Short cationic lipopeptides as effective antibacterial agents: Design, physicochemical properties and biological evaluation.

The spread of drug-resistant bacteria has imparted a sense of urgency in the search for new antibiotics. In an effort to develop a new generation of antibacterial agents, we have designed de novo charged lipopeptides inspired by natural antimicrobial peptides. These short lipopeptides are composed of cationic lysine and hydrophobic lipoamino acids that replicate the amphiphilic properties of natural antimicrobial peptides. The resultant lipopeptides were found to self-assemble into nanoparticles. Some were effective against a variety of Gram-positive bacteria, including strains resistant to methicillin, daptomycin and/or vancomycin. The lipopeptides were not toxic to human kidney and liver cell lines and were highly resistant to tryptic degradation. Transmission electron microscopy analysis of bacteria cells treated with lipopeptide showed membrane-damage and lysis with extrusion of cytosolic contents. With such properties in mind, these lipopeptides have the potential to be developed as new antibacterial agents against drug-resistant Gram-positive bacteria.

Semi-synthesis and NMR spectral assignments of flavonoid and chalcone derivatives.

Previous investigations of the aerial parts of the Australian plant Eremophila microtheca and Syzygium tierneyanum resulted in the isolation of the antimicrobial flavonoid jaceosidin (4) and 2',6'-dihydroxy-4'-methoxy-3',5'-dimethyl chalcone (7), respectively. In this current study, compounds 4 and 7 were derivatized by acetylation, pivaloylation, and methylation reactions. The final products, 5,7,4'-triacetoxy jaceosidin (10), 5,7,4'-tripivaloyloxy jaceosidin (11), 5,7,4'-trimethoxy jaceosidin (12), 2',6'-diacetoxy-4'-methoxy-3',5'-dimethyl chalcone (13), 2'-hydroxy-4'-methoxy-6'-pivaloyloxy-3',5'-dimethyl chalcone (14), and 2'-hydroxy-4',6'-dimethoxy-3',5'-dimethyl chalcone (15) were all fully characterized by NMR and MS. Derivatives 10 and 13 have been previously reported but were only partially characterized. This is the first reported synthesis of 11 and 14. The natural products and their derivatives were evaluated for their antibacterial and antifungal properties, and the natural product, jaceosidin (4) and the acetylated derivative, 5,7,4'-triacetoxy jaceosidin (10), showed modest antibacterial activity (32-128 µg/ml) against Staphylococcus aureus strains. Copyright © 2016 John Wiley & Sons, Ltd.

An azido-oxazolidinone antibiotic for live bacterial cell imaging and generation of antibiotic variants.

An azide-functionalised analogue of the oxazolidinone antibiotic linezolid was synthesised and shown to retain antimicrobial activity. Using facile 'click' chemistry, this versatile intermediate can be further functionalised to explore antimicrobial structure-activity relationships or conjugated to fluorophores to generate fluorescent probes. Such probes can report bacteria and their location in a sample in real time. Modelling of the structures bound to the cognate 50S ribosome target demonstrates binding to the same site as linezolid is possible. The fluorescent probes were successfully used to image Gram-positive bacteria using confocal microscopy.

A lipoglycopeptide antibiotic for Gram-positive biofilm-related infections.

Drug-resistant Gram-positive bacterial infections are still a substantial burden on the public health system, with two bacteria (Staphylococcus aureus and Streptococcus pneumoniae) accounting for over 1.5 million drug-resistant infections in the United States alone in 2017. In 2019, 250,000 deaths were attributed to these pathogens globally. We have developed a preclinical glycopeptide antibiotic, MCC5145, that has excellent potency (MIC90 ≤ 0.06 µg/ml) against hundreds of isolates of methicillin-resistant S. aureus (MRSA) and other Gram-positive bacteria, with a greater than 1000-fold margin over mammalian cell cytotoxicity values. The antibiotic has therapeutic in vivo efficacy when dosed subcutaneously in multiple murine models of established bacterial infections, including thigh infection with MRSA and blood septicemia with S. pneumoniae, as well as when dosed orally in an antibiotic-induced Clostridioides difficile infection model. MCC5145 exhibited reduced nephrotoxicity at microbiologically active doses in mice compared to vancomycin. MCC5145 also showed improved activity against biofilms compared to vancomycin, both in vitro and in vivo, and a low propensity to select for drug resistance. Characterization of drug action using a transposon library bioinformatic platform showed a mechanistic distinction from other glycopeptide antibiotics.

The antimicrobial potential of cannabidiol.

