Building a Better Quaternary Ammonium Compound (QAC): Branched Tetracationic Antiseptic Amphiphiles

Quaternary ammonium salts based on caprylic acid as antimicrobial and surface-active agents

In this work, amidequats and esterquats based on caprylic acid were investigated as promising compounds with surface properties and biological activity that are in harmony with the principles of green chemistry. Herein, caprylic acid, which is an essential component of the above compounds, is a noteworthy natural resource. Structural analysis was performed with the amphiphilic cations of the tested amidequats and esterquats, revealing two distinct factors, i.e., the elongation of the alkyl chain and the presence of two different functional groups; these factors undoubtedly affect the desired biological activity. These compounds were synthesized and characterized in terms of their physicochemical properties, among which surface activity is pivotal. In addition, the surfaces of the tested compounds were investigated through a detailed topographical analysis. The obtained results suggested that the esterquats exhibited higher surface activity, wettability and foamability than the amidequats. Antimicrobial studies, on the other hand, are not as conclusive. For shorter chains, esterquats are more active than amidequats, while for longer chains (over C12), the trend was the opposite. The amidequats and esterquats presented in this research may be a potential good replacement for antimicrobial formulations or as alternatives to surface-active agents used in industry.

Combinatorial Screening of Cationic Lipidoids Reveals How Molecular Conformation Affects Membrane-Targeting Antimicrobial Activity

The search for next-generation antibacterial compounds that overcome the development of resistance can be facilitated by identifying how to target the cell membrane of bacteria. Understanding the key molecular features that enable interactions with lipids and lead to membrane disruption is therefore crucial. Here, we employ a library of lipid-like compounds (lipidoids) comprising modular structures with tunable hydrophobic and hydrophilic architecture to shed light on how the chemical functionality and molecular shape of synthetic amphiphilic compounds determine their activity against bacterial membranes. Synthesized from combinations of 8 different polyamines as headgroups and 13 acrylates as tails, 104 different lipidoids are tested for activity against a model Gram-positive bacterial strain (Bacillus subtilis). Results from the combinatorial screening assay show that lipidoids with the most potent antimicrobial properties (down to 2 μM) have intermediate tail hydrophobicity (i.e., c log P values between 3 and 4) and lower headgroup charge density (i.e., longer spacers between charged amines). However, the most important factor appeared to be the ability of a lipidoid to self-assemble into an inverse hexagonal liquid crystalline phase, as observed by small-angle X-ray scattering (SAXS) analysis. The lipidoids active at lowest concentrations, which induced the most significant membrane damage during propidium iodide (PI) permeabilization assays, were those that aggregated into highly curved inverse hexagonal liquid crystal phases. These observations suggest that the introduction of strong curvature stress into the membrane is one way to maximize membrane disruption and lipidoid antimicrobial activity. Lipidoids that demonstrated the ability to furnish this phase consisted of either (i) branched or linear headgroups with shorter linear tails or (ii) cyclic headgroups with 4 bulky nonlinear tails. On the contrary, lipidoids previously observed to adopt disc-like conformations that pack into bicontinuous cubic phases were significantly less effective against B. subtilis. The discovery of these structure-property relationships demonstrates that it is not simply a balance of hydrophobic and hydrophilic moieties that govern membrane-active antibacterial activity, but also their intrinsic curvature and collective behavior.

Structure-Activity Relationships of Cationic Lipidoids against Escherichia coli

Membrane-active molecules provide a promising strategy to target and kill pathogenic bacteria. Understanding how specific molecular features drive interactions with membrane components and subsequently cause disruption that leads to antimicrobial activity is a crucial step in designing next-generation treatments. Here, we test a library of lipid-like compounds (lipidoids) against Gram-negative bacteria Escherichia coli to garner in-depth structure-activity relationships using antimicrobial assays. Modular lipidoid molecules were synthesized in high-throughput, such that we could analyze 104 compounds with variable combinations of hydrophobic tails and cationic headgroups. Antibacterial activity was strongly correlated to specific structural features, including tail hydrophobicity and headgroup charge density, and also to the overall molecular shape and propensity for self-assembly into curved liquid crystalline phases. Dye permeabilization assays showed that E. coli membranes were permeabilized by lipidoids, confirming their membrane-active nature. The reduced permeabilization, as compared to Gram-positive Bacillus subtilis, alludes to the challenge of permeabilizing the additional outer membrane layer of E. coli. The effect of headgroup solubility in gemini-type lipidoids was also demonstrated, revealing that a headgroup with a more hydrophilic spacer between amine groups had enhanced activity against B. subtilis but not E. coli. This provides insight into features enabling outer membrane penetration and governing selectivity between bacterial species.

