Small cationic antimicrobial peptides delocalize peripheral membrane proteins

Mechanistic Studies of Antibiotic Adjuvants Reducing Kidney's Bacterial Loads upon Systemic Monotherapy

We describe the design and attributes of a linear pentapeptide-like derivative (C_14(ω5)OOc₁₀O) screened for its ability to elicit bactericidal competences of plasma constituents against Gram-negative bacteria (GNB). In simpler culture media, the lipopeptide revealed high aptitudes to sensitize resilient GNB to hydrophobic and/or efflux-substrate antibiotics, whereas in their absence, C_14(ω5)OOc₁₀O only briefly delayed bacterial proliferation. Instead, at low micromolar concentrations, the lipopeptide has rapidly lowered bacterial proton and ATP levels, although significantly less than upon treatment with its bactericidal analog. Mechanistic studies support a two-step scenario providing a plausible explanation for the lipopeptide’s biological outcomes against GNB: initially, C_14(ω5)OOc₁₀O permeabilizes the outer membrane similarly to polymyxin B, albeit in a manner not necessitating as much LPS-binding affinity. Subsequently, C_14(ω5)OOc₁₀O would interact with the inner membrane gently yet intensively enough to restrain membrane-protein functions such as drug efflux and/or ATP generation, while averting the harsher inner membrane perturbations that mediate the fatal outcome associated with bactericidal peers. Preliminary in vivo studies where skin wound infections were introduced in mice, revealed a significant efficacy in affecting bacterial viability upon topical treatment with creams containing C_14(ω5)OOc₁₀O, whereas synergistic combination therapies were able to secure the pathogen’s eradication. Further, capitalizing on the finding that C_14(ω5)OOc₁₀O plasma-potentiating concentrations were attainable in mice blood at sub-maximal tolerated doses, we used a urinary tract infection model to acquire evidence for the lipopeptide’s systemic capacity to reduce the kidney’s bacterial loads. Collectively, the data establish the role of C_14(ω5)OOc₁₀O as a compelling antibacterial potentiator and suggest its drug-like potential.

Adaptive responses of Pseudomonas aeruginosa to treatment with antibiotics

Pseudomonas aeruginosa is among the highest priority pathogens for drug development, because of its resistance to antibiotics, extraordinary adaptability, and persistence. Anti-pseudomonal research is strongly encouraged to address the acute scarcity of innovative antimicrobial lead structures. In an effort to understand the physiological response of P. aeruginosa to clinically relevant antibiotics, we investigated the proteome after exposure to ciprofloxacin, levofloxacin, rifampicin, gentamicin, tobramycin, azithromycin, tigecycline, polymyxin B, colistin, ceftazidime, meropenem, and piperacillin/tazobactam. We further investigated the response to CHIR-90, which represents a promising class of lipopolysaccharide biosynthesis inhibitors currently under evaluation. Radioactive pulse-labeling of newly synthesized proteins followed by 2D-PAGE was used to monitor the acute response of P. aeruginosa to antibiotic treatment. The proteomic profiles provide insights into the cellular defense strategies for each antibiotic. A mathematical comparison of these response profiles based on upregulated marker proteins revealed similarities of responses to antibiotics acting on the same target area. This study provides insights into the effects of commonly used antibiotics on P. aeruginosa and lays the foundation for the comparative analysis of the impact of novel compounds with precedented and unprecedented modes of action.

Cationic Peptides and Their Cu(II) and Ni(II) Complexes: Coordination and Biological Characteristics

Antimicrobial peptides are a promising group of compounds used for the treatment of infections. In some cases, metal ions are essential to activate these molecules. Examples of metalloantibiotics are, for instance, bleomycin and dermcidin. This study is focused on three new pseudopeptides with potential biological activity. The coordination behavior of all ligands with Cu(II) and Ni(II) ions has been examined. Various analytical methods such as potentiometric titration, UV-Vis and CD spectroscopies, and mass spectrometry were used. All compounds are convenient chelators for metal ion-binding. Two of the ligands tested have histidine residues. Surprisingly, imidazole nitrogen is not involved in the coordination of the metal ion. The N-terminal amino group, Dab side chains, and amide nitrogen atoms of the peptide bonds coordinated Cu(II) and Ni(II) in all the complexes formed. The cytotoxicity of three pseudopeptides and their complexes was evaluated. Moreover, their other model allowed for assessing the attenuation of LPS-induced cytotoxicity and anti-inflammatory activities were also evaluated, the results of which revealed to be very promising.

Evaluation of antimicrobial peptide LL-37 for treatment of Staphylococcus aureus biofilm on titanium plate

The antimicrobial peptide LL-37 belongs to the cathelicidin family and is one of the few human bactericidal peptides with potent antistaphylococcal activity. Staphylococcus aureus is one of the main infection bacteria in orthopedic implant therapy. Biofilm formation after bacterial infection brings more and more severe test for clinical antiinfection treatment.However, there are few studies on LL-37 in S. aureus infection of prosthesis. In this work, addition to research the antibacterial activity and the inhibitory effect on bacterial adhesion of LL-37, an in vitro model of S. aureus biofilm formation on titanium alloy surface was established to observe the inhibitory effect of LL-37.The results showed that LL-37 has a strong antibacterial effect on S. aureus in vitro, and the minimum inhibitory concentration (MIC) is about 0.62 μΜ. Moreover, LL-37 has significant impact on the adhesion of S. aureus when the concentration ≥0.16 μM and significant anti-staphylococcal biofilm effects on static biofilm models at the concentration of 0.31 to 10 μM. Additionally, LL-37 at 5 μM had a significant destructive effect on S. aureus biofilm (P < .05) that formed on the titanium alloy surface.This study further confirmed the role of LL-37 in the process of S. aureus infection, including antimicrobial activities, inhibition of bacterial adhesion, and inhibition of mature biofilm. LL-37 can significantly destroy the stable biofilm structure on the titanium alloy surface in vitro, which may provide a new way for refractory infection caused by S. aureus in titanium alloy prosthesis infection.

Mechanism of Antimicrobial Peptides: Antimicrobial, Anti-Inflammatory and Antibiofilm Activities

Antimicrobial peptides (AMPs) are regarded as a new generation of antibiotics. Besides antimicrobial activity, AMPs also have antibiofilm, immune-regulatory, and other activities. Exploring the mechanism of action of AMPs may help in the modification and development of AMPs. Many studies were conducted on the mechanism of AMPs. The present review mainly summarizes the research status on the antimicrobial, anti-inflammatory, and antibiofilm properties of AMPs. This study not only describes the mechanism of cell wall action and membrane-targeting action but also includes the transmembrane mechanism of intracellular action and intracellular action targets. It also discusses the dual mechanism of action reported by a large number of investigations. Antibiofilm and anti-inflammatory mechanisms were described based on the formation of biofilms and inflammation. This study aims to provide a comprehensive review of the multiple activities and coordination of AMPs in vivo, and to fully understand AMPs to realize their therapeutic prospect.

