirmed using ethidium bromide (EtBr) accumulation and efflux inhibition assays. In time-kill kinetics, the combination treatment with butoconazole engendered a marked increase in the bactericidal capacity of tetracycline. When assessing the off-target effects of the azoles, we observed no disruption of bacterial membrane permeability and polarization. Finally, the combination of azoles with tetracycline led to a significant eradication of preformed mature biofilms. This study demonstrates that azoles can be repurposed as putative Tet(K) EPIs and to reduce biofilm formation at clinically relevant concentrations. IMPORTANCE Staphylococcus aureus uses efflux pumps to transport antibiotics out of the cell and thus increases the dosage at which it endures antibiotics. Also, efflux pumps play a role in biofilm formation by the excretion of extracellular matrix molecules. One way to combat these pathogens may be to reduce the activity of efflux pumps and thereby increase pathogen sensitivity to existing antibiotics. We describe the in silico-based screen of clinically approved drugs that identified antifungal azoles inhibiting Tet(K), a pump that belongs to the major facilitator superfamily, and showed that these compounds bind to and block the activity of the Tet(K) pump. Azoles enhanced the susceptibility of tetracycline against S. aureus and its methicillin-resistant strains. The combination of azoles with tetracycline led to a significant reduction in preformed biofilms. Repurposing approved drugs may help solve the classical toxicity issues related to efflux pump inhibitors.

Enhanced Synergism and Mechanism of Action Studies of Synthetic Antimicrobial Metallopeptides

Antimicrobial peptides (AMPs) are found throughout most kingdoms of life, are an important part of host immunity, and have been shown to act synergistically in various organisms to ameliorate bacterial infections. Herein, we report the synergistic behavior observed between two AMPs, Sub5 and CP10A, against E. coli. In addition, enhanced synergistic activity against E. coli and MRSA 43300 for two derivatives of Sub5, extended with the amino-terminal copper and nickel (ATCUN) binding motif, is observed when dosed together with CP10A, while displaying little cytotoxicity towards human dermal fibroblasts. All three combinations of peptides co-localized within bacterial cells as evidenced by fluorescence confocal microscopy. Investigations into the mechanism of synergy shows that all peptides indirectly damage DNA within cells, while only the ATCUN derivatives can oxidize phospholipids. Combinations of peptides were also shown to upregulate the concentration of reactive oxygen species within both E. coli and MRSA 43300. These results suggest that the production of reactive oxygen species is an important aspect mechanistically and further highlights the potential of these metallopeptides to aid in the treatment of antibiotic-resistant infections.

Antimicrobial Peptides and Copper(II) Ions: Novel Therapeutic Opportunities

The emergence of new pathogens and multidrug resistant bacteria is an important public health issue that requires the development of novel classes of antibiotics. Antimicrobial peptides (AMPs) are a promising platform with great potential for the identification of new lead compounds that can combat the aforementioned pathogens due to their broad-spectrum antimicrobial activity and relatively low rate of resistance emergence. AMPs of multicellular organisms made their debut four decades ago thanks to ingenious researchers who asked simple questions about the resistance to bacterial infections of insects. Questions such as "Do fruit flies ever get sick?", combined with pioneering studies, have led to an understanding of AMPs as universal weapons of the immune system. This review focuses on a subclass of AMPs that feature a metal binding motif known as the amino terminal copper and nickel (ATCUN) motif. One of the metal-based strategies of hosts facing a pathogen, it includes wielding the inherent toxicity of copper and deliberately trafficking this metal ion into sites of infection. The sudden increase in the concentration of copper ions in the presence of ATCUN-containing AMPs (ATCUN-AMPs) likely results in a synergistic interaction. Herein, we examine common structural features in ATCUN-AMPs that exist across species, and we highlight unique features that deserve additional attention. We also present the current state of knowledge about the molecular mechanisms behind their antimicrobial activity and the methods available to study this promising class of AMPs.