Does thermotropic liquid crystalline self-assembly control biological activity in amphiphilic amino acids? - tyrosine ILCs as a case study

Amphiphilic amino acids represent promising scaffolds for biologically active soft matter. In order to understand the bulk self-assembly of amphiphilic amino acids into thermotropic liquid crystalline phases and their biological properties a series of tyrosine ionic liquid crystals (ILCs) was synthesized, carrying a benzoate unit with 0-3 alkoxy chains at the tyrosine unit and a cationic guanidinium head group. Investigation of the mesomorphic properties by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (WAXS, SAXS) revealed smectic A bilayers (SmA_d) for ILCs with 4-alkoxy- and 3,4-dialkoxybenzoates, whereas ILCs with 3,4,5-trisalkoxybenzoates showed hexagonal columnar mesophases (Col_h), while different counterions had only a minor influence. Dielectric measurements revealed a slightly higher dipole moment of non-mesomorphic tyrosine-benzoates as compared to their mesomorphic counterparts. The absence of lipophilic side chains on the benzoate unit was important for the biological activity. Thus, non-mesomorphic tyrosine benzoates and crown ether benzoates devoid of additional side chains at the benzoate unit displayed the highest cytotoxicities (against L929 mouse fibroblast cell line) and antimicrobial activity (against Escherichia coli ΔTolC and Staphylococcus aureus) and promising selectivity ratio in favour of antimicrobial activity.

The Antimicrobial and Antibiofilm Potential of New Water-Soluble Tris-Quaternary Ammonium Compounds

The invention and innovation of highly effective antimicrobials are always crucial tasks for medical and organic chemistry, especially at the current time, when there is a serious threat of shortages of effective antimicrobials following the pandemic. In the study presented in this article, we established a new approach to synthesizing three novel series of bioactive water-soluble tris-quaternary ammonium compounds using an optimized one-pot method, and we assessed their antimicrobial and antibiofilm potential. Five pathogenic microorganisms of the ESKAPE group, including highly resistant clinical isolates, were used as the test samples. Moreover, we highlighted the dependence of antibacterial activity from the hydrophilic-hydrophobic balance of the QACs and noted the significant performance of the desired products on biofilms with MBEC as low as 16 mg/L against bacteria and 8 mg/L against fungi. Particularly notable was the high activity against Pseudomonas aeruginosa and Acinetobacter baumannii, which are among the most resilient bacteria known. The presented work will provide useful insights for future research on the topic.

Multicationic Quaternary Ammonium Compounds: A Framework for Combating Bacterial Resistance

During previous stages of research, high biocidal activity toward microorganism archival strains has been used as the main indicator in the development of new antiseptic formulations. Although this factor remains one of the most important characteristics of biocide efficiency, the scale of antimicrobial resistance spread causes serious concern. Therefore, focus shifts toward the development of formulations with a stable effect even in the case of prolonged contact with pathogens. Here, we introduce an original isocyanuric acid alkylation method with the use of available alkyl dichlorides, which opened access to a wide panel of multi-QACs with alkyl chains of various lengths between the nitrogen atoms of triazine and pyridine cycles. We used a complex approach for the resulting series of 17 compounds, including their antibiofilm properties, bacterial tolerance development, and antimicrobial activity toward multiresistant pathogenic strains. As a result of these efforts, available compounds have shown higher levels of antibacterial activity against ESKAPE pathogens than widely used commercial QACs. Hit compounds possessed high activity toward clinical bacterial strains and have also demonstrated a long-term biocidal effect without significant development of microorganism tolerance. The overall results indicated a high level of antibacterial activity and the broad application prospects of multi-QACs based on isocyanuric acid against multiresistant bacterial strains.

A Dicationic BODIPY-Based Fluorescent Bactericide to Combat Infectious Diseases and to Eradicate Bacterial Biofilms

Increased bacterial resistance against extensively used common disinfectants has begun to emerge. The discovery of disinfectants substituting the current commercially available ones is strongly needed. For this purpose, a dicationic BODIPY-based fluorescent amphiphile has been synthesized by specific molecular design. This quaternized BODIPY behaves as a broad-spectrum disinfectant against both Gram-positive and Gram-negative bacteria strains. It exhibits potent antimicrobial activity against tested microorganisms when compared with structurally similar disinfectant benzalkonium chloride (BAC). Moreover, it shows antibiofilm activity against Staphylococcus epidermidis with a minimum biofilm eradication concentration as low as 16 μg/mL. The interaction of this compound with the bacterial cell and genomic DNA was further evaluated by fluorescence spectroscopy and microscopy to follow cell internationalization and to clarify the mechanism of antibacterial action.