A novel NK-lysin in hybrid crucian carp can exhibit cytotoxic activity in fish cells and confer protection against Aeromonas hydrophila infection in comparison with Carassius cuvieri and Carassius auratus red var

NK-lysin, an effector of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs), not only exhibits cytotoxic effect in fish cells, but also participates in the immune defense against pathogenic infection. In this study, ORF sequences of RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin were 369 bp. Tissue-specific analysis revealed that the highest expressions of RCC-NK-lysin and WCC-NK-lysin were observed in gill, while the peaked level of WR-NK-lysin mRNA was observed in spleen. A. hydrophila infection sharply increased RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin mRNA expression in liver, trunk kidney and spleen. In addition, elevated levels of NK-lysin mRNA were observed in cultured fin cell lines of red crucian carp (RCC), white crucian carp (WCC) and their hybrid offspring (WR) after Lipopolysaccharide (LPS) challenge. RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin exerted regulatory roles in inducing ROS generation, modulating mitochondrial membrane potential, decreasing fish cell viability and antagonizing survival signalings, respectively. RCC/WCC/WR-NK-lysin-overexpressing fish could up-regulate expressions of inflammatory cytokines and decrease bacterial loads in spleen. These results indicated that NK-lysin in hybrid fish contained close sequence similarity to those of its parents, possessing the capacities of cytotoxicity and immune defense against bacterial infection.

Functional biomedical materials derived from proteins in the acquired salivary pellicle

In the oral environment, the acquired salivary pellicle (ASP) on the tooth surface comprises proteins, glycoproteins, carbohydrates, and lipids. The ASP can specifically and rapidly adsorb on the enamel surface to provide effective lubrication, protection, hydration, and remineralisation, as well as be recognised by various bacteria to form a microbial biofilm (plaque). The involved proteins, particularly various phosphoproteins such as statherins, histatins, and proline-rich proteins, are vital to their specific functions. This review first describes the relationship between the biological functions of these proteins and their structures. Subsequently, recent advances in functional biomedical materials derived from these proteins are reviewed in terms of dental/bone therapeutic materials, antibacterial materials, tissue engineering materials, and coatings for medical devices. Finally, perspectives and challenges regarding the rational design and biomedical applications of ASP-derived materials are discussed.

Solid-phase synthesis and evaluation of linear and cyclic ferrocenoyl/ruthenocenoyl water-soluble hexapeptides as potential antibacterial compounds

A series of novel water-soluble short peptide-bioconjugates containing a ferrocenoyl (Fc) or ruthenocenoyl (Rc) unit was synthesized and characterized to combine the unique activity of ferrocene and the isoelectronic ruthenocene with precisely designed peptide structures. We aim at evaluating these bioconjugates as a new class of OrganoMetallic Short AntiMicrobial Peptides (OM-SAMPs). The series of OM-SAMPs was designed with a set of linear and "head-to-tail" cyclic metallocene-based hexapeptides derived from the homo-sequence H-KKKKKK-NH₂ by substitution of lysine (K) by tryptophan (W) and by orthogonal derivatization of the ε-N-amine group of lysine by a metallocene moiety. Peptide conjugates were characterized by RP-HPLC, mass spectrometry (ESI and MALDI-TOF) and circular dichroism (CD) spectroscopy. Gram-positive and Gram-negative antibacterial activity testings were carried out to explore the role of insertion of the metallocene fragment into the peptide, and the effect of the modification of the cationic charge and aromatic residues on the physiochemical properties of these OM-SAMPs. These results show that the insertion of two tryptophan residues and ferrocenoyl/ruthenocenoyl moieties into a linear homo-sequence peptides increase significantly their antibacterial activity with minimum inhibitory concentration values as low as 5 μM for the most active compounds. However, "head-to-tail" cyclic metallocene-based hexapeptides were not active against Gram-negative bacteria up to concentrations of 50 μM. These studies provide a better understanding of the role of structural modifications to enhance antibacterial peptide activity, which is promising for their therapeutic application.

De novo design of a pH-triggered self-assembled β-hairpin nanopeptide with the dual biological functions for antibacterial and entrapment

BACKGROUND

Acid-tolerant enteric pathogens can evade small intestinal acid barriers, colonize and infect the intestinal tract. However, broad-spectrum antibiotics are not the best therapeutic strategy because of the disruption of intestinal flora caused by its indiscriminate antimicrobial activity against beneficial and harmful bacteria. So that is what inspired us to combine pH regulation with nanotechnology to develop a pH-triggered site-targeted antimicrobial peptide with entrapping function.

RESULTS

A pH-triggered dual biological functional self-assembled peptide (SAP) was designed according to the features of amino-acid building blocks and the diagonal cation-π interaction principle. The results of characterization experiments showed that changes in pH conditions could trigger microstructural transformation of the nanopeptide from nanospheres to nanofibers. The subsequent antibacterial and toxicity experiments determined that SAP had great antimicrobial activity against Escherichia coli, Salmonella typhimurium, Listeria monocytogenes, and Bacillus cereus above 15.6 μg/mL under acidic conditions by disrupting bacterial membrane integrity, excellent biocompatibility in vitro even at 250 μg/mL and high tolerance in physical environment. Moreover, at peptide concentrations greater than 62.5 μg/mL, SAP showed the entrapment property, which played an important role in phagocytic clearance in infection forces. Meanwhile, the in vivo results revealed that SAP possessed excellent therapeutic effect and good biosafety.

CONCLUSIONS

Our study revealed the antibacterial activity of a short β-hairpin forming self-assembled peptide, and established an innovative design strategy for peptide-based nanomaterials and a new treatment strategy for gastrointestinal bacterial infections.

Modified histidine containing amphipathic ultrashort antifungal peptide, His[2-p-(n-butyl)phenyl]-Trp-Arg-OMe exhibits potent anticryptococcal activity

In pursuit of ultrashort peptide-based antifungals, a new structural class, His(2-aryl)-Trp-Arg is reported. Structural changes were investigated on His-Trp-Arg scaffold to demonstrate the impact of charge and lipophilic character on the biological activity. The presence and size of the aryl moiety on imidazole of histidine modulated overall amphiphilic character, and biological activity. Peptides exhibited IC₅₀ of 0.37-9.66 μg/mL against C. neoformans. Peptide 14f [His(2-p-(n-butyl)phenyl)-Trp-Arg-OMe] exhibited two-fold potency (IC₅₀ = 0.37 μg/mL, MIC = 0.63 μg/mL) related to amphotericin B, without any cytotoxic effects up to 10 μg/mL. Peptide 14f act by nuclear fragmentation, membranes permeabilization, disruption and pore formations in the microbial cells as determined by the mechanistic studies employing Trp-quenching, CLSM, SEM, and HR-TEM. The amalgamation of short sequence, presence of appropriate aryl group on l-histidine, potent anticryptococcal activity, no cytotoxicity, and detailed mechanistic studies directed to the identification of 14f as a new antifungal structural lead.