Antibacterial Activity and Mode of Action of Lactoquinomycin A from Streptomyces bacillaris

This study aims to isolate and identify the structure of antibacterial compounds having potent activity on methicillin-resistant Staphylococcus aureus (MRSA) from marine actinomycetes, and also to identify their mode of action. Lactoquinomycin A (LQM-A) (compound 1) and its derivatives (2–4) were isolated from marine-derived Streptomyces bacillaris strain MBTC38, and their structures were determined using extensive spectroscopic methods. These compounds showed potent antibacterial activities against Gram-positive bacteria, with MIC values of 0.06–4 μg/mL. However, the tested compounds exhibited weak inhibitory activity against Gram-negative bacteria, although they were effective against Salmonella enterica (MIC = 0.03–1 μg/mL). LQM-A exhibited the most significant inhibitory activity against methicillin-resistant Staphylococcus aureus (MRSA) (MIC = 0.25–0.5 μg/mL), with a low incidence of resistance. An in vivo dual-reporter assay designed to distinguish between compounds that inhibit translation and those that induce DNA damage was employed to assess the mode of action of LQM-A. LQM-A-induced DNA damage and did not inhibit protein synthesis. The gel mobility shift assay showed that LQM-A switched plasmid DNA from the supercoiled to relaxed form in a time- and concentration-dependent manner. These data suggest that LQM-A intercalated into double-stranded DNA and damaged DNA repair.

The Strategies of Pathogen-Oriented Therapy on Circumventing Antimicrobial Resistance

The emerging antimicrobial resistance (AMR) poses serious threats to the global public health. Conventional antibiotics have been eclipsed in combating with drug-resistant bacteria. Moreover, the developing and deploying of novel antimicrobial drugs have trudged, as few new antibiotics are being developed over time and even fewer of them can hit the market. Alternative therapeutic strategies to resolve the AMR crisis are urgently required. Pathogen-oriented therapy (POT) springs up as a promising approach in circumventing antibiotic resistance. The tactic underling POT is applying antibacterial compounds or materials directly to infected regions to treat specific bacteria species or strains with goals of improving the drug efficacy and reducing nontargeting and the development of drug resistance. This review exemplifies recent trends in the development of POTs for circumventing AMR, including the adoption of antibiotic-antibiotic conjugates, antimicrobial peptides, therapeutic monoclonal antibodies, nanotechnologies, CRISPR-Cas systems, and microbiota modulations. Employing these alternative approaches alone or in combination shows promising advantages for addressing the growing clinical embarrassment of antibiotics in fighting drug-resistant bacteria.

Cyclic Derivative of Host-Defense Peptide IDR-1018 Improves Proteolytic Stability, Suppresses Inflammation, and Enhances In Vivo Activity

Host-defense peptides have drawn significant attention as new drugs or drug adjuvants to combat multidrug-resistant bacteria. In this study, we report the development of cyclic derivatives of the immunomodulatory and antibiofilm innate defense regulator peptide (IDR)-1018 based on three different synthetic strategies including head-to-tail cyclization (C1), side-chain-to-tail cyclization (C2), and a disulfide bond cross-linkage (C3). The generated mimetics showed enhanced proteolytic stability and reduced aggregation in vitro and in vivo. The C2 derivative exhibited exceptional ability to suppress inflammation and significantly reduce bacterial loads in a high-density Staphylococcus aureus murine skin infection model. The findings describe different routes to the creation of enzymatically stable mimetics of IDR-1018 and identify a promising new cyclic analogue against bacterial infections.

Revealing the Specificity of a Range of Antimicrobial Peptides in Lipid Nanodiscs by Native Mass Spectrometry

Antimicrobial peptides (AMPs) interact directly with lipid membranes of pathogens and may have the potential to combat antibiotic resistance. Although many AMPs are thought to form toxic oligomeric pores, their interactions within lipid membranes are not well understood. Here, we used native mass spectrometry to measure the incorporation of a range of different AMPs in lipoprotein nanodiscs. We found that the truncation of human LL37 increases the lipid specificity but decreases the specificity of complex formation. We also saw that the reduction of disulfide bonds can have a dramatic effect on the ability of AMPs to interact with lipid bilayers. Finally, by examining a wider range of peptides we discovered that AMPs tend to interact specifically with anionic lipids but form nonspecific complexes with wide oligomeric state distributions. Overall, these data reveal that each AMP has unique behaviors but some common trends apply to many AMPs.

Antimicrobial peptides as a promising treatment option against Acinetobacter baumannii infections

BACKGROUND

With the increasing rate of antibiotic resistance in Acinetobacter, the World Health Organization introduced the carbapenem-resistant isolates in the priority pathogens list for which innovative new treatments are urgently needed. Antimicrobial peptides (AMPs) are one of the antimicrobial agents with high potential to produce new anti-Acinetobacter drugs. This review aims to summarize recent advances and compare AMPs with anti-Acinetobacter baumannii activity.