From Antibacterial to Antibiofilm Targeting: An Emerging Paradigm Shift in the Development of Quaternary Ammonium Compounds (QACs)

In a previous development stage, mostly individual antibacterial activity was a target in the optimization of biologically active compounds and antiseptic agents. Although this targeting is still valuable, a new trend has appeared since the discovery of superhigh resistance of bacterial cells upon their aggregation into groups. Indeed, it is now well established that the great majority of pathogenic germs are found in the environment as surface-associated microbial communities called biofilms. The protective properties of biofilms and microbial resistance, even to high concentrations of biocides, cause many chronic infections in medical settings and lead to serious economic losses in various areas. A paradigm shift from individual bacterial targeting to also affecting more complex cellular frameworks is taking place and involves multiple strategies for combating biofilms with compounds that are effective at different stages of microbiome formation. Quaternary ammonium compounds (QACs) play a key role in many of these treatments and prophylactic techniques on the basis of both the use of individual antibacterial agents and combination technologies. In this review, we summarize the literature data on the effectiveness of using commercially available and newly synthesized QACs, as well as synergistic treatment techniques based on them. As an important focus, techniques for developing and applying antimicrobial coatings that prevent the formation of biofilms on various surfaces over time are discussed. The information analyzed in this review will be useful to researchers and engineers working in many fields, including the development of a new generation of applied materials; understanding biofilm surface growth; and conducting research in medical, pharmaceutical, and materials sciences. Although regular studies of antibacterial activity are still widely conducted, a promising new trend is also to evaluate antibiofilm activity in a comprehensive study in order to meet the current requirements for the development of highly needed practical applications.

Does Hypochlorous Acid Cause Ototoxicity? An Experimental Study

AIM

Hypochlorous acid (HOCl) is a weak acid that ionizes in water. It is an effective antiseptic exhibiting low toxicity on living tissues. We aimed to investigate the ototoxic effects of HOCl on an animal model by using electrophysiological and histological methods.

MATERIALS AND METHODS

The study comprised 32 Sprague-Dawley rats, which were separated into four groups: control group (A), saline solution group (B), 70% isopropyl alcohol + 2% chlorhexidine group (C), and HOCl group (D). After recording the auditory brainstem response (ABR) for basal hearing thresholds (8, 16, 24, and 32 kHz), 0.03 ml of the aforementioned materials was injected intratympanically three times every 2 days in groups B, C, and D. ABR measurements were repeated on the 7th and 21st days. All animals were sacrificed, and temporal bones were prepared for examinations of cochlear histology and vascular endothelial growth factor immunohistochemistry.

RESULTS

Basal hearing levels were normal across all frequencies and groups, with no statistical differentiation. On the 7th and 21st days after the ABR test, all other groups demonstrated a significant deterioration in hearing levels compared with group A. When the results from 7th and 21st days were compared within group D, a partial recovery was observed. In histopathology, groups C and D demonstrated moderate and severe cochlear degeneration, along with decreased immunoreactivity in the organ of Corti, stria vascularis, and spiral ligament.

CONCLUSION

This is the first study to evaluate the safety of using HOCl in otology. Although HOCI is less ototoxic than the disinfectant used, it may have a toxic effect on cochlea.Level of Evidence: Animal Research.

Quaternary Phosphonium Compound Unveiled as a Potent Disinfectant against Highly Resistant Acinetobacter baumannii Clinical Isolates