BING, a novel antimicrobial peptide isolated from Japanese medaka plasma, targets bacterial envelope stress response by suppressing cpxR expression

Antimicrobial peptides (AMPs) have emerged as a promising alternative to small molecule antibiotics. Although AMPs have previously been isolated in many organisms, efforts on the systematic identification of AMPs in fish have been lagging. Here, we collected peptides from the plasma of medaka (Oryzias latipes) fish. By using mass spectrometry, 6399 unique sequences were identified from the isolated peptides, among which 430 peptides were bioinformatically predicted to be potential AMPs. One of them, a thermostable 13-residue peptide named BING, shows a broad-spectrum toxicity against pathogenic bacteria including drug-resistant strains, at concentrations that presented relatively low toxicity to mammalian cell lines and medaka. Proteomic analysis indicated that BING treatment induced a deregulation of periplasmic peptidyl-prolyl isomerases in gram-negative bacteria. We observed that BING reduced the RNA level of cpxR, an upstream regulator of envelope stress responses. cpxR is known to play a crucial role in the development of antimicrobial resistance, including the regulation of genes involved in drug efflux. BING downregulated the expression of efflux pump components mexB, mexY and oprM in P. aeruginosa and significantly synergised the toxicity of antibiotics towards these bacteria. In addition, exposure to sublethal doses of BING delayed the development of antibiotic resistance. To our knowledge, BING is the first AMP shown to suppress cpxR expression in Gram-negative bacteria. This discovery highlights the cpxR pathway as a potential antimicrobial target.

Development of a Potent Antimicrobial Peptide With Photodynamic Activity

The emergence of antibiotic-resistant bacteria poses a serious challenge to medical practice worldwide. A small peptide with sequence RWRWRW was previously identified as a core antimicrobial peptide with limited antimicrobial spectrum to bacteria, especially Gram-positive bacteria. By conjugating this peptide and its analogs with lipophilic phthalocyanine (Pc), we identified a new antibiotic peptide [PcG₃K₅(RW)₃]. The peptide demonstrates increased antimicrobial effect to both Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. In addition, Pc also provides added and potent antimicrobial effect upon red light illumination. The inhibitory efficacy of PcG₃K₅(RW)₃ was increased by ~140-fold to nanomolar range upon illumination. Moreover, PcG₃K₅(RW)₃ was safe for mammalian cell and promoted wound healing in the mouse infection model. Our work provides a new direction to optimize antimicrobial peptides to enhance antimicrobial efficacy.

Physicochemical Features and Peculiarities of Interaction of AMP with the Membrane

Antimicrobial peptides (AMPs) are anti-infectives that have the potential to be used as a novel and untapped class of biotherapeutics. Modes of action of antimicrobial peptides include interaction with the cell envelope (cell wall, outer- and inner-membrane). A comprehensive understanding of the peculiarities of interaction of antimicrobial peptides with the cell envelope is necessary to perform a rational design of new biotherapeutics, against which working out resistance is hard for microbes. In order to enable de novo design with low cost and high throughput, in silico predictive models have to be invoked. To develop an efficient predictive model, a comprehensive understanding of the sequence-to-function relationship is required. This knowledge will allow us to encode amino acid sequences expressively and to adequately choose the accurate AMP classifier. A shared protective layer of microbial cells is the inner, plasmatic membrane. The interaction of AMP with a biological membrane (native and/or artificial) has been comprehensively studied. We provide a review of mechanisms and results of interactions of AMP with the cell membrane, relying on the survey of physicochemical, aggregative, and structural features of AMPs. The potency and mechanism of AMP action are presented in terms of amino acid compositions and distributions of the polar and apolar residues along the chain, that is, in terms of the physicochemical features of peptides such as hydrophobicity, hydrophilicity, and amphiphilicity. The survey of current data highlights topics that should be taken into account to come up with a comprehensive explanation of the mechanisms of action of AMP and to uncover the physicochemical faces of peptides, essential to perform their function. Many different approaches have been used to classify AMPs, including machine learning. The survey of knowledge on sequences, structures, and modes of actions of AMP allows concluding that only possessing comprehensive information on physicochemical features of AMPs enables us to develop accurate classifiers and create effective methods of prediction. Consequently, this knowledge is necessary for the development of design tools for peptide-based antibiotics.

Cationic Glycosylated Block Co-β-peptide Acts on the Cell Wall of Gram-Positive Bacteria as Anti-biofilm Agents

Antimicrobial resistance is a global threat. In addition to the emergence of resistance to last resort drugs, bacteria escape antibiotics killing by forming complex biofilms. Strategies to tackle antibiotic resistance as well as biofilms are urgently needed. Wall teichoic acid (WTA), a generic anionic glycopolymer present on the cell surface of many Gram-positive bacteria, has been proposed as a possible therapeutic target, but its druggability remains to be demonstrated. Here we report a cationic glycosylated block co-β-peptide that binds to WTA. By doing so, the co-β-peptide not only inhibits biofilm formation, it also disperses preformed biofilms in several Gram-positive bacteria and resensitizes methicillin-resistant Staphylococcus aureus to oxacillin. The cationic block of the co-β-peptide physically interacts with the anionic WTA within the cell envelope, whereas the glycosylated block forms a nonfouling corona around the bacteria. This reduces physical interaction between bacteria-substrate and bacteria-biofilm matrix, leading to biofilm inhibition and dispersal. The WTA-targeting co-β-peptide is a promising lead for the future development of broad-spectrum anti-biofilm strategies against Gram-positive bacteria.

Design, Synthesis, and Biological Evaluation of Membrane-Active Bakuchiol Derivatives as Effective Broad-Spectrum Antibacterial Agents

Infections caused by drug-resistant bacteria seriously endanger human health and global public health. Therefore, it is urgent to discover and develop novel antimicrobial agents to combat multidrug-resistant bacteria. In this study, we designed and synthesized a series of new membrane-active bakuchiol derivatives by biomimicking the structure and function of cationic antibacterial peptides. The most promising compound 28 displayed potent antibacterial activity against both Gram-positive bacteria (minimum inhibitory concentration, MIC = 1.56-3.125 μg/mL) and Gram-negative bacteria (MIC = 3.125 μg/mL), very weak hemolytic activity, and low cytotoxicity. Compound 28 had rapid bactericidal properties and avoided bacterial resistance. More importantly, compound 28 showed strong in vivo antibacterial efficacy against Staphylococcus aureus and Pseudomonas aeruginosa in murine corneal infection models. This design strategy is expected to provide an effective solution to the antibiotic crisis.

Identification of potential therapeutic antimicrobial peptides against Acinetobacter baumannii in a mouse model of pneumonia

Acinetobacter baumannii-induced nosocomial pneumonia has become a serious clinical problem because of high antibiotic resistance rates. Antimicrobial peptides (AMP) are an ideal alternative strategy due to their broad-spectrum of antimicrobial activity and low incidence of bacterial resistance. However, their application is limited by toxicity and stability in vivo. The present study used a mouse model to directly identify potential AMPs effective for treatment of A. baumannii-induced pneumonia. Fifty-eight AMPs were screened and two identified (SMAP-29 and TP4) to have prophylactic effects which prevented the death of mice with pneumonia. Furthermore, two TP4 derivatives (dN4 and dC4) were found to have therapeutic activity in pneumonia mouse models by peritoneal or intravenous administration. Both dN4 and dC4 also inhibited and/or eliminated A. baumannii biofilms at higher doses. Taken together, these data suggest the AMP derivatives dN4 and dC4 represent a potential treatment strategy for A. baumannii-induced pneumonia.

