Outer-membrane-acting peptides and lipid II-targeting antibiotics cooperatively kill Gram-negative pathogens

Membrane Modification and Adaptation of Metabolism by Acinetobacter baumannii Prompts Resistance to Antimicrobial Activity of Outer Membrane Perturbing Peptide L8

Multidrug resistant (MDR) Acinetobacter baumannii has emerged as one of the most concerning nosocomial pathogens worldwide. One approach to target MDR A. baumannii is treatment with synergistic combinations of outer membrane-permeabilizing antimicrobial peptides (AMP) and antibiotics that otherwise only act against Gram-positive bacteria. Resistance against AMPs is rarely observed, especially when administered in combination with other drugs. Recently, we described the synergistic antimicrobial activity of AMPs L8 and L8S1 with rifampicin against a clinical isolate of A. baumannii. In the current work we explore the mechanisms of action of these peptides. We demonstrate that L8 and L8S1 perturb the cell envelope of A. baumannii. Moreover, we show that resistance against peptide L8 could be acquired in vitro either by increasing the amount of PE lipid on the surface or by increasing biofilm formation. Interestingly, the resistance to the antimicrobial activity of the peptides did not affect membrane perturbation or synergistic activity of the peptides with rifampicin, suggesting a dual mechanism of action for these peptides.

Therapeutic potential of celastrol in bacterial infections: Current research advancements and future perspectives

Drug-resistant bacterial infections and their associated inflammatory diseases constitute a deadly threat to global health. Celastrol is one of the main effective components extracted from the traditional Chinese medicine Tripterygium wilfordii Hook.F (TWHF). An increasing number of researchers have been focusing on the pharmacological properties of celastrol in the context of bacterial infection and associated inflammatory complications. This paper presents a comprehensive review of the pharmacological activity and mechanisms of celastrol in the treatment of bacterial infectious diseases. Celastrol has been demonstrated to possess a range of antibacterial, anti-biofilm, anti-virulence and synergistic antibacterial properties with antibiotics, mediated through diverse molecular mechanisms. Several potential targets of celastrol, such as Δ1-Pyrroline-5-Carboxylate Dehydrogenase (P5CDH), Filamenting temperature-sensitive mutant Z (FtsZ), and GdpP, have been identified. By acting on these proteins, celastrol can disrupt bacterial structure (e.g., cell walls and membranes), inhibit macromolecular synthesis (protein, RNA, and DNA), and interfere with metabolic pathways. Furthermore, celastrol exerts dual immunomodulatory effects against bacterial infections through the coordinated regulation of host-pathogen interactions: by suppressing critical bacterial virulence factors staphyloxanthin (STX) and chemotaxis inhibitory protein of S. aureus (CHIPS) to counteract immune evasion mechanisms, while simultaneously activating nuclear respiratory factor 1 (Nrf1), nuclear factor kappa-B (NF-κB), mitogen-activated protein kinase (MAPK), and various signaling pathways of host immune cells to attenuate infection-induced hyperinflammatory responses and immunocyte-derived tissue damage. Finally, a review and discussion of the therapeutic potential of celastrol is presented, with particular attention to its future development as an effective therapeutic agent for treating diseases associated with bacterial infections.

Combination of antimicrobial peptide and clinical antibiotic shows enhanced potency toward Acinetobacter baumannii infection

The continued development of novel antimicrobial treatment strategies is crucial for maintaining an effective therapeutic arsenal, and antimicrobial peptides (AMPs) exhibit promising activities against a wide range of pathogens. In this study, we tested the combined effects of an AMP, TP4-3, and meropenem on Acinetobacter baumannii, which is responsible for many severe infections and is associated with high rates of overall mortality and morbidity. This study aimed to develop an effective combination therapy for microbial infections. TP4-3 and meropenem were found to act synergistically toward Acinetobacter baumannii and exhibit an activity profile better than those of the individual compounds. TP4-3 is an antimicrobial peptide with proven activity, low toxicity and extended stability. This AMP was combined with meropenem and tested for efficacy against A. baumannii using a panel of in vitro and in vivo tests. The combination of TP4-3 and meropenem exhibited robust activity against Acinetobacter baumannii pathogens. In particular, the combined treatment demonstrated significant antibiofilm properties and a lower degree of induced resistance than meropenem alone. Additionally, the combination showed an excellent activity profile in in vivo studies. Thus, the combination of TP4-3 and meropenem appears to be an effective strategy to mitigate the detrimental consequences of infections caused by this clinically relevant pathogen. Since the combination of TP4-3 and meropenem displayed better activity than the individual compounds, this strategy of combining AMPs with clinical antibiotics may be suitable for development of clinical treatments targeting various microbial infections.

Kiam wood, Cotylelobium lanceotatum, extract as a natural antimicrobial agent: protecting Pacific white shrimp, Penaeus vannamei, against vibriosis

Vibriosis, caused by Vibrio parahaemolyticus, is a major bacterial disease in shrimp aquaculture, often managed with antibiotics that contribute to antimicrobial resistance and environmental concerns. This study investigated the antimicrobial properties of Kiam wood (Cotylelobium lanceotatum) extract and its potential as a dietary supplement to enhance the disease resistance of Pacific white shrimp, Penaeus vannamei. Kiam wood extracts were prepared using ethanol-water mixture at different ratios, and their antimicrobial activity was evaluated against V. parahaemolyticus. The water extract (KWE) exhibited the strongest anti-Vibrio activity as indicated by the widest clearance zone (15.65 mm), with a minimum inhibitory concentration (MIC) of 256 µg/mL and a minimum bactericidal concentration (MBC) of 512 µg/mL. Further assays demonstrated that KWE effectively inhibited biofilm formation and restricted bacterial motility at 512 µg/mL (MBC). Scanning electron microscopic images revealed significant cell-wall damages in treated bacteria as shown by membrane disruption and pore formations. Liquid chromatography-mass spectrometry (LC-MS) analysis identified Amuresins D (C42H30O9), Pauciflorol A (C24H32O9), Vaticasides A (C48H42O14), Vaticanols B (C56H42O12), and Cotylelophenol B (C24H30O10) as key bioactive compounds. For the in vivo trials, P. vannamei was firstly infected by feeding them a V. parahaemolyticus-impregnated diet (1.5 × 10⁴ CFU/g) for two days, followed by a 15-day feeding period with KWE-supplemented diets at 2MBC (1 mg/g). Shrimp-fed KWE diets showed a significant reduction in intestinal Vibrio loads, enhanced immune responses (total hemocyte count, semi-granulocyte, and granulocyte levels), and improved survival rates. Notably, the KWE diet achieved the highest survival rate (85%), compared to 52% in the control group. These findings highlight KWE as a promising natural antimicrobial agent with immunostimulatory properties, offering a sustainable alternative to synthetic antibiotics for managing vibriosis in shrimp aquaculture. Further studies should explore its mode of action and long-term effects on shrimp health and aquaculture environments.

Novel Antibacterial Approaches and Therapeutic Strategies

The increase in multidrug-resistant organisms worldwide is a major public health threat driven by antibiotic overuse, horizontal gene transfer (HGT), environmental drivers, and deficient infection control in hospitals. In this article, we discuss these factors and summarize the new drugs and treatment strategies suggested to combat the increasing challenges of multidrug-resistant (MDR) bacteria. New treatments recently developed involve targeting key processes involved in bacterial growth, such as riboswitches and proteolysis, and combination therapies to improve efficacy and minimize adverse effects. It also tackles the challenges of the Gram-negative bacterial outer membrane, stressing that novel strategies are needed to evade permeability barriers, efflux pumps, and resistance mechanisms. Other approaches, including phage therapy, AMPs, and AI in drug discovery, are also discussed as potential alternatives. Finally, this review points out the urgency for continued research and development (R&D), industry-academic partnerships, and financial engines to ensure that MDR microbes do not exceed the value of antibacterial therapies.