Acinetobacter baumannii is classified as a highest threat pathogen, urgently necessitating novel antimicrobials that evade resistance to combat its spread. Quaternary ammonium compounds (QACs) have afforded a valuable first line of defense against antimicrobial resistant pathogens as broad-spectrum amphiphilic disinfectant molecules. However, a paucity of innovation in this space has driven the emergence of QAC resistance. Through this work, we sought to identify next-generation disinfectant molecules with efficacy against highly resistant A. baumannii clinical isolates. We selected 12 best-in-class molecules from our previous investigations of quaternary ammonium and quaternary phosphonium compounds (QPCs) to test against a panel of 35 resistant A. baumannii clinical isolates. The results highlighted the efficacy of our next-generation compounds over leading commercial QACs, with our best-in-class QAC (2Pyr-11,11) and QPC (P6P-10,10) displaying improved activities with a few exceptions. Furthermore, we elucidated a correlation between colistin resistance and QAC resistance, wherein the only pan-resistant isolate of the panel, also harboring colistin resistance, exhibited resistance to all tested QACs. Notably, P6P-10,10 maintained efficacy against this strain with an IC₉₀ of 3 μM. In addition, P6P-10,10 displayed minimum biofilm eradication concentrations as low as 32 μM against extensively drug resistant clinical isolates. Lastly, examining the development of disinfectant resistance and cross-resistance, we generated QAC-resistant A. baumannii mutants and observed the development of QAC cross-resistance. In contrast, neither disinfectant resistance nor cross-resistance was observed in A. baumannii under P6P-10,10 treatment. Taken together, the results of this work illustrate the need for novel disinfectant compounds to treat resistant pathogens, such as A. baumannii, and underscore the promise of QPCs, such as P6P-10,10, as viable next-generation disinfectant molecules.

Antibacterial quaternary ammonium agents: Chemical diversity and biological mechanism

Bacterial infections have seriously threatened public health especially with the increasing resistance and the cliff-like decline of the number of newly approved antibacterial agents. Quaternary ammonium compounds (QACs) possess potent medicinal properties with 95 successfully marketed drugs, which also have a long history as antibacterial agents. In this review, we summarize the chemical diversity of antibacterial QACs, divided into chain-like and aromatic ring, reported over the past decade (2012 to mid-2022). Additionally, the structure-activity relationships, mainly covering hydrophobicity, charges and skeleton features, are discussed. In the cases where sufficient information is available, antibacterial mechanisms including biofilm, cell membrane, and intracellular targets are presented. It is hoped that this review will provide sufficient information for medicinal chemists to discover the new generation of antibacterial agents based on QACs.

Diptool-A Novel Numerical Tool for Membrane Interactions Analysis, Applying to Antimicrobial Detergents and Drug Delivery Aids

The widespread problem of resistance development in bacteria has become a critical issue for modern medicine. To limit that phenomenon, many compounds have been extensively studied. Among them were derivatives of available drugs, but also alternative novel detergents such as Gemini surfactants. Over the last decade, they have been massively synthesized and studied to obtain the most effective antimicrobial agents, as well as the most selective aids for nanoparticles drug delivery. Various protocols and distinct bacterial strains used in Minimal Inhibitory Concentration experimental studies prevented performance benchmarking of different surfactant classes over these last years. Motivated by this limitation, we designed a theoretical methodology implemented in custom fast screening software to assess the surfactant activity on model lipid membranes. Experimentally based QSAR (quantitative structure-activity relationship) prediction delivered a set of parameters underlying the Diptool software engine for high-throughput agent-membrane interactions analysis. We validated our software by comparing score energy profiles with Gibbs free energy from the Adaptive Biasing Force approach on octenidine and chlorhexidine, popular antimicrobials. Results from Diptool can reflect the molecule behavior in the lipid membrane and correctly predict free energy of translocation much faster than classic molecular dynamics. This opens a new venue for searching novel classes of detergents with sharp biologic activity.

Pyridinium Modified Anthracenes and Their Endoperoxides Provide a Tunable Scaffold with Activity against Gram-Positive and Gram-Negative Bacteria

Due to the emergence of multidrug resistant bacteria, the development of new antibiotics is required. We introduce here asymmetrically modified positively charged bis(methylpyridinium) anthracenes as a novel tunable scaffold, in which the two positive charges can be placed at a defined distance and angle. Our structure-activity relationship reveals that coupling the methylpyridiniums with alkynyl linkers to the central anthracene unit yields antibacterial compounds against a wide range of bacteria, including Escherichia coli, Staphylococcus aureus, and Staphylococcus epidermidis. Also, different mycobacteria, such as Mycobacterium smegmatis and Mycobacterium tuberculosis, are efficiently targeted by these compounds. The antibacterial activity depends on the number of alkynyl linkers and consequently also on the distance of the positive charges in the rigid anthracene scaffold. Additionally, the formation of an anthracene endoperoxide further increases the antibacterial activity, likely due to the release of toxic singlet oxygen that converts the endoperoxide back to the antibacterial anthracene scaffold with half-lives of several hours.