Bioreporters for direct mode of action-informed screening of antibiotic producer strains

A big challenge in natural product research of today is rapid dereplication of already known substances, to free capacities for the exploration of new agents. Prompt information on bioactivities and mode of action (MOA) speeds up the lead discovery process and is required for rational compound optimization. Here, we present a bioreporter approach as a versatile strategy for combined bioactivity- and MOA-informed primary screening for antimicrobials. The approach is suitable for directly probing producer strains grown on agar, without need for initial compound enrichment or purification, and works along the entire purification pipeline with culture supernatants, extracts, fractions, and pure substances. The technology allows for MOA-informed purification to selectively prioritize activities of interest. In combination with high-resolution mass spectrometry, the biosensor panel is an efficient and sensitive tool for compound deconvolution. Concomitant information on the affected metabolic pathway enables the selection of appropriate follow-up assays to elucidate the molecular target.

A flat embedding method for transmission electron microscopy reveals an unknown mechanism of tetracycline

Transmission electron microscopy of cell sample sections is a popular technique in microbiology. Currently, ultrathin sectioning is done on resin-embedded cell pellets, which consumes milli- to deciliters of culture and results in sections of randomly orientated cells. This is problematic for rod-shaped bacteria and often precludes large-scale quantification of morphological phenotypes due to the lack of sufficient numbers of longitudinally cut cells. Here we report a flat embedding method that enables observation of thousands of longitudinally cut cells per single section and only requires microliter culture volumes. We successfully applied this technique to Bacillus subtilis, Escherichia coli, Mycobacterium bovis, and Acholeplasma laidlawii. To assess the potential of the technique to quantify morphological phenotypes, we monitored antibiotic-induced changes in B. subtilis cells. Surprisingly, we found that the ribosome inhibitor tetracycline causes membrane deformations. Further investigations showed that tetracycline disturbs membrane organization and localization of the peripheral membrane proteins MinD, MinC, and MreB. These observations are not the result of ribosome inhibition but constitute a secondary antibacterial activity of tetracycline that so far has defied discovery.

Thymine Sensitizes Gram-Negative Pathogens to Antibiotic Killing

Diminished antibiotic susceptibility of bacterial pathogens is an increasingly serious threat to human and animal health. Alternative strategies are required to combat antibiotic refractory bacteria. Bacterial metabolic state has been shown to play a critical role in its susceptibility to antibiotic killing. However, the adjuvant potential of nucleotides in combination with antibiotics to kill Gram-negative pathogens remains unknown. Herein, we found that thymine potentiated ciprofloxacin killing against both sensitive and resistant-E. coli in a growth phase-independent manner. Similar promotion effects were also observed for other bactericidal antibiotics, including ampicillin and kanamycin, in the fight against four kinds of Gram-negative bacteria. The mechanisms underlying this finding were that exogenous thymine could upregulate bacterial metabolism including increased TCA cycle and respiration, which thereby promote the production of ATP and ROS. Subsequently, metabolically inactive bacteria were converted to active bacteria and restored its susceptibility to antibiotic killing. In Galleria mellonella infection model, thymine effectively improved ciprofloxacin activity against E. coli. Taken together, our results demonstrated that thymine potentiates bactericidal antibiotics activity against Gram-negative pathogens through activating bacterial metabolism, providing a universal strategy to overcome Gram-negative pathogens.

Curbing gastrointestinal infections by defensin fragment modifications without harming commensal microbiota

The occurrence and spread of multidrug-resistant pathogens, especially bacteria from the ESKAPE panel, increases the risk to succumb to untreatable infections. We developed a novel antimicrobial peptide, Pam-3, with antibacterial and antibiofilm properties to counter this threat. The peptide is based on an eight-amino acid carboxyl-terminal fragment of human β-defensin 1. Pam-3 exhibited prominent antimicrobial activity against multidrug-resistant ESKAPE pathogens and additionally eradicated already established biofilms in vitro, primarily by disrupting membrane integrity of its target cell. Importantly, prolonged exposure did not result in drug-resistance to Pam-3. In mouse models, Pam-3 selectively reduced acute intestinal Salmonella and established Citrobacter infections, without compromising the core microbiota, hence displaying an added benefit to traditional broad-spectrum antibiotics. In conclusion, our data support the development of defensin-derived antimicrobial agents as a novel approach to fight multidrug-resistant bacteria, where Pam-3 appears as a particularly promising microbiota-preserving candidate.

Antimicrobial Peptides From Lycosidae (Sundevall, 1833) Spiders

Antimicrobial peptides (AMPs) have been found in all organism taxa and may play an essential role as a host defense system. AMPs are organized in various conformations, such as linear peptides, disulfide bond-linked peptides, backbone-linked peptides and circular peptides. AMPs apparently act primarily on the plasma membrane, although an increasing number of works have shown that they may also target various intracellular sites. Spider venoms are rich sources of biomolecules that show several activities, including modulation or blockage of ion channels, anti-insect, anti-cancer, antihypertensive and antimicrobial activities, among others. In spider venoms from the Lycosidae family there are many linear AMPs with a wide range of activities against several microorganisms. Due to these singular activities, some Lycosidae AMPs have been modified to improve or decrease desirable or undesirable effects, respectively. Such modifications, especially with the aim of increasing their antibiotic activity, have led to the filing of many patent applications. This review explores the abundance of Lycosidae venom AMPs and some of their derivatives, and their use as new drug models.

Synthesis and biological evaluation of stilbene-based peptoid mimics against the phytopathogenic bacterium Xanthomonas citri pv. citri

BACKGROUND

The emergence of drug-resistant phytopathogenic bacteria and the need for new types of biological disease-control agents have accelerated efforts toward searching for alternative candidates with a low propensity for resistance development. In this study, a new series of stilbene-based peptoid mimics were synthesized, and their biological activities were evaluated against citrus pathogenic bacteria in vitro and in vivo.

RESULTS

Antibacterial bioassay results showed that the dicationic peptoid mimics 9a and 9b displayed excellent bioactivity against Xanthomonas citri pv. citri, with the minimum inhibitory concentration values of 25 μM, which were superior to those of commercial copper biocides Delite (200 μM) and Kasumin Bordeaux (100 μM). In vivo bioassay further confirmed their control efficacy against plant bacterial diseases. In addition, the antibacterial mechanism of action elucidated their membrane-disruption effects resulting in the leakage of the bacterial membranes, which was similar to that of antimicrobial peptides. Moreover, the inhibition effect on biofilm formation of peptoid mimics has also been demonstrated.

CONCLUSION

Stilbene-based peptoid mimics synthesized in this study showed promising antibacterial activity with a potent membrane-disruptive mechanism. The results suggested that stilbene-based peptoid mimics have the potential as a candidate new type of bactericide for citrus disease protection.