Engineered Hybrid Lantibiotic that Selectively Combats Infections Caused by Staphylococcus aureus

The rapid emergence of antibiotic-resistant strains of Staphylococcus aureus presents a substantial challenge to global public health, underscoring the urgent need for novel antibiotics with diverse mechanisms of action. In this study, we conducted mutagenesis on the C-terminal region of the lantibiotic ripcin C to enhance its antimicrobial efficacy against S. aureus. The resulting optimized variant, ripcin CP23A, demonstrated potent and selective antimicrobial activity, with a minimal inhibitory concentration of 2-4 mg/L against S. aureus. Beyond its strong antimicrobial properties, ripcin CP23A exhibited significant antibiofilm activity against methicillin-resistant S. aureus (MRSA). Mechanistic studies revealed that, in addition to targeting lipid II, ripcin CP23A disrupts bacterial membranes, a capability absent in ripcin C, which may contribute to its superior antimicrobial and antibiofilm effects. Moreover, ripcin CP23A displayed favorable biosafety and plasma stability profiles. Notably, in a mouse model of MRSA-induced mastitis, ripcin CP23A effectively reduced bacterial load, alleviated inflammation, and preserved the normal histomorphology of mammary glands. This study introduces ripcin CP23A as a promising antibiotic candidate for the treatment of MRSA-related infections.

Guanidine-modified polysaccharide conditioning layer designed for regulating bacterial attachment behaviors

Biofouling has been persisting as a global problem due to the difficulties in finding efficient and environmentally friendly antifouling coatings for long-term applications. Initial attachment of bacteria on material surface and subsequent formation of biofilm are the predominate phenomena accounting for subsequent occurrence of biofouling. Among the various factors influencing the bacterial attachment, conditioning layer formed by organic macromolecules usually plays the key role in mediating bacterial attachment through altering physicochemical properties of substrate surface. In this study, a guanidine-modified polysaccharide conditioning layer with the capability of tuning the bacterial attachment is constructed and characterized. Dextran, a polysaccharide widespread in bacteria extracellular polymeric substances (EPS), is oxidized by sodium periodate, and cationic polymer polyhexamethylene guanidine hydrochloride (PHMG) is anchored to oxidized dextran (ODEX) by Schiff base reaction. AFM characterization reveals morphological changes of the polysaccharide conditioning layer from tangled chain to island conformation after the PHMG modification. The guanidine-based dextran conditioning layer promotes attachment of both P. aeruginosa and S. aureus and disrupted bacterial cytomembranes are seen for the attached bacteria due to electrostatic interaction of the electropositive guanidine group with the electronegative bacteria. The guanidine-based dextran conditioning layer shows a low survival ratio of 22 %-34 % and 1 %-4 % for P. aeruginosa and S. aureus respectively after incubation in the bacterial suspension for 72 hours. The results would give insight into further exploring the potential applications of the newly designed polysaccharides conditioning layer for combating occurrence of biofouling.

Bacteriocin diversity, function, discovery and application as antimicrobials

Bacteriocins are potent antimicrobial peptides that are produced by bacteria. Since their discovery almost a century ago, diverse peptides have been discovered and described, and some are currently used as commercial food preservatives. Many bacteriocins exhibit extensively post-translationally modified structures encoded on complex gene clusters, whereas others have simple linear structures. The molecular structures, mechanisms of action and resistance have been determined for a number of bacteriocins, but most remain incompletely characterized. These gene-encoded peptides are amenable to bioengineering strategies and heterologous expression, enabling metagenomic mining and modification of novel antimicrobials. The ongoing global antimicrobial resistance crisis demands that novel therapeutics be developed to combat infectious pathogens. New compounds that are target-specific and compatible with the resident microbiota would be valuable alternatives to current antimicrobials. As bacteriocins can be broad or narrow spectrum in nature, they are promising tools for this purpose. However, few bacteriocins have gone beyond preclinical trials and none is currently used therapeutically in humans. In this Review, we explore the broad diversity in bacteriocin structure and function, describe identification and optimization methods and discuss the reasons behind the lack of translation beyond the laboratory of these potentially valuable antimicrobials.

Optimizing Bothropstoxin-I-Derived Peptides: Exploring the Antibacterial Potential of p-BthW

Antimicrobial peptides are an emerging class of antibiotics that present a series of advantageous characteristics such as wide structural variety, broad spectrum of activity, and low propensity to select for resistance. They are found in all classes of life as defense molecules. A group of peptides derived from the protein Bothropstoxin-I has been previously studied as an alternative treatment against multi-drug-resistant bacteria. The peptide p-BthTX-I (sequence: KKYRYHLKPFCKK) and its homodimer, linked by disulfide oxidation through the residues of Cys11 and the serum degradation product [sequence: (KKYRYHLKPFC)2], were evaluated and showed similar antimicrobial activity. In this study, we synthesized an analogue of p-BthTX-I that uses the strategy of Fmoc-Lys(Fmoc)-OH in the C-terminal region for dimerization and tryptophan for all aromatic amino acids to provide better membrane interactions. This analogue, named p-BthW, displayed potent antibacterial activity at lower concentrations and maintained the same hemolytic levels as the original molecule. Our assessment revealed that p-BthW has a quick in vitro bactericidal action and prolonged post-antibiotic effect, comparable to the action of polymyxin B. The mode of action of p-BthW seems to rely not only on membrane depolarization but also on necrosis-like effects, especially in Gram-negative bacteria. Overall, the remarkable results regarding the propensity to develop resistance reaffirmed the great potential of the developed molecule.

Efficacy of Short Novel Antimicrobial Peptides in a Mouse Model of Staphylococcus pseudintermedius Skin Infection

As the clinical application of antibiotics for bacterial skin infections in companion animals becomes increasingly prevalent, the issue of bacterial resistance has become more pronounced. Antimicrobial peptides, as a novel alternative to traditional antibiotics, have garnered widespread attention. In our study, synthetic peptides ADD-A and CBD3-ABU were tested against Staphylococcus pseudintermedius skin infections in KM mice. ADD-A was applied topically and through intraperitoneal injection, compared with control groups and treatments including CBD3-ABU, ampicillin sodium, and saline. Wound contraction, bacterial counts and histology were assessed on days 3 and 11 post-infection. ADD-A and ampicillin treatments significantly outperformed saline in wound healing (p < 0.0001 and p < 0.001, respectively). ADD-A also showed a markedly lower bacterial count than ampicillin (p < 0.0001). Histologically, ADD-A-applied wounds had better epidermal continuity and a thicker epidermis than normal, with restored follicles and sebaceous glands. ADD-A's effectiveness suggests it as a potential alternative to antibiotics for treating skin infections in animals.

Antimicrobial peptides as promising antibiotic adjuvants to combat drug-resistant pathogens

The widespread antimicrobial resistance (AMR) calls for the development of new antimicrobial strategies. Antibiotic adjuvant rescues antibiotic activity and increases the life span of the antibiotics, representing a more productive, timely, and cost-effective strategy in fighting drug-resistant pathogens. Antimicrobial peptides (AMPs) from synthetic and natural sources are considered new-generation antibacterial agents. Besides their direct antimicrobial activity, growing evidence shows that some AMPs effectively enhance the activity of conventional antibiotics. The combinations of AMPs and antibiotics display an improved therapeutic effect on antibiotic-resistant bacterial infections and minimize the emergence of resistance. In this review, we discuss the value of AMPs in the age of resistance, including modes of action, limiting evolutionary resistance, and their designing strategies. We summarise the recent advances in combining AMPs and antibiotics against antibiotic-resistant pathogens, as well as their synergistic mechanisms. Lastly, we highlight the challenges and opportunities associated with the use of AMPs as potential antibiotic adjuvants. This will shed new light on the deployment of synergistic combinations to address the AMR crisis.