Quaternary Ammonium Compounds (QACs) and Ionic Liquids (ILs) as Biocides: From Simple Antiseptics to Tunable Antimicrobials

Quaternary ammonium compounds (QACs) belong to a well-known class of cationic biocides with a broad spectrum of antimicrobial activity. They are used as essential components in surfactants, personal hygiene products, cosmetics, softeners, dyes, biological dyes, antiseptics, and disinfectants. Simple but varied in their structure, QACs are divided into several subclasses: Mono-, bis-, multi-, and poly-derivatives. Since the beginning of the 20th century, a significant amount of work has been dedicated to the advancement of this class of biocides. Thus, more than 700 articles on QACs were published only in 2020, according to the modern literature. The structural variability and diverse biological activity of ionic liquids (ILs) make them highly prospective for developing new types of biocides. QACs and ILs bear a common key element in the molecular structure-quaternary positively charged nitrogen atoms within a cyclic or acyclic structural framework. The state-of-the-art research level and paramount demand in modern society recall the rapid development of a new generation of tunable antimicrobials. This review focuses on the main QACs exhibiting antimicrobial and antifungal properties, commercial products based on QACs, and the latest discoveries in QACs and ILs connected with biocide development.

Antimicrobial Peptides Derived From Insects Offer a Novel Therapeutic Option to Combat Biofilm: A Review

Biofilms form a complex layer with defined structures, that attach on biotic or abiotic surfaces, are tough to eradicate and tend to cause some resistance against most antibiotics. Several studies confirmed that biofilm-producing bacteria exhibit higher resistance compared to the planktonic form of the same species. Antibiotic resistance factors are well understood in planktonic bacteria which is not so in case of biofilm producing forms. This may be due to the lack of available drugs with known resistance mechanisms for biofilms. Existing antibiotics cannot eradicate most biofilms, especially of ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species). Insects produce complex and diverse set of chemicals for survival and defense. Antimicrobial peptides (AMPs), produced by most insects, generally have a broad spectrum of activity and the potential to bypass the resistance mechanisms of classical antibiotics. Besides, AMPs may well act synergistically with classical antibiotics for a double-pronged attack on infections. Thus, AMPs could be promising alternatives to overcome medically important biofilms, decrease the possibility of acquired resistance and treatment of multidrug-resistant pathogens including ESKAPE. The present review focuses on insect-derived AMPs with special reference to anti-biofilm-based strategies. It covers the AMP composition, pathways and mechanisms of action, the formation of biofilms, impact of biofilms on human diseases, current strategies as well as therapeutic options to combat biofilm with antimicrobial peptides from insects. In addition, the review also illustrates the importance of bioinformatics tools and molecular docking studies to boost the importance of select bioactive peptides those can be developed as drugs, as well as suggestions for further basic and clinical research.

An Efficient Synthesis of 3-Alkylpyridine Alkaloids Enables Their Biological Evaluation

3-Alkylpyridine alkaloids (3-APAs) isolated from the arctic sponge Haliclona viscosa are a promising group of bioactive marine alkaloids. However, due to limited bioavailability, investigations of their bioactivity have been hampered. Additionally, synthesis of a common intermediate requires the use of protecting groups and harsh conditions. In this work, we developed a simple and concise two-step route to nine different natural and synthetic haliclocyclins. These compounds displayed modest antibiotic activity against several Gram-positive bacterial strains.

Advancements in the Development of Non-Nitrogen-Based Amphiphilic Antiseptics to Overcome Pathogenic Bacterial Resistance

The prevalence of quaternary ammonium compounds (QACs) as common disinfecting agents for the past century has led bacteria to develop resistance to such compounds. Given the alarming increase in resistant strains, new strategies are required to combat this rise in resistance. Recent efforts to probe and combat bacterial resistance have focused on studies of multiQACs. Relatively unexplored, however, have been changes to the primary atom bearing positive charge in these antiseptics. Here we review the current state of the field of both phosphonium and sulfonium amphiphilic antiseptics, both of which hold promise as novel means to address bacterial resistance.

New details of assembling bioactive films from dispersions of amphiphilic molecules on titania surfaces

Tailoring the surface properties of materials for biomedical applications is important to avoid clinical complications. Forming thin layers of amphiphilic molecules with apolar regions that facilitate attractive intermolecular interactions, can be a suitable and versatile approach to achieve hydrophobic surface modification and provide functional antibacterial properties. Aiming to correlate layer structure and properties starting from film formation, octadecylphosphonic acid (ODPA) and dimethyloctadecyl (3-trimethoxysilylpropyl) ammonium chloride (DMOAP) layers were adsorbed onto smooth titania surfaces. Then the films were studied by atomic force microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS), and their interactions with aqueous environments were characterized by contact angle and zeta potential measurements. In addition, antibacterial assays were performed using E. coli and S. mutants to reveal the antibacterial properties effected by the surface modification. Immediately after sputter deposition, titania was hydrophilic; however, after air storage and adsorption of DMOAP or ODPA, an increase in the water contact angle was observed. XPS investigations after layer formation and after antibacterial tests revealed that the attachment of layers assembled from ODPA on titania substrates is considerably stronger and more stable than that observed for DMOAP films. Heat treatment strongly affects DMOAP layers. Furthermore, DMOAP layers are not stable under biological conditions.