Photothermally Active Cryogel Devices for Effective Release of Antimicrobial Peptides: On-Demand Treatment of Infections

There has been significant interest in the use of peptides as antimicrobial agents, and peptide containing hydrogels have been proposed as biological scaffolds for various applications. Limited stability and rapid clearance of small molecular weight peptides pose challenges to their widespread implementation. As a common approach, antibacterial peptides are physically loaded into hydrogel scaffolds, which leads to continuous release through the passive mode with spatial control but provides limited control over drug dosage. Although utilization of peptide covalent linkage onto hydrogels addresses partially this problem, the peptide release is commonly too slow. To alleviate these challenges, in this work, maleimide-modified antimicrobial peptides are covalently conjugated onto furan-based cryogel (CG) scaffolds via the Diels-Alder cycloaddition at room temperature. The furan group offers a handle for specific loading of the peptides, thus minimizing passive and burst drug release. The porous nature of the CG matrix provides rapid loading and release of therapeutic peptides, apart from high water uptake. Interfacing the peptide adduct containing a CG matrix with a reduced graphene oxide-modified Kapton substrate allows "on-demand" photothermal heating upon near-infrared (NIR) irradiation. A fabricated photothermal device enables tunable and efficient peptide release through NIR exposure to kill bacteria. Apart from spatial confinement offered by this CG-based bandage, the selective ablation of planktonic Staphylococcus aureus is demonstrated. It can be envisioned that this modular "on-demand" peptide-releasing device can be also employed for other topical applications by appropriate choice of therapeutic peptides.

Revealing the Mechanisms of Synergistic Action of Two Magainin Antimicrobial Peptides

The study of peptide-lipid and peptide-peptide interactions as well as their topology and dynamics using biophysical and structural approaches have changed our view how antimicrobial peptides work and function. It has become obvious that both the peptides and the lipids arrange in soft supramolecular arrangements which are highly dynamic and able to change and mutually adapt their conformation, membrane penetration, and detailed morphology. This can occur on a local and a global level. This review focuses on cationic amphipathic peptides of the magainin family which were studied extensively by biophysical approaches. They are found intercalated at the membrane interface where they cause membrane thinning and ultimately lysis. Interestingly, mixtures of two of those peptides namely magainin 2 and PGLa which occur naturally as a cocktail in the frog skin exhibit synergistic enhancement of antimicrobial activities when investigated together in antimicrobial assays but also in biophysical experiments with model membranes. Detailed dose-response curves, presented here for the first time, show a cooperative behavior for the individual peptides which is much increased when PGLa and magainin are added as equimolar mixture. This has important consequences for their bacterial killing activities and resistance development. In membranes that carry unsaturations both peptides align parallel to the membrane surface where they have been shown to arrange into mesophases involving the peptides and the lipids. This supramolecular structuration comes along with much-increased membrane affinities for the peptide mixture. Because this synergism is most pronounced in membranes representing the bacterial lipid composition it can potentially be used to increase the therapeutic window of pharmaceutical formulations.

Macrel: antimicrobial peptide screening in genomes and metagenomes

Motivation

Antimicrobial peptides (AMPs) have the potential to tackle multidrug-resistant pathogens in both clinical and non-clinical contexts. The recent growth in the availability of genomes and metagenomes provides an opportunity for in silico prediction of novel AMP molecules. However, due to the small size of these peptides, standard gene prospection methods cannot be applied in this domain and alternative approaches are necessary. In particular, standard gene prediction methods have low precision for short peptides, and functional classification by homology results in low recall.

Results

Here, we present Macrel (for metagenomic AMP classification and retrieval), which is an end-to-end pipeline for the prospection of high-quality AMP candidates from (meta)genomes. For this, we introduce a novel set of 22 peptide features. These were used to build classifiers which perform similarly to the state-of-the-art in the prediction of both antimicrobial and hemolytic activity of peptides, but with enhanced precision (using standard benchmarks as well as a stricter testing regime). We demonstrate that Macrel recovers high-quality AMP candidates using realistic simulations and real data.

Availability

Macrel is implemented in Python 3. It is available as open source at https://github.com/BigDataBiology/macrel and through bioconda. Classification of peptides or prediction of AMPs in contigs can also be performed on the webserver: https://big-data-biology.org/software/macrel.

Comparison of Proteomic Responses as Global Approach to Antibiotic Mechanism of Action Elucidation

New antibiotics are urgently needed to address the mounting resistance challenge. In early drug discovery, one of the bottlenecks is the elucidation of targets and mechanisms. To accelerate antibiotic research, we provide a proteomic approach for the rapid classification of compounds into those with precedented and unprecedented modes of action. We established a proteomic response library of Bacillus subtilis covering 91 antibiotics and comparator compounds, and a mathematical approach was developed to aid data analysis.

New antibiotics are urgently needed to address the mounting resistance challenge. In early drug discovery, one of the bottlenecks is the elucidation of targets and mechanisms. To accelerate antibiotic research, we provide a proteomic approach for the rapid classification of compounds into those with precedented and unprecedented modes of action. We established a proteomic response library of Bacillus subtilis covering 91 antibiotics and comparator compounds, and a mathematical approach was developed to aid data analysis. Comparison of proteomic responses (CoPR) allows the rapid identification of antibiotics with dual mechanisms of action as shown for atypical tetracyclines. It also aids in generating hypotheses on mechanisms of action as presented for salvarsan (arsphenamine) and the antirheumatic agent auranofin, which is under consideration for repurposing. Proteomic profiling also provides insights into the impact of antibiotics on bacterial physiology through analysis of marker proteins indicative of the impairment of cellular processes and structures. As demonstrated for trans-translation, a promising target not yet exploited clinically, proteomic profiling supports chemical biology approaches to investigating bacterial physiology.

The revitalization of antimicrobial peptides in the resistance era

The antibiotic resistance crisis is becoming incredibly thorny due to the indiscriminate employment of antibiotics in agriculture and aquaculture, such as growth promoters, and the emergence of bacteria that are capable of enduring antibiotic treatment in an endless stream. Hence, to reverse this situation, vigorous efforts should be made in the process of identifying other alternative strategies with a lower frequency of resistance. Antimicrobial peptides (AMPs), originated from host defense peptides, are generally produced by a variety of organisms as defensive weapons to protect the host from other pathogenic bacteria. The unique ability of AMPs to control bacterial infections, as well as low propensity to acquire resistance, provides the basis for it to become one of the promising antibacterial substances. Herein, we present new insights into the biological functions, structural properties, distinct mechanisms of action of AMPs and their resistance determinants. Besides, we separately discuss natural and synthetic AMPs, including their source, screening pathway and antibacterial activity. Lastly, challenges and perspectives to identify novel potent AMPs are highlighted, which will expand our understanding of the chemical space of antimicrobials and provide a pipeline for discovering the next-generation of AMPs.

Innate immune function of serine/threonine-protein kinase from Macrobrachium rosenbergii in response to host-pathogen interactions

The occurrences of multiple drug-resistant strains have been relentlessly increasing in recent years. The aquaculture industry has encountered major disease outbreaks and crucially affected by this situation. The usage of non-specific chemicals and antibiotics expedites the stimulation of resistant strains. Triggering the natural defense mechanism would provide an effective and safest way of protecting the host system. Hence, we have investigated the innate immune function of serine/threonine-protein kinase (STPK) in Macrobrachium rosenbergii (Mr). The in-silico protein analysis resulted in the identification of cationic antimicrobial peptide, MrSL-19, with interesting properties from STPK of M. rosenbergii. Antimicrobial assay, FACS and SEM analysis demonstrated that the peptide potentially inhibits Staphylococcus aureus by interacting with its membrane. The toxic study on MrSL-19 demonstrated that the peptide is not toxic against HEK293 cells as well as human erythrocytes. This investigation showed the significant innate immune property of an efficient cationic antimicrobial peptide, MrSL-19 of STPK from M. rosenbergii.