Recent advances in antimicrobial peptide-based therapy

Antimicrobial peptides (AMPs) are a group of polypeptide chains that have the property to target and kill a myriad of microbial organisms including viruses, bacteria, protists, etc. The first discovered AMP was named gramicidin, an extract of aerobic soil bacteria. Further studies discovered that these peptides are present not only in prokaryotes but in eukaryotes as well. They play a vital role in human innate immunity and wound repair. Consequently, they have maintained a high level of intrigue among scientists in the field of immunology, especially so with the rise of antibiotic-resistant pathogens decreasing the reliability of antibiotics in healthcare. While AMPs have promising potential to substitute for common antibiotics, their use as effective replacements is barred by certain limitations. First, they have the potential to be cytotoxic to human cells. Second, they are unstable in the blood due to action by various proteolytic agents and ions that cause their degradation. This review provides an overview of the mechanism of AMPs, their limitations, and developments in recent years that provide techniques to overcome those limitations. We also discuss the advantages and drawbacks of AMPs as a replacement for antibiotics as compared to other alternatives such as synthetically modified bacteriophages, traditional medicine, and probiotics.

Optimization of sequence and chiral content enhances therapeutic potential of tilapia piscidin peptides

Because antimicrobial peptides (AMPs) often exhibit broad-spectrum bactericidal potency, we sought to develop peptide-based antimicrobials for potential clinical use against drug-resistant pathogens. To accomplish this goal, we first optimized the amino acid sequence of a broad-spectrum AMP known as Tilapia Piscidin 4 (TP4). Then, we used the optimized sequence to create a pair of heterochiral variants (TP4-α and TP4-β) with different percentages of D-enantiomers, as poly-L peptides often exhibit poor pharmacokinetic profiles. The conformations of the peptide pair exhibited inverted chirality according to CD and NMR spectroscopic analyses. Both heterochiral peptides displayed enhanced stability and low hemolysis activities. Irrespective of their different d-enantiomer contents, both heterochiral peptides exhibited bactericidal activities in the presence of human serum or physiological enzymes. However, the peptide with higher d-amino acid content (TP4-β) caused better bacterial clearance when tested in mice infected with NDM-1 K. pneumoniae. In addition, we observed a relatively higher hydrogen bonding affinity in a simulation of the interaction between TP4-β and a model bacterial membrane. In sum, our results demonstrate that the current design strategy may be applicable for development of new molecules with enhanced stability and in vivo antimicrobial activity.

Interaction with lipopolysaccharide is key to efficacy of tryptophan- and arginine-rich α-melanocyte-stimulating hormone analogs against Gram-negative bacteria

Aim: In order to search for novel antibacterial therapeutics against Gram-negative bacteria, the antibacterial efficacies and mechanism of action of tryptophan- and arginine-rich α-melanocyte-stimulating hormone analogs were investigated. Materials & methods: We performed a killing assay to determine their efficacy; fluorescence, microscopic studies were used to understand their mechanism and peptide-lipopolysaccharide interaction. A checkerboard assay was used to find the effective combination of peptide and antibiotics. Results: Ana-peptides displayed good killing activity against Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. Their strong interaction with lipopolysaccharide damaged the bacterial membranes and led to their subsequent death. Ana-5, the highest cationic and hydrophobic analog, emerged as the most potent peptide, showing synergistic action with rifampicin and erythromycin. Conclusion: Ana-5 can be presented as an important therapeutic candidate against bacterial infections.

Brevicidine acts as an effective sensitizer of outer membrane-impermeable conventional antibiotics for Acinetobacter baumannii treatment

The antibiotic resistance of Acinetobacter baumannii poses a significant threat to global public health, especially those strains that are resistant to carbapenems. Therefore, novel strategies are desperately needed for the treatment of infections caused by antibiotic-resistant A. baumannii. In this study, we report that brevicidine, a bacterial non-ribosomally produced cyclic lipopeptide, shows synergistic effects with multiple outer membrane-impermeable conventional antibiotics against A. baumannii. In particular, brevicidine, at a concentration of 1 μM, lowered the minimum inhibitory concentration of erythromycin, azithromycin, and rifampicin against A. baumannii strains by 32-128-fold. Furthermore, mechanistic studies were performed by employing erythromycin as an example of an outer membrane-impermeable conventional antibiotic, which showed the best synergistic effects with brevicidine against the tested A. baumannii strains in the present study. The results demonstrate that brevicidine disrupted the outer membrane of A. baumannii at a concentration range of 0.125-4 μM in a dose-dependent manner. This capacity of brevicidine could help the tested outer membrane-impermeable antibiotics enter A. baumannii cells and thereafter exert their antimicrobial activity. In addition, the results show that brevicidine-erythromycin combination exerted strong A. baumannii killing capacity by the enhanced inhibition of adenosine triphosphate biosynthesis and accumulation of reactive oxygen species, which are the main mechanisms causing the death of bacteria. Interestingly, brevicidine and erythromycin combination showed good therapeutic effects on A. baumannii-induced mouse peritonitis-sepsis models. These findings demonstrate that brevicidine is a promising sensitizer candidate of outer membrane-impermeable conventional antibiotics for treating A. baumannii infections in the post-antibiotic age.

Synthesis and antimicrobial activity of aminoalkyl resveratrol derivatives inspired by cationic peptides

Antimicrobial resistance is a global concern, far from being resolved. The need of new drugs against new targets is imminent. In this work, we present a family of aminoalkyl resveratrol derivatives with antibacterial activity inspired by the properties of cationic amphipathic antimicrobial peptides. Surprisingly, the newly designed molecules display modest activity against aerobically growing bacteria but show surprisingly good antimicrobial activity against anaerobic bacteria (Gram-negative and Gram-positive) suggesting specificity towards this bacterial group. Preliminary studies into the action mechanism suggest that activity takes place at the membrane level, while no cross-resistance with traditional antibiotics is observed. Actually, some good synergistic relations with existing antibiotics were found against Gram-negative pathogens. However, some cytotoxicity was observed, despite their low haemolytic activity. Our results show the importance of the balance between positively charged moieties and hydrophobicity to improve antimicrobial activity, setting the stage for the design of new drugs based on these molecules.

Chemical genetic approaches for the discovery of bacterial cell wall inhibitors

Antimicrobial resistance (AMR) in bacterial pathogens is a worldwide health issue. The innovation gap in discovering new antibiotics has remained a significant hurdle in combating the AMR problem. Currently, antibiotics target various vital components of the bacterial cell envelope, nucleic acid and protein biosynthesis machinery and metabolic pathways essential for bacterial survival. The critical role of the bacterial cell envelope in cell morphogenesis and integrity makes it an attractive drug target. While a significant number of in-clinic antibiotics target peptidoglycan biosynthesis, several components of the bacterial cell envelope have been overlooked. This review focuses on various antibacterial targets in the bacterial cell wall and the strategies employed to find their novel inhibitors. This review will further elaborate on combining forward and reverse chemical genetic approaches to discover antibacterials that target the bacterial cell envelope.