METHODS

Active AMPs against Acinetobacter were considered, and essential features, including structure, mechanism of action, anti-A. baumannii potent, and other prominent characteristics, were investigated and compared to each other. In this regard, the Google Scholar search engine and databases of PubMed, Scopus, and Web of Science were used.

RESULTS

Forty-six anti-Acinetobacter peptides were identified and classified into ten groups: Cathelicidins, Defensins, Frog AMPs, Melittin, Cecropins, Mastoparan, Histatins, Dermcidins, Tachyplesins, and computationally designed AMPs. According to the Minimum Inhibitory Concentration (MIC) reports, six peptides of Melittin, Histatin-8, Omega76, AM-CATH36, Hymenochirin, and Mastoparan have the highest anti-A. baumannii power against sensitive and antibiotic-resistant isolates. All anti-Acinetobacter peptides except Dermcidin have a net positive charge. Most of these peptides have alpha-helical structure; however, β-sheet and other structures have been observed among them. The mechanism of action of these antimicrobial agents is divided into two categories of membrane-based and intracellular target-based attack.

CONCLUSION

Evidence from this review indicates that AMPs would be likely among the main anti-A. baumannii drugs in the post-antibiotic era. Also, the application of computer science to increase anti-A. baumannii activity and reduce toxicity could be helpful.

Expression of Melittin in Fusion with GST in Escherichia coli and Its Purification as a Pure Peptide with Good Bacteriostatic Efficacy

The expression and purification of melittin (MET) in microbials are difficult because of its antibacterial activities. In this work, MET was fused with a glutathione-S-transferase (GST) tag and expressed in Escherichia coli to overcome its lethality to host cells. The fusion protein GST-MET was highly expressed and then purified by glutathione sepharose high-performance affinity chromatography, digested with prescission protease, and further purified by Superdex Peptide 10/300 GL chromatography. Finally, 3.5 mg/L recombinant melittin (rMET) with a purity of >90% was obtained; its antibacterial activities against Gram-positive Bacillus pumilus and Staphylococcus pasteuri were similar to those of commercial MET. A circular dichroism spectroscopic assay showed that the rMET peptide secondary structure was similar to those of the commercial form. To our knowledge, this is the report of the preparation of active pure rMET with no tags. The successful expression and purification of rMET will enable large-scale, industrial biosynthesis of MET.

How do cyclic antibiotics with activity against Gram-negative bacteria permeate membranes? A machine learning informed experimental study

All antibiotics have to engage bacterial amphiphilic barriers such as the lipopolysaccharide-rich outer membrane or the phospholipid-based inner membrane in some manner, either by disrupting them outright and/or permeating them and thereby allow the antibiotic to get into bacteria. There is a growing class of cyclic antibiotics, many of which are of bacterial origin, that exhibit activity against Gram-negative bacteria, which constitute an urgent problem in human health. We examine a diverse collection of these cyclic antibiotics, both natural and synthetic, which include bactenecin, polymyxin B, octapeptin, capreomycin, and Kirshenbaum peptoids, in order to identify what they have in common when they interact with bacterial lipid membranes. We find that they virtually all have the ability to induce negative Gaussian curvature (NGC) in bacterial membranes, the type of curvature geometrically required for permeation mechanisms such as pore formation, blebbing, and budding. This is interesting since permeation of membranes is a function usually ascribed to antimicrobial peptides (AMPs) from innate immunity. As prototypical test cases of cyclic antibiotics, we analyzed amino acid sequences of bactenecin, polymyxin B, and capreomycin using our recently developed machine-learning classifier trained on α-helical AMP sequences. Although the original classifier was not trained on cyclic antibiotics, a modified classifier approach correctly predicted that bactenecin and polymyxin B have the ability to induce NGC in membranes, while capreomycin does not. Moreover, the classifier was able to recapitulate empirical structure-activity relationships from alanine scans in polymyxin B surprisingly well. These results suggest that there exists some common ground in the sequence design of hybrid cyclic antibiotics and linear AMPs.