Antimicrobial resistance threatens the viability of modern medicine, which is largely dependent on the successful prevention and treatment of bacterial infections. Unfortunately, there are few new therapeutics in the clinical pipeline, particularly for Gram-negative bacteria. We now present a detailed evaluation of the antimicrobial activity of cannabidiol, the main non-psychoactive component of cannabis. We confirm previous reports of Gram-positive activity and expand the breadth of pathogens tested, including highly resistant Staphylococcus aureus, Streptococcus pneumoniae, and Clostridioides difficile. Our results demonstrate that cannabidiol has excellent activity against biofilms, little propensity to induce resistance, and topical in vivo efficacy. Multiple mode-of-action studies point to membrane disruption as cannabidiol's primary mechanism. More importantly, we now report for the first time that cannabidiol can selectively kill a subset of Gram-negative bacteria that includes the 'urgent threat' pathogen Neisseria gonorrhoeae. Structure-activity relationship studies demonstrate the potential to advance cannabidiol analogs as a much-needed new class of antibiotics.

In vitro Antimicrobial Activity of Acne Drugs Against Skin-Associated Bacteria.

Acne is a common skin affliction that involves excess sebum production and modified lipid composition, duct blockage, colonization by bacteria, and inflammation. Acne drugs target one or more of these steps, with antibiotics commonly used to treat the microbial infection for moderate to severe cases. Whilst a number of other acne therapies are purported to possess antimicrobial activity, this has been poorly documented in many cases. We conducted a comparative analysis of the activity of common topical acne drugs against the principal etiological agent associated with acne: the aerotolerant anaerobic Gram-positive organism Propionibacterium acnes (recently renamed as Cutibacterium acnes). We also assessed their impact on other bacteria that could also be affected by topical treatments, including both antibiotic-sensitive and antibiotic-resistant strains, using broth microdilution assay conditions. Drugs designated specifically as antibiotics had the greatest potency, but lost activity against resistant strains. The non-antibiotic acne agents did possess widespread antimicrobial activity, including against resistant strains, but at substantially higher concentrations. Hence, the antimicrobial activity of non-antibiotic acne agents may provide protection against a background of increased drug-resistant bacteria.

Protein-inspired antibiotics active against vancomycin- and daptomycin-resistant bacteria.

The public health threat posed by a looming 'post-antibiotic' era necessitates new approaches to antibiotic discovery. Drug development has typically avoided exploitation of membrane-binding properties, in contrast to nature's control of biological pathways via modulation of membrane-associated proteins and membrane lipid composition. Here, we describe the rejuvenation of the glycopeptide antibiotic vancomycin via selective targeting of bacterial membranes. Peptide libraries based on positively charged electrostatic effector sequences are ligated to N-terminal lipophilic membrane-insertive elements and then conjugated to vancomycin. These modified lipoglycopeptides, the 'vancapticins', possess enhanced membrane affinity and activity against methicillin-resistant Staphylococcus aureus (MRSA) and other Gram-positive bacteria, and retain activity against glycopeptide-resistant strains. Optimised antibiotics show in vivo efficacy in multiple models of bacterial infection. This membrane-targeting strategy has potential to 'revitalise' antibiotics that have lost effectiveness against recalcitrant bacteria, or enhance the activity of other intravenous-administered drugs that target membrane-associated receptors.

Structure-Activity and -Toxicity Relationships of the Antimicrobial Peptide Tachyplesin-1.

Tachyplesin-1 (TP1; 1) is a cationic ß-hairpin antimicrobial peptide with a membranolytic mechanism of action. While it possesses broad-spectrum, potent antimicrobial activity, 1 is highly hemolytic against mammalian erythrocytes, which precludes it from further development. In this study, we report a template-based approach to investigate the structure-function and structure-toxicity relationships of each amino acid of 1. We modulated charge and hydrophobicity by residue modification and truncation of the peptide. Antimicrobial activity was then assessed against six key bacterial pathogens and two fungi, with toxicity profiled against mammalian cells. The internal disulfide bridge Cys7-Cys12 of 1 was shown to play an important role in broad-spectrum antimicrobial activity against all pathogenic strains tested. Novel peptides based on the progenitor were then designed, including 5 (TP1[F4A]), 12 (TP1[I11A]), and 19 (TP1[C3A,C16A]). These had 26- to 64-fold improved activity/toxicity indices and show promise for further development. Structural studies of 5 (TP1[F4A]) and 12 (TP1[I11A]) identified a conserved ß-hairpin secondary structure motif correlating with their very high stablility in mouse and human plasma. Membrane binding affinity determined by surface plasmon resonance confirmed their selectivity toward bacterial membranes, but the degree of membrane binding did not correlate with the degree of hemolysis, suggesting that other factors may drive toxicity.

Contribution of Amphipathicity and Hydrophobicity to the Antimicrobial Activity and Cytotoxicity of ß-Hairpin Peptides.