LL37-Derived Fragments Improve the Antibacterial Potential of Penicillin G and Ampicillin against Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) infections are a severe threat to public health. Antimicrobial peptides (AMPs) are novel and potential antimicrobials with specific antibacterial mechanisms. Our aim was to study the potential of LL37, FK16, and FK13 to enhance the anti-MRSA activity of antibiotics in vitro, particularly penicillin G and ampicillin. Our results showed that FK16 and FK13 have more synergistic inhibitory effects to MRSA strains when combined with penicillin G and ampicillin. In addition, AMPs exhibited strong membrane permeabilizing properties, and membrane permeabilizing effects can provide a possible explanation for the improved antibacterial effects of antibiotics, since permeabilizing AMPs have the potential to increase the access of antibiotics. To further study the electrostatic interactions among cationic AMPs with negatively charged bacteria, we measured the zeta potentials of three MRSA strains and also neutralized three MRSA strains with the addition of cationic AMPs. Further, we demonstrated the connection between membrane permeabilization and zeta potential neutralization. Finally, we treated MRSA strains with AMPs and characterized the MICs of penicillin G and ampicillin. FK16 was the most promising AMP among the three AMPs, since exposure to FK16 decreased the MICs of both penicillin G and ampicillin for all MRSA strains and also demonstrated more synergistic combinations when combined with antibiotics. AMP exposure and subsequent membrane permeabilization provide a possible pathway to re-sensitize drug-resistant bacteria to traditional antibiotics. Re-sensitization may help preserve the effectiveness of traditional antibiotics, thus providing a potential new strategy for fighting MRSA infections.

Seeing the Invisibles: Detection of Peptide Enantiomers, Diastereomers, and Isobaric Ring Formation in Lanthipeptides Using Nanopores

Mass spectrometry (MS) is widely used in proteomic analysis but cannot differentiate between molecules with the same mass-to-charge ratio. Nanopore technology might provide an alternative method for the rapid and cost-effective analysis and sequencing of proteins. In this study, we demonstrate that nanopore currents can distinguish between diastereomeric and enantiomeric differences in l- and d-peptides, not observed by conventional MS analysis, down to individual d-amino acids in small opioid peptides. Molecular dynamics simulations suggest that similar to chiral chromatography the resolution likely arises from multiple chiral interactions during peptide transport across the nanopore. Additionally, we used nanopore recordings to rapidly assess 4- and 11-amino acid ring formation in lanthipeptides, a process used in the synthesis of pharmaceutical peptides. The cyclization step requires distinguishing between constitutional isomers, which have identical MS signals and typically involve numerous tedious experiments to confirm. Hence, nanopore technology offers new possibilities for the rapid and cost-effective analysis of peptides, including those that cannot be easily differentiated by mass spectrometry.

Elucidating the Mechanism of Action of the Gram-Negative-Pathogen-Selective Cyclic Antimicrobial Lipopeptide Brevicidine

Due to the accelerated appearance of antimicrobial-resistant (AMR) pathogens in clinical infections, new first-in-class antibiotics, operating via novel modes of action, are desperately needed. Brevicidine, a bacterial nonribosomally produced cyclic lipopeptide, has shown potent and selective antimicrobial activity against Gram-negative pathogens. However, before our investigations, little was known about how brevicidine exerts its potent bactericidal effect against Gram-negative pathogens. In this study, we find that brevicidine has potent antimicrobial activity against AMR Enterobacteriaceae pathogens, with MIC values ranging between 0.5 μM (0.8 mg/L) and 2 μM (3.0 mg/L). In addition, brevicidine showed potent antibiofilm activity against the Enterobacteriaceae pathogens, with the same 100% inhibition and 100% eradication concentration of 4 μM (6.1 mg/L). Further mechanistic studies showed that brevicidine exerts its potent bactericidal activity by interacting with lipopolysaccharide in the outer membrane, targeting phosphatidylglycerol and cardiolipin in the inner membrane, and dissipating the proton motive force of bacteria. This results in metabolic perturbation, including the inhibition of ATP synthesis; the inhibition of the dehydrogenation of NADH; the accumulation of reactive oxygen species in bacteria; and the inhibition of protein synthesis. Finally, brevicidine showed a good therapeutic effect in a mouse peritonitis-sepsis model. Our findings pave the way for further research on the clinical applications of brevicidine to combat prevalent infections caused by AMR Gram-negative pathogens worldwide.

Rational Design of Stapled Antimicrobial Peptides to Enhance Stability and In Vivo Potency against Polymicrobial Sepsis

In this work, we sought to develop a TP4-based stapled peptide that can be used to counter polymicrobial sepsis. First, we segregated the TP4 sequence into hydrophobic and cationic/hydrophilic zones and substituted the preferred residue, lysine, as the sole cationic amino acid. These modifications minimized the intensity of cationic or hydrophobic characteristics within small segments. Then, we incorporated single or multiple staples into the peptide chain, bracketing the cationic/hydrophilic segments to improve pharmacological suitability. Using this approach, we were able to develop an AMP with low toxicity and notable in vivo efficacy. IMPORTANCE In our in vitro studies, one dual stapled peptide out of the series of candidates (TP4-3: FIIXKKSXGLFKKKAGAXKKKXIKK) showed significant activity, low toxicity, and high stability (in 50% human serum). When tested in cecal ligation and puncture (CLP) mouse models of polymicrobial sepsis, TP4-3 improved survival (87.5% on day 7). Furthermore, TP4-3 enhanced the activity of meropenem against polymicrobial sepsis (100% survival on day 7) compared to meropenem alone (37.5% survival on day 7). Molecules such as TP4-3 may be well suited for a wide variety of clinical applications.

Antimicrobial Activity of an Fmoc-Plantaricin 149 Derivative Peptide against Multidrug-Resistant Bacteria

Antimicrobial resistance poses a major threat to public health. Given the paucity of novel antimicrobials to treat resistant infections, the emergence of multidrug-resistant bacteria renewed interest in antimicrobial peptides as potential therapeutics. This study designed a new analog of the antimicrobial peptide Plantaricin 149 (Pln149-PEP20) based on previous Fmoc-peptides. The minimal inhibitory concentrations of Pln149-PEP20 were determined for 60 bacteria of different species and resistance profiles, ranging from 1 mg/L to 128 mg/L for Gram-positive bacteria and 16 to 512 mg/L for Gram-negative. Furthermore, Pln149-PEP20 demonstrated excellent bactericidal activity within one hour. To determine the propensity to develop resistance to Pln149-PEP20, a directed-evolution in vitro experiment was performed. Whole-genome sequencing of selected mutants with increased MICs and wild-type isolates revealed that most mutations were concentrated in genes associated with membrane metabolism, indicating the most likely target of Pln149-PEP20. Synchrotron radiation circular dichroism showed how this molecule disturbs the membranes, suggesting a carpet mode of interaction. Membrane depolarization and transmission electron microscopy assays supported these two hypotheses, although a secondary intracellular mechanism of action is possible. The molecule studied in this research has the potential to be used as a novel antimicrobial therapy, although further modifications and optimization remain possible.