A How-To Guide for Mode of Action Analysis of Antimicrobial Peptides

Antimicrobial peptides (AMPs) are a promising alternative to classical antibiotics in the fight against multi-resistant bacteria. They are produced by organisms from all domains of life and constitute a nearly universal defense mechanism against infectious agents. No drug can be approved without information about its mechanism of action. In order to use them in a clinical setting, it is pivotal to understand how AMPs work. While many pore-forming AMPs are well-characterized in model membrane systems, non-pore-forming peptides are often poorly understood. Moreover, there is evidence that pore formation may not happen or not play a role in vivo. It is therefore imperative to study how AMPs interact with their targets in vivo and consequently kill microorganisms. This has been difficult in the past, since established methods did not provide much mechanistic detail. Especially, methods to study membrane-active compounds have been scarce. Recent advances, in particular in microscopy technology and cell biological labeling techniques, now allow studying mechanisms of AMPs in unprecedented detail. This review gives an overview of available in vivo methods to investigate the antibacterial mechanisms of AMPs. In addition to classical mode of action classification assays, we discuss global profiling techniques, such as genomic and proteomic approaches, as well as bacterial cytological profiling and other cell biological assays. We cover approaches to determine the effects of AMPs on cell morphology, outer membrane, cell wall, and inner membrane properties, cellular macromolecules, and protein targets. We particularly expand on methods to examine cytoplasmic membrane parameters, such as composition, thickness, organization, fluidity, potential, and the functionality of membrane-associated processes. This review aims to provide a guide for researchers, who seek a broad overview of the available methodology to study the mechanisms of AMPs in living bacteria.

Parietal Structures of Escherichia coli Can Impact the D-Cateslytin Antibacterial Activity

Bacterial resistance to conventional antibiotics is of major concern. Antimicrobial peptides (AMPs) are considered excellent alternatives. Among them, D-cateslytin (D-Ctl, derivative of a host defense peptide) has shown high efficiency against a broad spectrum of bacteria. The first target of AMPs is the outer membrane of the bacterium. However, the role of bacterial cell-wall structures on D-Ctl's mechanism of action has not yet been understood. In this study, we investigated the activity of D-Ctl on two isogenic strains of E. coli: one is devoid of any parietal structures; the other constitutively overexpresses only type 1 fimbriae. We studied the damage caused by D-Ctl at several initial concentrations of bacteria and D-Ctl, and exposure times to D-Ctl were examined using a combination of epifluorescence microscopy, atomic force microscopy (AFM), and Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATR-FTIR). The analysis of nanomechanical and spectrochemical properties related to the antibacterial mechanism showed a concentration dependent activity. Whereas the membrane permeabilization was evidenced for all concentrations of D-Ctl and both mutants, no pore formation was observed. The bacterial stiffness is modified dramatically concomitantly to major membrane damage and changes in the spectral fingerprints of the bacteria. In the case of the occurrence of type 1 fimbriae only, an intracellular activity was additionally detected. Our results evidenced that D-Ctl activity is highly impacted by the cell-wall external structures and surface properties of the bacteria.

Mechanisms of action of ionic liquids on living cells: the state of the art

Ionic liquids (ILs) are a relatively new class of organic electrolytes composed of an organic cation and either an organic or inorganic anion, whose melting temperature falls around room-temperature. In the last 20 years, the toxicity of ILs towards cells and micro-organisms has been heavily investigated with the main aim to assess the risks associated with their potential use in (industrial) applications, and to develop strategies to design greener ILs. Toxicity, however, is synonym with affinity, and this has stimulated, in turn, a series of biophysical and chemical-physical investigations as well as few biochemical studies focused on the mechanisms of action (MoAs) of ILs, key step in the development of applications in bio-nanomedicine and bio-nanotechnology. This review has the intent to present an overview of the state of the art of the MoAs of ILs, which have been the focus of a limited number of studies but still sufficient enough to provide a first glimpse on the subject. The overall picture that emerges is quite intriguing and shows that ILs interact with cells in a variety of different mechanisms, including alteration of lipid distribution and cell membrane viscoelasticity, disruption of cell and nuclear membranes, mitochondrial permeabilization and dysfunction, generation of reactive oxygen species, chloroplast damage (in plants), alteration of transmembrane and cytoplasmatic proteins/enzyme functions, alteration of signaling pathways, and DNA fragmentation. Together with our earlier review work on the biophysics and chemical-physics of IL-cell membrane interactions (Biophys. Rev. 9:309, 2017), we hope that the present review, focused instead on the biochemical aspects, will stimulate a series of new investigations and discoveries in the still new and interdisciplinary field of "ILs, biomolecules, and cells."

Potent Broad-Spectrum Antibacterial Activity of Amphiphilic Peptides against Multidrug-Resistant Bacteria

The emergence and prevalence of multidrug-resistant (MDR) bacteria particularly Gram-negative bacteria presents a global crisis for human health. Colistin and tigecycline were recognized as the last resort of defenses against MDR Gram-negative pathogens. However, the emergence and prevalence of MCR or Tet(X)-mediated acquired drug resistance drastically impaired their clinical efficacy. It has been suggested that antimicrobial peptides might act a crucial role in combating antibiotic resistant bacteria owing to their multiple modes of action and characteristics that are not prone to developing drug resistance. Herein, we report a safe and stable tryptophan-rich amphiphilic peptide termed WRK-12 with broad-spectrum antibacterial activity against various MDR bacteria, including MRSA, colistin and tigecycline-resistant Escherichia coli. Mechanistical studies showed that WRK-12 killed resistant E. coli through permeabilizing the bacterial membrane, dissipating membrane potential and triggering the production of reactive oxygen species (ROS). Meanwhile, WRK-12 significantly inhibited the formation of an E. coli biofilm in a dose-dependent manner. These findings revealed that amphiphilic peptide WRK-12 is a promising drug candidate in the fight against MDR bacteria.

Overview of Host Defense Peptides and Their Applications for Plastic and Reconstructive Surgeons

BACKGROUND

Host defense peptides are a family of endogenous short peptides that are found in all living beings and play a critical role in innate immunity against infection.

METHODS

A nonsystematic review of host defense peptides was conducted with specific interest in properties and applications relevant to plastic and reconstructive surgery.

RESULTS

In addition to their direct antimicrobial actions against pathogens, including multidrug-resistant bacteria, they also demonstrate important functions in immunomodulation, tumor cell lysis, and tissue regeneration. These properties have made them a topic of clinical interest for plastic surgeons because of their potential applications as novel antibiotics, wound healing medications, and cancer therapies. The rising clinical interest has led to a robust body of literature describing host defense peptides in great depth and breadth. Numerous mechanisms have been observed to explain their diverse functions, which rely on specific structural characteristics. However, these peptides remain mostly experimental, with limited translation to clinical practice because of numerous failures to achieve acceptable results in human trials.