Bacteria have acquired extensive resistance mechanisms to protect themselves against antibiotic action. Today the bacterial membrane has become one of the "final frontiers" in the search for new compounds acting on novel targets to address the threat of multi-drug resistant (MDR) and XDR bacterial pathogens. ß-Hairpin antimicrobial peptides are amphipathic, membrane-binding antibiotics that exhibit a broad range of activities against Gram-positive, Gram-negative, and fungal pathogens. However, most members of the class also possess adverse cytotoxicity and hemolytic activity that preclude their development as candidate antimicrobials. We examined peptide hydrophobicity, amphipathicity, and structure to better dissect and understand the correlation between antimicrobial activity and toxicity, membrane binding, and membrane permeability. The hydrophobicity, pI, net charge at physiological pH, and amphipathic moment for the ß-hairpin antimicrobial peptides tachyplesin-1, polyphemusin-1, protegrin-1, gomesin, arenicin-3, and thanatin were determined and correlated with key antimicrobial activity and toxicity data. These included antimicrobial activity against five key bacterial pathogens and two fungi, cytotoxicity against human cell lines, and hemolytic activity in human erythrocytes. Observed antimicrobial activity trends correlated with compound amphipathicity and, to a lesser extent, with overall hydrophobicity. Antimicrobial activity increased with amphipathicity, but unfortunately so did toxicity. Of note, tachyplesin-1 was found to be 8-fold more amphipathic than gomesin. These analyses identify tachyplesin-1 as a promising scaffold for rational design and synthetic optimization toward an antibiotic candidate.

Activity and Predicted Nephrotoxicity of Synthetic Antibiotics Based on Polymyxin B.

The polymyxin lipodecapeptides colistin and polymyxin B have become last resort therapies for infections caused by highly drug-resistant Gram-negative bacteria. Unfortunately, their utility is compromised by significant nephrotoxicity and polymyxin-resistant bacterial strains. We have conducted a systematic activity-toxicity investigation by varying eight of the nine polymyxin amino acid free side chains, preparing over 30 analogues using a novel solid-phase synthetic route. Compounds were tested against a panel of Gram-negative bacteria and counter-screened for in vitro cell toxicity. Promising compounds underwent additional testing against primary kidney cells isolated from human kidneys to better predict their nephrotoxic potential. Many of the new compounds possessed equal or better antimicrobial potency compared to polymyxin B, and some were less toxic than polymyxin B and colistin against mammalian HepG2 cells and human primary kidney cells. These initial structure-activity and structure-toxicity studies set the stage for further improvements to the polymyxin class of antibiotics.

Metronidazole-triazole conjugates: activity against Clostridium difficile and parasites.

Metronidazole has been used clinically for over 50 years as an antiparasitic and broad-spectrum antibacterial agent effective against anaerobic bacteria. However resistance to metronidazole in parasites and bacteria has been reported, and improved second-generation metronidazole analogues are needed. The copper catalysed Huigsen azide-alkyne 1,3-dipolar cycloaddition offers a way to efficiently assemble new libraries of metronidazole analogues. Several new metronidazole-triazole conjugates (Mtz-triazoles) have been identified with excellent broad spectrum antimicrobial and antiparasitic activity targeting Clostridium difficile, Entamoeba histolytica and Giardia lamblia. Cross resistance to metronidazole was observed against stable metronidazole resistant C. difficile and G. lamblia strains. However for the most potent Mtz-triazoles, the activity remained in a therapeutically relevant window.

Antibacterial serrulatane diterpenes from the Australian native plant Eremophila microtheca.

Chemical investigations of the aerial parts of the Australian plant Eremophila microtheca resulted in the isolation of three serrulatane diterpenoids, 3-acetoxy-7,8-dihydroxyserrulat-14-en-19-oic acid (1), 3,7,8-trihydroxyserrulat-14-en-19-oic acid (2) and 3,19-diacetoxy-8-hydroxyserrulat-14-ene (3) as well as the previously reported compounds verbascoside (4) and jaceosidin (5). Acetylation and methylation of the major serrulatane diterpenoid 2 afforded 3,8-diacetoxy-7-hydroxyserrulat-14-en-19-oic acid (6) and 3,7,8-trihydroxyserrulat-14-en-19-oic acid methyl ester (7), respectively. The antibacterial activity of 1-7 was assessed against a panel of Gram-positive and Gram-negative bacterial isolates. All of the serrulatane compounds exhibited moderate activity against Streptococcus pyogenes (ATCC 12344) with minimum inhibitory concentrations (MICs) ranging from 64-128 µg/mL. Serrulatane 1 demonstrated activity against all Gram-positive bacterial strains (MICs 64-128 µg/mL) except for Enterococcus faecalis and Enterococcus faecium. This is the first report of natural products from E. microtheca.