Structural Characterization, Functional Profiling, and Mechanism Study of Four Antimicrobial Peptides for Antibacterial and Anticancer Applications

Antimicrobial peptides (AMPs) are potent compounds for treating bacterial infection and cancer, drawing ever-increasing interest. However, the function and mechanism of most AMPs remain to be explored. In this research, we focused on investigating the antibacterial and anticancer activities of four AMPs (Dhvar4, Lasioglossin-III, Macropin 1, and Temporin La) and the possible corresponding mechanisms. All four AMPs are cationic α-helical with moderate hydrophobicity and high helicity. They have broad-spectrum antibacterial capacities, among which the antibacterial activities of Dhvar4 and Temporin La are not as effective as Lasioglossin-III and Macropin 1. Macropin 1 exhibited the highest antibacterial effect with a pretty low minimal inhibitory concentration (MIC) of 2-8 μM. Meanwhile, Lasioglossin-III exhibited the strongest anticancer activities, displaying the IC₅₀ of 26.36 μM for A549 and 7.75 μM for HepG2. Although Dhvar4 possessed the highest positive charge and entered the bacterial and animal cells in large amounts, it displayed the lowest bactericidal and anticancer activities which might be ascribed to its lowest hydrophobicity and thus the weakest cell membrane damage capability. It seems that the positive charge and cell internalization play a supporting rather than a determined role in antibacterial and anticancer activities of AMPs. All the four AMPs damaged the bacterial cell membrane with Macropin 1 damaging the cell membrane of Escherichia coli the most and Lasioglossin-III destroying the cell membrane of Staphylococcus aureus the worst. In addition, the animal cellular internalization of the four peptides was temperature-dependent and mainly mediated by caveolae-mediated endocytosis, and they were distributed in lysosomes once inside the cells. These findings expand our knowledge on the function and mechanism of AMPs, laying the fundamental theoretical basis for designing and engineering AMPs for infection and cancer treatment.

Overcoming intrinsic resistance in gram-negative bacteria using small molecule adjuvants

Gram-negative bacteria are intrinsically resistant to many classes of antibiotics, predominantly due to the impermeability of the outer membrane and the presence of efflux pumps. Small molecule adjuvants that circumvent these resistance mechanisms have the potential to expand therapeutic options for treating Gram-negative infections to encompass antibiotic classes that are otherwise limited to treating Gram-positive infections. Adjuvants that effect increased antibiotic permeation, either by physical disruption of the outer membrane or through interference with synthesis, transport, or assembly of membrane components, and adjuvants that limit efflux, are discussed as potential avenues to overcoming intrinsic resistance in Gram-negative bacteria.

R R, L. F L, M.C.G DR, N.B C, S.C D, O. L F. Advances and perspectives for antimicrobial peptide and combinatory therapies

Antimicrobial peptides (AMPs) have shown cell membrane-directed mechanisms of action. This specificity can be effective against infectious agents that have acquired resistance to conventional drugs. The AMPs' membrane-specificity and their great potential to combat resistant microbes has brought hope to the medical/therapeutic scene. The high death rate worldwide due to antimicrobial resistance (AMR) has pushed forward the search for new molecules and product developments, mainly antibiotics. In the current scenario, other strategies including the association of two or more drugs have contributed to the treatment of difficult-to-treat infectious diseases, above all, those caused by bacteria. In this context, the synergistic action of AMPs associated with current antibiotic therapy can bring important results for the production of new and effective drugs to overcome AMR. This review presents the advances obtained in the last 5 years in medical/antibiotic therapy, with the use of products based on AMPs, as well as perspectives on the potentialized effects of current drugs combined with AMPs for the treatment of bacterial infectious diseases.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

New antimicrobial peptide-antibiotic combination strategy for Pseudomonas aeruginosa inactivation

Novel antimicrobials or new treatment strategies are urgently needed to treat Pseudomonas aeruginosa (P. aeruginosa) related infections and especially to address the problem of antibiotic resistance. We propose a novel strategy that combines the human antimicrobial peptide (AMP) LL37 with different antibiotics to find synergistic AMP-antibiotic combinations against P. aeruginosa strains in vitro. Our results showed that LL37 exhibited synergistic inhibitory and bactericidal effects against P. aeruginosa strains PAO1 and PA103 when combined with the antibiotics vancomycin, azithromycin, polymyxin B, and colistin. In addition, LL37 caused strong outer membrane permeabilization, as demonstrated through measurement of an increased uptake of the fluorescent probe N-phenyl-1-naphthylamine. The membrane permeabilization effects appear to explain why it was easier to rescue the effectiveness of the antibiotic toward the bacteria because the outer membrane of P. aeruginosa exhibits barrier function for antibiotics. Furthermore, the change in the zeta potential was measured for P. aeruginosa strains with the addition of LL37. Zeta potentials for P. aeruginosa strains PAO1 and PA103 were -40.9 and -10.9 mV, respectively. With the addition of LL37, negative zeta potentials were gradually neutralized. We found that positively charged LL37 can interact with and neutralize the negatively charged bacterial outer membrane through electrostatic interactions, and the process of neutralization is believed to have contributed to the increase in outer membrane permeability. Finally, to further illustrate the relationship between outer membrane permeabilization and the uptake of antibiotics, we used LL37 to make the outer membrane of P. aeruginosa strains more permeable, and minimum inhibitory concentrations (MICs) for several antibiotics (colistin, gentamicin, polymyxin B, vancomycin, and azithromycin) were measured. The MICs decreased were twofold to fourfold, in general. For example, the MICs of azithromycin and vancomycin decreased more than fourfold when against P. aeruginosa strain PAO1, which were the greatest decrease of any of the antibiotics tested in this experiment. As for PA103, the MIC of polymyxin B2 decreased fourfold, which was the strongest decrease seen for any of the antibiotics tested in this experiment. The increased uptake of antibiotics not only demonstrates the barrier role of the outer membrane but also validates the mechanism of synergistic effects that we have proposed. These results indicate the great potential of an LL37-antibiotic combination strategy and provide possible explanations for the mechanisms behind this synergy.

Recent Advances in the Development of Semisynthetic Glycopeptide Antibiotics: 2014-2022

The accelerated appearance of drug-resistant bacteria poses an ever-growing threat to modern medicine's capacity to fight infectious diseases. Gram-positive species such as methicillin-resistant Staphylococcus aureus (MRSA) and Streptococcus pneumoniae continue to contribute significantly to the global burden of antimicrobial resistance. For decades, the treatment of serious Gram-positive infections relied upon the glycopeptide family of antibiotics, typified by vancomycin, as a last line of defense. With the emergence of vancomycin resistance, the semisynthetic glycopeptides telavancin, dalbavancin, and oritavancin were developed. The clinical use of these compounds is somewhat limited due to toxicity concerns and their unusual pharmacokinetics, highlighting the importance of developing next-generation semisynthetic glycopeptides with enhanced antibacterial activities and improved safety profiles. This Review provides an updated overview of recent advancements made in the development of novel semisynthetic glycopeptides, spanning the period from 2014 to today. A wide range of approaches are covered, encompassing innovative strategies that have delivered semisynthetic glycopeptides with potent activities against Gram-positive bacteria, including drug-resistant strains. We also address recent efforts aimed at developing targeted therapies and advances made in extending the activity of the glycopeptides toward Gram-negative organisms.

Chemically Tuning Resveratrol for the Effective Killing of Gram-Positive Pathogens

In the era of antimicrobial resistance, the identification of new compounds with strong antimicrobial activity and the development of alternative therapies to fight drug-resistant bacteria are urgently needed. Here, we have used resveratrol, a safe and well-known plant-derived stilbene with poor antimicrobial properties, as a scaffold to design several new families of antimicrobials by adding different chemical entities at specific positions. We have characterized the mode of action of the most active compounds prepared and have examined their synergistic antibacterial activity in combination with traditional antibiotics. Some alkyl- and silyl-resveratrol derivatives show bactericidal activity against Gram-positive bacteria in the same low micromolar range of traditional antibiotics, with an original mechanism of action that combines membrane permeability activity with ionophore-related activities. No cross-resistance or antagonistic effect was observed with traditional antibiotics. Synergism was observed for some specific general-use antibiotics, such as aminoglycosides and cationic antimicrobial peptide antibiotics. No hemolytic activity was observed at the active concentrations or above, although some low toxicity against an MRC-5 cell line was noted.