CONCLUSIONS

Despite the broad ranging potential of these peptides for use in the field of plastic and reconstructive surgery, they are rarely discussed in the literature or at scientific meetings. In this review, the authors provide a summary of the background, structure, function, bacterial resistance, and clinical applications of host defense peptides with the goal of stimulating host defense peptide-based innovation within the field of plastic and reconstructive surgery.

Design, Engineering and Discovery of Novel α-Helical and β-Boomerang Antimicrobial Peptides against Drug Resistant Bacteria

In an era where the pipeline of new antibiotic development is drying up, the continuous rise of multi-drug resistant (MDR) and extensively drug resistant (XDR) bacteria are genuine threats to human health. Although antimicrobial peptides (AMPs) may serve as promising leads against drug resistant bacteria, only a few AMPs are in advanced clinical trials. The limitations of AMPs, namely their low in vivo activity, toxicity, and poor bioavailability, need to be addressed. Here, we review engineering of frog derived short α-helical AMPs (aurein, temporins) and lipopolysaccharide (LPS) binding designed β-boomerang AMPs for further development. The discovery of novel cell selective AMPs from the human proprotein convertase furin is also discussed.

The impact of non-ideality of lipid mixing on peptide induced lipid clustering

The influence of several antimicrobial trivalent cyclic hexapeptides on the mixing behavior of bilayer lipid membranes containing phosphatidylglycerol (PG) and phosphatidylethanolamine (PE) with varying composition was studied using DSC and ITC. The peptides contained three arginines and three aromatic amino acids and had different sequences. All of them induce clustering of PG-rich clusters with bound peptides after binding. In a previous publication we could show that a correlation between clustering efficacy and the antimicrobial activity of the peptides exists (S. Finger et al., Biochim. Biophys. Acta 1848 (2015) 2998-3006). In the current study we investigated whether the non-ideality of the lipid mixture had any effect on the clustering efficacy and the critical peptide/lipid clustering ratio. We could show that for PG/PE membranes containing 1:1 M ratios and lipids with equal or unequal chain lengths, the amount of clustered PG depended only slightly on the absolute chain length and on the chain length difference between PG and PE. Much larger differences were observed when the PG/ PE mixing ratio was changed. In mixtures of DPPG/DPPE with high PG content, the amount of clustered PG per added peptide was much higher than in PE-rich mixtures. The ITC experiments showed that the critical peptide/lipid ratio for cluster formation is also strongly dependent on the PG/PE ratio in the mixture. In the PG/PE 3:1 mixture, the formation of clusters with bound peptide is much more likely than for mixtures with less PG. For 1:1 and 1:3 lipid mixtures, the critical peptide/lipid ratio for demixing is between 0.002 and 0.004. Therefore, even in these mixtures clustering occurs way below charge saturation of the PG in the mixture and the PG-rich clusters are not charge compensated either. The peptide concentration necessary for inducing clustering amounts to ~8 μM, a value well within the range of minimal inhibitory concentration values observed for the cyclic peptides studied here. Our results show that not only the structure of the cyclic peptide influences the clustering efficacy but also the mixing behavior of the lipids in the bilayers has an influence on the amount of clustering induced by binding of cyclic peptides.

Synthesis and biological evaluation of backbone-aminated analogues of gramicidin S

We report the parallel synthesis of gramicidin S derivatives featuring backbone N-amino substituents. Analogues were prepared by incorporation of N-amino dipeptide subunits on solid support. Nine backbone-aminated macrocycles were evaluated for growth inhibitory activity against ESKAPE pathogens and hemolytic activity against human red blood cells. Diamination of the Orn residues in the β-strand region of gramicidin S was found to enhance broad-spectrum antimicrobial activity without a corresponding increase in hemolytic activity.

Antimicrobial Mechanism of pBD2 against Staphylococcus aureus

Antimicrobial peptides (AMPs) show high antibacterial activity against pathogens, which makes them potential new therapeutics to prevent and cure diseases. Porcine beta defensin 2 (pBD2) is a newly discovered AMP and has shown antibacterial activity against different bacterial species including multi-resistant bacteria. In this study, the functional mechanism of pBD2 antibacterial activity against Staphylococcus aureus was investigated. After S. aureus cells were incubated with different concentrations of pBD2, the morphological changes in S. aureus and locations of pBD2 were detected by electron microscopy. The differentially expressed genes (DEGs) were also analyzed. The results showed that the bacterial membranes were broken, bulging, and perforated after treatment with pBD2; pBD2 was mainly located on the membranes, and some entered the cytoplasm. Furthermore, 31 DEGs were detected and confirmed by quantitative real-time PCR (qRT-PCR). The known functional DEGs were associated with transmembrane transport, transport of inheritable information, and other metabolic processes. Our data suggest that pBD2 might have multiple modes of action, and the main mechanism by which pBD2 kills S. aureus is the destruction of the membrane and interaction with DNA. The results imply that pBD2 is an effective bactericide for S. aureus, and deserves further study as a new therapeutic substance against S. aureus.

Cysteine Potentiates Bactericidal Antibiotics Activity Against Gram-Negative Bacterial Persisters

Purpose

Bacterial metabolism regulators offer a novel productive strategy in the eradication of antibiotic refractory bacteria, particularly bacterial persisters. However, the potential of amino acids in the fight against Gram-negative bacterial persisters has not been fully explored. The aim of this study is to investigate the potentiation of amino acids to antibiotics in combating Gram-negative bacterial persisters and to reveal the underlying mechanisms of action.

Methods

Bactericidal activity of antibiotics in the absence or presence of amino acids was evaluated through detecting the reduction of bacterial CFUs. The ratio of NAD⁺/NADH in E. coli B2 persisters was determined using assay kit with WST-8. Bacterial respiration and ROS production were measured by the reduction of iodonitrotetrazolium chloride and fluorescent probe 2′,7′-dichlorodihydrofluorescein diacetate, respectively.

Results

In this study, we found that cysteine possesses excellent synergistic bactericidal activity with ciprofloxacin against multiple Gram-negative bacterial persisters. Furthermore, the potentiation of cysteine was evaluated in exponential and stationary-phase E. coli ATCC 25922 and E. coli B2. Interestingly, cysteine significantly improves three bactericidal antibiotics killing against stationary-phase bacteria, but not exponential-phase bacteria, implying that the effect of cysteine correlates with the metabolic state of bacteria. Mechanistic studies revealed that cysteine accelerates the bacterial TCA cycle and promotes bacterial respiration and ROS production. These metabolic regulation effects of cysteine re-sensitive bacterial persisters to antibiotic killing.

Conclusion

Collectively, our study highlights the synergistic bactericidal activity of bacterial metabolism regulators such as cysteine with commonly used antibiotics against Gram-negative bacterial persisters.

The value of antimicrobial peptides in the age of resistance

Accelerating growth and global expansion of antimicrobial resistance has deepened the need for discovery of novel antimicrobial agents. Antimicrobial peptides have clear advantages over conventional antibiotics which include slower emergence of resistance, broad-spectrum antibiofilm activity, and the ability to favourably modulate the host immune response. Broad bacterial susceptibility to antimicrobial peptides offers an additional tool to expand knowledge about the evolution of antimicrobial resistance. Structural and functional limitations, combined with a stricter regulatory environment, have hampered the clinical translation of antimicrobial peptides as potential therapeutic agents. Existing computational and experimental tools attempt to ease the preclinical and clinical development of antimicrobial peptides as novel therapeutics. This Review identifies the benefits, challenges, and opportunities of using antimicrobial peptides against multidrug-resistant pathogens, highlights advances in the deployment of novel promising antimicrobial peptides, and underlines the needs and priorities in designing focused development strategies taking into account the most advanced tools available.