Biological Control of Tomato Bacterial Wilt, Kimchi Cabbage Soft Rot, and Red Pepper Bacterial Leaf Spot Using Paenibacillus elgii JCK-5075

The over and repeated use of chemical bactericides to control plant bacterial diseases has resulted in unwanted effects, such as environmental pollution, residual toxicity, and resistance buildup in bacterial pathogens. Many previous studies have aimed to develop biological control agents to replace chemical bactericides. In this study, the antibacterial efficacy of the fermentation broth of Paenibacillus elgii JCK-5075 and its antibacterial compounds were evaluated against plant pathogenic bacteria, using both in vitro and in vivo bioassays. Pelgipeptins (PGPs) A, B, C, and D that were isolated from P. elgii JCK-5075 displayed broad-spectrum antibacterial activity against various plant pathogenic bacteria. The fermentation broth of P. elgii JCK-5075, at 5-fold dilution, effectively suppressed the development of tomato bacterial wilt, Kimchi cabbage soft rot, and red pepper bacterial leaf spot in pot experiments with control values of 81, 84, and 67%, respectively. PGP-A and C, at 200 μg/ml, were also found to markedly reduce the development of Kimchi cabbage bacterial soft rot by 75% and tomato bacterial wilt by 83%, respectively, and their disease control efficacy was comparable to that of oxolinic acid with control values of 81 and 85%, respectively. Additionally, the antibacterial activity of PGP-C was found to be directly correlated with membrane damage mechanisms. These results indicates that P. elgii JCK-5075 producing PGPs could be used as a biocontrol agent for the control of plant bacterial diseases. This is the first report on the in vitro and in vivo antibacterial activity of PGPs against bacterial plant pathogens.

Synergy Pattern of Short Cationic Antimicrobial Peptides Against Multidrug-Resistant Pseudomonas aeruginosa

With the rise of various multidrug-resistant (MDR) pathogenic bacteria, worldwide health care is under pressure to respond. Conventional antibiotics are failing and the development of novel classes and alternative strategies is a major priority. Antimicrobial peptides (AMPs) cannot only kill MDR bacteria, but also can be used synergistically with conventional antibiotics. We selected 30 short AMPs from different origins and measured their synergy in combination with polymyxin B, piperacillin, ceftazidime, cefepime, meropenem, imipenem, tetracycline, erythromycin, kanamycin, tobramycin, amikacin, gentamycin, and ciprofloxacin. In total, 403 unique combinations were tested against an MDR Pseudomonas aeruginosa isolate (PA910). As a measure of the synergistic effects, fractional inhibitory concentrations (FICs) were determined using microdilution assays with FICs ranges between 0.25 and 2. A high number of combinations between peptides and polymyxin B, erythromycin, and tetracycline were found to be synergistic. Novel variants of indolicidin also showed a high frequency in synergist interaction. Single amino acid substitutions within the peptides can have a very strong effect on the ability to synergize, making it possible to optimize future drugs toward synergistic interaction.

Characterization of the Bioactivity and Mechanism of Bactenecin Derivatives Against Food-Pathogens

With the emergence of multidrug-resistant bacteria, antimicrobial peptides (AMPs) are regarded as potential alternatives to traditional antibiotics or chemicals. We designed and synthesized six derivatives of bactenecin (L₂C₃V₁₀C₁₁, RLCRIVVIRVCR), including R₂F₃W₁₀L₁₁ (RRFRIVVIRWLR), R₂W₃W₁₀R₁₁ (RRWRIVVIRWRR), K₂W₃V₁₀R₁₁ (RKWRIVVIRVRR), W₂R₃V₁₀R₁₁ (RWRRIVVIRVRR), W₂K₃K₁₀R₁₁ (RWKRIVVIRKRR), and K₂R₃R₁₀K₁₁ (RKRRIVVIRRKR), by amino acid substitution to increase the net charge and reduce hydrophobicity gradually. The bioactivity and mechanisms of action of the designed peptides were investigated. The results indicated that the antimicrobial activity of the designed peptides was higher than that of bactenecin. The hemolytic activity and cytotoxicity of the designed peptides were significantly lower than those of bactenecin. The designed peptides exhibited a wide range of antimicrobial activity against food-pathogens, particularly peptides K₂W₃V₁₀R₁₁ and W₂R₃V₁₀R₁₁; in addition, the activity was maintained under physiological salt and heat conditions. Mechanism studies indicated that AMPs interacted with negatively charged bacterial cell membranes, resulting in the destruction of cell membrane integrity by increasing membrane permeability and changing transmembrane potential, leading to cell death. The present study suggested that peptides K₂W₃V₁₀R₁₁ and W₂R₃V₁₀R₁₁ exhibited potential as alternatives to traditional antibiotics or chemicals for the treatment of food-pathogens. These findings lead to the development of a potential method for the design of novel AMPs.