Potentiation of Vancomycin: Creating Cooperative Membrane Lysis through a "Derivatization-for-Sensitization" Approach

Gram-negative bacteria, especially the ones with multidrug resistance, post dire challenges to antibiotic treatments due to the presence of the outer membrane (OM), which blocks the entry of many antibiotics. Current solutions for such permeability issues, namely lipophilic-cationic derivatization of antibiotics and sensitization with membrane-active agents, cannot effectively potentiate the large, globular, and hydrophilic antibiotics such as vancomycin, due to ineffective disruption of the OM. Here, we present our solution for high-degree OM binding of vancomycin via a hybrid "derivatization-for-sensitization" approach, which features a combination of LPS-targeting lipo-cationic modifications on vancomycin and OM disruption activity from a sensitizing adjuvant. 10⁶- to 10⁷-fold potentiation of vancomycin and 20-fold increase of the sensitizer's effectiveness were achieved with a combination of a vancomycin derivative and its sensitizer. Such potentiation is the result of direct membrane lysis through cooperative membrane binding for the sensitizer-antibiotic complex, which strongly promotes the uptake of vancomycin and adds to the extensive antiresistance effectiveness. The potential of such derivatization-for-sensitization approach was also supported by the combination's potent in vivo antimicrobial efficacy in mouse model studies, and the expanded application of such strategy on other antibiotics and sensitizer structures.

6-Bromo-2-naphthol from Silene armeria extract sensitizes Acinetobacter baumannii strains to polymyxin

The overuse of antibiotics has led to the emergence of multidrug-resistant bacteria, which are resistant to various antibiotics. Combination therapies using natural compounds with antibiotics have been found to have synergistic effects against several pathogens. Synergistic natural compounds can potentiate the effects of polymyxins for the treatment of Acinetobacter baumannii infection. Out of 120 types of plant extracts, only Silene armeria extract (SAE) showed a synergistic effect with polymyxin B (PMB) in our fractional inhibitory concentration and time-kill analyses. The survival rate of G. mellonella infected with A. baumannii ATCC 17978 increased following the synergistic treatment. Interestingly, the addition of osmolytes, such as trehalose, canceled the synergistic effect of SAE with PMB; however, the underlying mechanism remains unclear. Quadrupole time-of-flight liquid chromatography-mass spectrometry revealed 6-bromo-2-naphthol (6B2N) to be a major active compound that exhibited synergistic effects with PMB. Pretreatment with 6B2N made A. baumannii cells more susceptible to PMB exposure in a time- and concentration-dependent manner, indicating that 6B2N exhibits consequential synergistic action with PMB. Moreover, the exposure of 6B2N-treated cells to PMB led to higher membrane leakage and permeability. The present findings provide a promising approach for utilizing plant extracts as adjuvants to reduce the toxicity of PMB in A. baumannii infection.

Stapling of Peptides Potentiates: The Antibiotic Treatment of Acinetobacter baumannii In Vivo

The rising incidence of multidrug resistance in Gram-negative bacteria underlines the urgency for novel treatment options. One promising new approach is the synergistic combination of antibiotics with antimicrobial peptides. However, the use of such peptides is not straightforward; they are often sensitive to proteolytic degradation, which greatly limits their clinical potential. One approach to increase stability is to apply a hydrocarbon staple to the antimicrobial peptide, thereby fixing them in an α-helical conformation, which renders them less exposed to proteolytic activity. In this work we applied several different hydrocarbon staples to two previously described peptides shown to act on the outer membrane, L6 and L8, and tested their activity in a zebrafish embryo infection model using a clinical isolate of Acinetobacter baumannii as a pathogen. We show that the introduction of such a hydrocarbon staple to the peptide L8 improves its in vivo potentiating activity on antibiotic treatment, without increasing its in vivo antimicrobial activity, toxicity or hemolytic activity.

Development of Bactericidal Peptides against Multidrug-Resistant Acinetobacter baumannii with Enhanced Stability and Low Toxicity

Pathogenic superbugs are the root cause of untreatable complex infections with limited or no treatment options. These infections are becoming more common as clinical antibiotics have lost their effectiveness over time. Therefore, the development of novel antibacterial agents is urgently needed to counter these microbes. Antimicrobial peptides (AMPs) are a viable treatment option due to their bactericidal potency against multiple microbial classes. AMPs are naturally selected physiological microbicidal agents that are found in all forms of organisms. In the present study, we developed two tilapia piscidin 2 (TP2)-based AMPs for antimicrobial application. Unlike the parent peptide, the redesigned peptides showed significant antimicrobial activity against multidrug-resistant bacterial species. These peptides also showed minimal cytotoxicity. In addition, they were significantly active in the presence of physiological salts, 50% human serum and elevated temperature. The designed peptides also showed synergistic activity when combined with clinical antibiotics. The current approach demonstrates a fruitful strategy for developing potential AMPs for antimicrobial application. Such AMPs have potential for progression to further trials and drug development investigations.

Mycobacterial Membranes as Actionable Targets for Lipid-Centric Therapy in Tuberculosis

Infectious diseases remain significant health concerns worldwide, and resistance is particularly common in patients with tuberculosis caused by Mycobacterium tuberculosis. The development of anti-infectives with novel modes of action may help overcome resistance. In this regard, membrane-active agents, which modulate membrane components essential for the survival of pathogens, present attractive antimicrobial agents. Key advantages of membrane-active compounds include their ability to target slow-growing or dormant bacteria and their favorable pharmacokinetics. Here, we comprehensively review recent advances in the development of membrane-active chemotypes that target mycobacterial membranes and discuss clinically relevant membrane-active antibacterial agents that have shown promise in counteracting bacterial infections. We discuss the relationship between the membrane properties and the synthetic requirements within the chemical scaffold, as well as the limitations of current membrane-active chemotypes. This review will lay the chemical groundwork for the development of membrane-active antituberculosis agents and will foster the discovery of more effective antitubercular agents.

RW-BP100-4D, a Promising Antimicrobial Candidate With Broad-Spectrum Bactericidal Activity

With the rapid emergence and dissemination of antimicrobial resistance (AMR) genes in bacteria from animal, animal-derived food and human clinic, it is of great significance to develop new approaches to combat the multidrug-resistant bacteria. This study presented a short linear antimicrobial peptide RW-BP100-4D, which was derived from RW-BP100 (RRLFRRILRWL-NH2) by transforming the N-terminal 4th amino acid from L- to D-enantiomer. This modification remarkably reduced the peptide cytotoxicity to mammalian cells, as indicated by hemolytic and cytotoxicity assays. Meanwhile, the antimicrobial activity of RW-BP100-4D was improved against a more variety of Gram-positive and Gram-negative bacteria (sensitive and resistant) as well as fungi. Also, RW-BP100-4D showed strong in vitro anti-biofilm activity in a concentration-dependent manner, including inhibition of the biofilm-formation and dispersion of the mature biofilms of Staphylococcus aureus. RW-BP100-4D could be efficiently uptaken by bovine mammary epithelial cells (MAC-T) cells to eliminate the intracellular S. aureus ATCC29213 and Salmonella enterica ATCC13076. Moreover, RW-BP100-4D was highly effective in food disinfection of multiple bacterial contamination (including S. aureus, Listeria monocytogenesis, Escherichia coli O157: H7, Campylobacter jejuni, S. enterica, and Shewanella putrefaction, 3.61 ± 0.063 log reduction) on chicken meat, and could kill 99.99% of the methicillin-resistant Staphylococcus aureus (MRSA) strain in the mouse skin infection model. In summary, RW-BP100-4D is a promising antimicrobial candidate for application on food disinfection and local infection treatment. However, the protease-sensitivity of RW-BP100-4D and toxic effect at higher doses reduced the therapeutic effect of the candidate peptide in vivo and should be improved in the future studies.