Multitarget Approaches against Multiresistant Superbugs

Despite efforts to develop new antibiotics, antibacterial resistance still develops too fast for drug discovery to keep pace. Often, resistance against a new drug develops even before it reaches the market. This continued resistance crisis has demonstrated that resistance to antibiotics with single protein targets develops too rapidly to be sustainable. Most successful long-established antibiotics target more than one molecule or possess targets, which are encoded by multiple genes. This realization has motivated a change in antibiotic development toward drug candidates with multiple targets. Some mechanisms of action presuppose multiple targets or at least multiple effects, such as targeting the cytoplasmic membrane or the carrier molecule bactoprenol phosphate and are therefore particularly promising. Moreover, combination therapy approaches are being developed to break antibiotic resistance or to sensitize bacteria to antibiotic action. In this Review, we provide an overview of antibacterial multitarget approaches and the mechanisms behind them.

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.

Development of Therapeutic Gramicidin S Analogues Bearing Plastic β,γ-Diamino Acids

Gramicidin S (GS), one of the most widely investigated antimicrobial peptides (AMPs), is known for its robust antimicrobial activity. However, it is restricted to topical application due to undesired hemolytic activity. With the aim of obtaining nontoxic GS analogues, we describe herein a molecular approach in which the native GS β-turn region is replaced by synthetic β,γ-diamino acids (β,γ-DiAAs). Four β,γ-DiAA diastereomers were employed to mimic the β-turn structure to afford GS analogues GS3-6, which exhibit diminished hemolytic activity. A comparative structural study demonstrates that the (βR,γS)-DiAA is the most-stable β-turn mimic. To further improve the therapeutic index (e. g., high antibacterial activity and low hemolytic activity) and to extend the molecular diversity, GS5 and GS6 were used as structural scaffolds to introduce additional hydrophobic or hydrophilic groups. We show that GS6K, GS6F and GS display comparable antibacterial activity, and GS6K and GS6F have significantly decreased toxicity. Moreover, antibacterial mechanism studies suggest that GS6K kills bacteria mainly through the disruption of the membrane.

The Antimicrobial Peptide Human Beta-Defensin 2 Inhibits Biofilm Production of Pseudomonas aeruginosa Without Compromising Metabolic Activity

Biofilm production is a key virulence factor that facilitates bacterial colonization on host surfaces and is regulated by complex pathways, including quorum sensing, that also control pigment production, among others. To limit colonization, epithelial cells, as part of the first line of defense, utilize a variety of antimicrobial peptides (AMPs) including defensins. Pore formation is the best investigated mechanism for the bactericidal activity of AMPs. Considering the induction of human beta-defensin 2 (HBD2) secretion to the epithelial surface in response to bacteria and the importance of biofilm in microbial infection, we hypothesized that HBD2 has biofilm inhibitory activity. We assessed the viability and biofilm formation of a pyorubin-producing Pseudomonas aeruginosa strain in the presence and absence of HBD2 in comparison to the highly bactericidal HBD3. At nanomolar concentrations, HBD2 - independent of its chiral state - significantly reduced biofilm formation but not metabolic activity, unlike HBD3, which reduced biofilm and metabolic activity to the same degree. A similar discrepancy between biofilm inhibition and maintenance of metabolic activity was also observed in HBD2 treated Acinetobacter baumannii, another Gram-negative bacterium. There was no evidence for HBD2 interference with the regulation of biofilm production. The expression of biofilm-related genes and the extracellular accumulation of pyorubin pigment, another quorum sensing controlled product, did not differ significantly between HBD2 treated and control bacteria, and in silico modeling did not support direct binding of HBD2 to quorum sensing molecules. However, alterations in the outer membrane protein profile accompanied by surface topology changes, documented by atomic force microscopy, was observed after HBD2 treatment. This suggests that HBD2 induces structural changes that interfere with the transport of biofilm precursors into the extracellular space. Taken together, these data support a novel mechanism of biofilm inhibition by nanomolar concentrations of HBD2 that is independent of biofilm regulatory pathways.

Antagonizing Vancomycin Resistance in Enterococcus by Surface Localized Antimicrobial Display-Derived Peptides

Decreasing the therapeutic pipeline for vancomycin-resistant Enterococci (VRE) calls for novel strategies to enhance our antibacterial arsenal. Herein, we investigated the potential applications of surface localized antimicrobial display (SLAY)-derived cationic peptides in the fight against VanA operon mediated vancomycin-resistant Enterococcus. Through determining their antibacterial spectrum, we found that SLAY peptide 1/2 displayed moderate bactericidal activity against Enterococcus with minimal inhibitory concentration (MIC) values of 2-8 μg/mL. Furthermore, we observed a significant synergistic activity between SLAY-P1 and vancomycin against VRE. Mechanistic studies demonstrated that SLAY-P1 specifically inhibits transcription of the vanRS two-component system, thereby restoring vancomycin activity and resulting in the accumulation of the cell wall precursor. Meaningfully, the combination of SLAY-P1 and vancomycin prevents the emergence of vancomycin resistance. Consistent with in vitro synergistic results, the addition of SLAY-P1 significantly enhanced the survival rates of Galleria mellonella larvae compared with vancomycin monotherapy. Taken together, these results suggested that SLAY-derived cationic peptides not only display antibacterial activity against VRE but also reverse vancomycin resistance in Enterococcus, providing promising candidates for combating vancomycin-resistant pathogens.

Ethyl-N-dodecanoyl-l-arginate hydrochloride combats pathogens with low-resistance generation by membrane attack and modifies gut microbiota structure

Ethyl-N-dodecanoyl-l-arginate hydrochloride (LAE, ethyl lauroyl arginate HCl) is a cationic surfactant used as a food preservative with broad-spectrum antibacterial activities. However, its resistance development, influences on gut microbiome and molecular target are unclear. In this study, bacteria were stimulated by LAE for 30 days to test the bacterial resistance. Several infected animal models were used to evaluate the antibacterial effect of LAE in vivo. Mice were orally treated with LAE to test its effect on animal growth. The influence of LAE on mice gut microbiome was analysed by 16S rDNA sequencing. The results indicated that Escherichia coli did not develop resistance to LAE. LAE significantly combats bacterial infection in mice, ducklings and piglets. Moreover, LAE promotes mouse weight gain without changing body composition or reducing animal vitality, and induces lower hepatotoxicity than ampicillin. In the mouse gut microbiome assessment and characterization, LAE modifies host gut microbiota structure. Mechanistically, LAE specifically binds to acidic phospholipids including phosphatidylserine, depolarizes the membrane and disrupts the bacterial membrane followed by bacterial growth inhibition. This study investigates the molecular mechanism of LAE as well as its antibacterial functions in poultry and livestock. Our data suggest LAE is a potential antibacterial agent in animal health.