Longistylin A, a natural stilbene isolated from the leaves of Cajanus cajan, exhibits significant anti-MRSA activity

Longistylin A (LLA) is an abundant stilbene isolated from the leaves of Cajanus cajan (L.) Millsp. However, the antibacterial effect of LLA is not yet understood. Therefore, in this study, a detailed investigation of the antibacterial effect of LLA, particularly against methicillin-resistant Staphylococcus aureus (MRSA), was conducted. In vitro, LLA exhibited strong antibacterial activity against MRSA with a minimum inhibitory concentration (MIC) of 1.56 µg/mL and displayed much more rapid bactericidal activity (3-log decrease in MRSA survival within 8 h) than vancomycin. A membrane-targeting experiment suggested that the antibacterial activity of LLA is associated with perturbation of the bacterial membrane potential and increased membrane permeability. Notably, LLA had relatively weak cytotoxicity to murine macrophages [50% cytotoxic concentration (CC₅₀) = 8.61 ± 0.57 µg/mL]. In vivo, topical treatment of a skin injury with LLA improved wound healing and closure in an MRSA-infected wound healing mouse model. After 3 days treatment, LLA decreased MRSA bacterial counts in the wounded region, reduced the accumulation of immune cells at the injury site, and alleviated induction of the inflammatory cytokines tumour necrosis factor-alpha (180.74 ± 10.78 pg/mL vs. 606.57 ± 68.99 pg/mL) and interleukin-6 (87.25 ± 10.19 pg/mL vs. 280.58 ± 42.27 pg/mL) in serum.

An Orbitide from Ratibida columnifera Seed Containing 16 Amino Acid Residues

Cyclic peptides are abundant in plants and have attracted interest due to their bioactivity and potential as drug scaffolds. Orbitides are head-to-tail cyclic peptides that are ribosomally synthesized, post-translationally modified, and lack disulfide bonds. All known orbitides contain 5-12 amino acid residues. Here we describe PLP-53, a novel orbitide from the seed of Ratibida columnifera. PLP-53 consists of 16 amino acids, four residues larger than any known orbitide. NMR structural studies showed that, compared to previously characterized orbitides, PLP-53 is more flexible and, under the studied conditions, did not adopt a single ordered conformation based on analysis of NOEs and chemical shifts.

Callistemonols A and B, Potent Antimicrobial Acylphloroglucinol Derivatives with Unusual Carbon Skeletons from Callistemon viminalis

A phytochemical investigation on the leaves of Callistemon viminalis resulted in the isolation of two unusual compounds, callistemonols A (1) and B (2). Callistemonol A (1) possesses a novel skeleton of a furan ring fusing both an α,β-triketone and a phloroglucinol unit, while callistemonol B (2) is an acylphloroglucinol derivative featuring two methyl substituents on a five-membered ring and an isovaleryl side chain. Their structures were fully characterized on the basis of extensive spectroscopic analysis, including 1D and 2D NMR parameters, as well as the IR and HRESIMS data. Callistemonol A (1) represents an example of a natural dibenzofuran with two phenyl moieties, and a plausible biogenetic pathway to generate this novel dibenzofuran through a C-C bond-forming radical SAM enzyme is proposed. Moreover, antimicrobial assays, in conjunction with time-killing and biophysical studies, revealed that 1 and 2 exert potent bactericidal activities against a panel of methicillin-resistant pathogenic microbes.

Rosanortriterpenes A-B, two new nortriterpenes from the fruits of Rosa laevigata var. leiocapus

One new oleanane-type nortriterpene, rosanortriterpene A (1), and one new ursane-type nortriterpene, rosanortriterpene B (2), were isolated from the fruits of Rosa laevigata var. leiocapus. The structures of 1-2 were fully characterised on the basis of extensive spectroscopic analysis, including IR, HRESIMS, as well as 1D and 2D NMR spectral data (HSQC, ¹H-¹H COSY, and HMBC). To the best of our knowledge, this represents the first study on the chemical constituents of R. laevigata var. leiocapus. Compounds 1-2 exhibited moderate inhibitory effects on NO production in LPS stimulated RAW264.7 cells with IC₅₀ values of 29.29 ± 3.64 and 14.28 ± 1.20 μM, respectively.