Rationally designed foldameric adjuvants enhance antibiotic efficacy via promoting membrane hyperpolarization

The negative membrane potential of bacterial cells influences crucial cellular processes. Inspired by the molecular scaffold of the antimicrobial peptide PGLa, we have developed antimicrobial foldamers with a computer-guided design strategy. The novel PGLa analogues induce sustained membrane hyperpolarization. When co-administered as an adjuvant, the resulting compounds - PGLb1 and PGLb2 - have substantially reduced the level of antibiotic resistance of multi-drug resistant Escherichia coli, Klebsiella pneumoniae and Shigella flexneri clinical isolates. The observed antibiotic potentiation was mediated by hyperpolarization of the bacterial membrane caused by the alteration of cellular ion transport. Specifically, PGLb1 and PGLb2 are selective ionophores that enhance the Goldman-Hodgkin-Katz potential across the bacterial membrane. These findings indicate that manipulating bacterial membrane electrophysiology could be a valuable tool to overcome antimicrobial resistance.

G-Quadruplex DNA as a Target in Pathogenic Bacteria: Efficacy of an Extended Naphthalene Diimide Ligand and Its Mode of Action

Guanidine DNA quadruplex (G4-DNA) structures convey a distinctive layer of epigenetic information that is critical for regulating key biological activities and processes as transcription, replication, and repair in living cells. The information regarding their role and use as therapeutic drug targets in bacteria is still scarce. Here, we tested the biological activity of a G4-DNA ligand library, based on the naphthalene diimide (NDI) pharmacophore, against both Gram-positive and Gram-negative bacteria. For the best compound identified, NDI-10, a different action mechanism was described for Gram-positive or negative bacteria. This asymmetric activity profile could be related to the different prevalence of putative G4-DNA structures in each group, the influence that they can exert on gene expression, and the different roles of the G4 structures in these bacteria, which seem to promote transcription in Gram-positive bacteria and repress transcription in Gram-negatives.

Inhibition of MurA Enzyme from Escherichia coli and Staphylococcus aureus by Diterpenes from Lepechinia meyenii and Their Synthetic Analogs

Enzymes MurA and MurF, involved in bacterial cell wall synthesis, have been validated as targets for the discovery of novel antibiotics. A panel of plant-origin antibacterial diterpenes and synthetic analogs derived therefrom were investigated for their inhibitory properties on these enzymes from Escherichia coli and Staphylococcus aureus. Six compounds were proven to be effective for inhibiting MurA from both bacteria, with IC50 values ranging from 1.1 to 25.1 µM. To further mechanistically investigate the nature of binding and to explain the activity, these compounds were docked into the active site of MurA from E. coli. The aromatic ring of the active compounds showed a T-shaped π-π interaction with the phenyl ring of Phe328, and at least one hydrogen bond was formed between the hydroxy groups and Arg120 and/or Arg91. The results disclosed here establish new chemical scaffolds for the development of novel entities targeting MurA as potential antibiotics to combat the threat of pathogenic bacteria, particularly resistant strains.

New potentiators of ineffective antibiotics: Targeting the Gram-negative outer membrane to overcome intrinsic resistance

Because of the rise in antibiotic resistance and the dwindling pipeline of effective antibiotics, it is imperative to explore avenues that breathe new life into existing drugs. This is particularly important for intrinsically resistant Gram-negative bacteria, which are exceedingly difficult to treat. The Gram-negative outer membrane (OM) prevents the entry of a plethora of antibiotics that are effective against Gram-positive bacteria, despite the presence of the targets of these drugs. Uncovering molecules that increase the permeability of the OM to sensitize Gram-negative bacteria to otherwise ineffective antibiotics is an approach that has recently garnered increased attention in the field. In this review, we survey chemical matter which has been shown to potentiate antibiotics against Gram-negative bacteria by perturbing the OM. These include peptides, nanoparticles, macromolecules, antibiotic conjugates, and small molecules.

Novel Small Molecule Growth Inhibitor Affecting Bacterial Outer Membrane Reduces Extraintestinal Pathogenic Escherichia coli (ExPEC) Infection in Avian Model

Avian pathogenic Escherichia coli (APEC), a subgroup of extraintestinal pathogenic E. coli (ExPEC), causes colibacillosis in chickens and is reportedly implicated in urinary tract infections and meningitis in humans. A major limitation for the current ExPEC antibiotic therapy is the development of resistance, and antibacterial drugs that can circumvent this problem are critically needed. Here, we evaluated eight novel membrane-affecting anti-APEC small molecule growth inhibitors (GIs), identified in our previous study, against APEC infection in chickens. Among the GIs tested, GI-7 (the most effective), when administered orally (1 mg/kg of body weight), reduced the mortality (41.7%), severity of lesions (62.9%), and APEC load (2.6 log) in chickens. Furthermore, GI-7 administration at an optimized dose (60 mg/liter) in drinking water also reduced the mortality (14.7%), severity of lesions (29.5%), and APEC load (2.2 log) in chickens. The abundances of Lactobacillus and oleate were increased in the cecum and serum, respectively, of GI-7-treated chickens. Pharmacokinetic analysis revealed that GI-7 was readily absorbed with minimal accumulation in the tissues. Earlier, we showed that GI-7 induced membrane blebbing and increased membrane permeability in APEC, suggesting an effect on the APEC membrane. Consistent with this finding, the expression of genes essential for maintaining outer membrane (OM) integrity was downregulated in GI-7-treated APEC. Furthermore, decreased levels of lipopolysaccharide (LPS) transport (Lpt) proteins and LPS were observed in GI-7-treated APEC. However, the mechanism of action of GI-7 currently remains unknown and needs further investigation. Our studies suggest that GI-7 represents a promising novel lead compound that can be developed to treat APEC infection in chickens and related human ExPEC infections. IMPORTANCE APEC is a subgroup of ExPEC, and genetic similarities of APEC with human ExPECs, including uropathogenic E. coli (UPEC) and neonatal meningitis E. coli (NMEC), have been reported. Our study identified a novel small molecule growth inhibitor, GI-7, effective in reducing APEC infection in chickens with an efficacy similar to that of the currently used antibiotic sulfadimethoxine, notably with an 8-times-lower dose. GI-7 affects the OM integrity and decreases the Lpt protein and LPS levels in APEC, an antibacterial mechanism that can overcome the antibiotic resistance problem. Overall, GI-7 represents a promising lead molecule/scaffold for the development of novel antibacterial therapies that could have profound implications for treating APEC infections in chickens, as well as human infections caused by ExPECs and other related Gram-negative bacteria. Further elucidation of the mechanism of action of GI-7 and identification of its target(s) in APEC will benefit future novel antibacterial development efforts.

The cathelicidin-derived close-to-nature peptide D-11 sensitises Klebsiella pneumoniae to a range of antibiotics in vitro, ex vivo and in vivo

The outer membrane of Gram-negative bacteria constitutes a permeability barrier that prevents certain antibiotics reaching their target, thus conferring a high tolerance to a wide range of antibiotics. Combined therapies of antibiotics and outer membrane-perturbing drugs have been proposed as an alternative treatment to extend the use of antibiotics active against Gram-positive bacteria to Gram-negative bacteria. Among the outer membrane-active compounds, the outer membrane-permeabilising peptides play a prominent role. They form a group of small cationic and amphipathic molecules with the ability to insert specifically into bacterial membranes, inducing their permeabilisation and/or disruption. Here we assessed the combined effect of several compounds belonging to the main antibiotic families and the cathelicidin close-to-nature outer membrane peptide D-11 against four clinically relevant Gram-negative bacteria. The results showed that peptide D-11 displays strong synergistic activity with several antibiotics belonging to different families, in particular against Klebsiella pneumoniae, even better than some other outer membrane-active peptides that are currently in clinical trials, such as SPR741. Notably, we observed this activity in vitro, ex vivo in a newly designed bacteraemia model, and in vivo in a mouse abscess infection model. Overall, our results suggest that D-11 is a good candidate to repurpose the activity of traditional antibiotics against K. pneumoniae.

Vancomyxins: Vancomycin-Polymyxin Nonapeptide Conjugates That Retain Anti-Gram-Positive Activity with Enhanced Potency against Gram-Negative Strains

Vancomycin functions by binding to lipid II, the penultimate bacterial cell wall building block used by both Gram-positive and Gram-negative species. However, vancomycin is generally only able to exert its antimicrobial effect against Gram-positive strains as it cannot pass the outer membrane (OM) of Gram-negative bacteria. To address this challenge, we here describe efforts to conjugate vancomycin to the OM disrupting polymyxin E nonapeptide (PMEN) to yield the hybrid “vancomyxins”. In designing these hybrid antibiotics, different spacers and conjugation sites were explored for connecting vancomycin and PMEN. The vancomyxins show improved activity against Gram-negative strains compared with the activity of vancomycin or vancomycin supplemented with PMEN separately. In addition, the vancomyxins maintain the antimicrobial effect of vancomycin against Gram-positive strains and, in some cases, show enhanced activity against vancomycin-resistant strains. The hybrid antibiotics described here have reduced nephrotoxicity when compared with clinically used polymyxin antibiotics. This study demonstrates that covalent conjugation to an OM disruptor contributes to sensitizing Gram-negative strains to vancomycin while retaining anti-Gram-positive activity.

Elucidating the mechanism by which synthetic helper peptides sensitize Pseudomonas aeruginosa to multiple antibiotics

The emergence and rapid spread of multi-drug resistant (MDR) bacteria pose a serious threat to the global healthcare. There is an urgent need for new antibacterial substances or new treatment strategies to deal with the infections by MDR bacterial pathogens, especially the Gram-negative pathogens. In this study, we show that a number of synthetic cationic peptides display strong synergistic antimicrobial effects with multiple antibiotics against the Gram-negative pathogen Pseudomonas aeruginosa. We found that an all-D amino acid containing peptide called D-11 increases membrane permeability by attaching to LPS and membrane phospholipids, thereby facilitating the uptake of antibiotics. Subsequently, the peptide can dissipate the proton motive force (PMF) (reducing ATP production and inhibiting the activity of efflux pumps), impairs the respiration chain, promotes the production of reactive oxygen species (ROS) in bacterial cells and induces intracellular antibiotics accumulation, ultimately resulting in cell death. By using a P. aeruginosa abscess infection model, we demonstrate enhanced therapeutic efficacies of the combination of D-11 with various antibiotics. In addition, we found that the combination of D-11 and azithromycin enhanced the inhibition of biofilm formation and the elimination of established biofilms. Our study provides a realistic treatment option for combining close-to-nature synthetic peptide adjuvants with existing antibiotics to combat infections caused by P. aeruginosa.

Brevibacillin 2V, a Novel Antimicrobial Lipopeptide With an Exceptionally Low Hemolytic Activity

Bacterial non-ribosomally produced peptides (NRPs) form a rich source of antibiotics, including more than 20 of these antibiotics that are used in the clinic, such as penicillin G, colistin, vancomycin, and chloramphenicol. Here we report the identification, purification, and characterization of a novel NRP, i.e., brevibacillin 2V (lipo-tridecapeptide), from Brevibacillus laterosporus DSM 25. Brevibacillin 2V has a strong antimicrobial activity against Gram-positive bacterial pathogens (minimum inhibitory concentration = 2 mg/L), including difficult-to-treat antibiotic-resistant Enterococcus faecium, Enterococcus faecalis, and Staphylococcus aureus. Notably, brevibacillin 2V has a much lower hemolytic activity (HC₅₀ > 128 mg/L) and cytotoxicity (CC₅₀ = 45.49 ± 0.24 mg/L) to eukaryotic cells than previously reported NRPs of the lipo-tridecapeptide family, including other brevibacillins, which makes it a promising candidate for antibiotic development. In addition, our results demonstrate that brevibacillins display a synergistic action with established antibiotics against Gram-negative bacterial pathogens. Probably due to the presence of non-canonical amino acids and D-amino acids, brevibacillin 2V showed good stability in human plasma. Thus, we identified and characterized a novel and promising antimicrobial candidate (brevibacillin 2V) with low hemolytic activity and cytotoxicity, which can be used either on its own or as a template for further total synthesis and modification.

Identification of Unique Quinazolone Thiazoles as Novel Structural Scaffolds for Potential Gram-Negative Bacterial Conquerors

A class of quinazolone thiazoles was identified as new structural scaffolds for potential antibacterial conquerors to tackle dreadful resistance. Some prepared compounds exhibited favorable bacteriostatic efficiencies on tested bacteria, and the most representative 5j featuring the 4-trifluoromethylphenyl group possessed superior performances against Escherichia coli and Pseudomonas aeruginosa to norfloxacin. Further studies revealed that 5j with inappreciable hemolysis could hinder the formation of bacterial biofilms and trigger reactive oxygen species generation, which could take responsibility for emerging low resistance. Subsequent paralleled exploration discovered that 5j not only disintegrated outer and inner membranes to induce leakage of cytoplasmic contents but also broke the metabolism by suppressing dehydrogenase. Meanwhile, derivative 5j could intercalate into DNA to exert powerful antibacterial properties. Moreover, compound 5j gave synergistic effects against some Gram-negative bacteria in combination with norfloxacin. These findings indicated that this novel structural type of quinazolone thiazoles showed therapeutic foreground in struggling with Gram-negative bacterial infections.

Thrombin-Derived Peptides Potentiate the Activity of Gram-Positive-Specific Antibiotics against Gram-Negative Bacteria

The continued rise of antibiotic resistance threatens to undermine the utility of the world’s current antibiotic arsenal. This problem is particularly troubling when it comes to Gram-negative pathogens for which there are inherently fewer antibiotics available. To address this challenge, recent attention has been focused on finding compounds capable of disrupting the Gram-negative outer membrane as a means of potentiating otherwise Gram-positive-specific antibiotics. In this regard, agents capable of binding to the lipopolysaccharide (LPS) present in the Gram-negative outer membrane are of particular interest as synergists. Recently, thrombin-derived C-terminal peptides (TCPs) were reported to exhibit unique LPS-binding properties. We here describe investigations establishing the capacity of TCPs to act as synergists with the antibiotics erythromycin, rifampicin, novobiocin, and vancomycin against multiple Gram-negative strains including polymyxin-resistant clinical isolates. We further assessed the structural features most important for the observed synergy and characterized the outer membrane permeabilizing activity of the most potent synergists. Our investigations highlight the potential for such peptides in expanding the therapeutic range of antibiotics typically only used to treat Gram-positive infections.