Structure–property relationship of amphiphilic molecules on smooth substrates was explored through a multi-step approach and its influence on biological activity.

Biological Activity of Quaternary Ammonium Salts and Their Derivatives

Besides their positive role, microorganisms are related to a number of undesirable effects, including many diseases, biodeterioration and food spoilage, so when their presence is undesired, they must be controlled. Numerous biocides limiting the development of microorganisms have been proposed, however, in this paper the biocidal and inhibitory activity of quaternary ammonium salts (QASs) and their zwitterionic derivatives is addressed. This paper presents the current state of knowledge about the biocidal activity of QAS and their derivatives. Moreover, the known mechanisms of antimicrobial activity and the problem of emerging resistance to QAS are discussed. The latest trends in the study of surfactants and their potential use are also presented.

Synthesis, structure-activity relationship and biological evaluation of tetracationic gemini Dabco-surfactants for transdermal liposomal formulations

In this study, we report the relationship between structure, self-assembly behavior and antimicrobial activity of multicationic gemini surfactants and their successful use as stabilizers of a new liposomal formulation for transdermal drug delivery. New surfactants containing natural moiety 1,4-diazabicyclo[2.2.2]octane with four charges and two hydrophobic chains (n-Dabco-s-Dabco-n, where s = 2, 6, 12 and n = 12, 14, 16, 18) were synthesized. A linear dependence of the CMC decrease, with the increase of the number of carbon atoms in alkyl groups (slope 0.23) was shown. The aggregation numbers of n-Dabco-2-Dabco-n are smaller than 30 and they decrease with increasing alkyl chain length. This is in compliance with the larger surface area per n-Dabco-2-Dabco-n molecule. New liposomal formulations loading Rhodamine B phosphatidylcholine (with mean size about 100 nm and increased zeta potential from -7 ± 2 mV to +55 ± 2 mV) have been successfully stabilized by n-Dabco-s-Dabco-n surfactants. These formulations were designed to improve the bioavailability and skin permeation of loaded compound. The antibacterial activity of Dabco-surfactants was shown to be strongly affected by their structure (alkyl chain length and number of charged nitrogen). 12-Dabco-2-Dabco-12 was the most active (MIC = 0.48, 0.98 and 15.6 µg/mL against S. aureus, B. cereus and E. coli, respectively) without hemolytic activity at 3.1 µg/mL concentration. PC/14-Dabco-2-Dabco-14-liposomes were shown to be the best formulation, with the highest antibacterial activity against Sa (MIC = 7.8 μg‧mL^-1) and lowest cytotoxicity (IC₅₀ > 125). The modification of liposomes by Dabco-surfactants stabilizes the membrane of the vesicles, preventing the release of rhodamine B and impairing the penetration of the dye across Strat-M® membrane. Cellular uptake of rhodamine B-loaded PC/12-Dabco-2-Dabco-12-liposomes was also reported. This is the first example of cationic mixed liposomes containing Dabco-surfactants of potential interest for transdermal drug delivery.

Bacterial Biofilm Eradication Agents: A Current Review

Most free-living bacteria can attach to surfaces and aggregate to grow into multicellular communities encased in extracellular polymeric substances called biofilms. Biofilms are recalcitrant to antibiotic therapy and a major cause of persistent and recurrent infections by clinically important pathogens worldwide (e.g., Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus). Currently, most biofilm remediation strategies involve the development of biofilm-inhibition agents, aimed at preventing the early stages of biofilm formation, or biofilm-dispersal agents, aimed at disrupting the biofilm cell community. While both strategies offer some clinical promise, neither represents a direct treatment and eradication strategy for established biofilms. Consequently, the discovery and development of biofilm eradication agents as comprehensive, stand-alone biofilm treatment options has become a fundamental area of research. Here we review our current understanding of biofilm antibiotic tolerance mechanisms and provide an overview of biofilm remediation strategies, focusing primarily on the most promising biofilm eradication agents and approaches. Many of these offer exciting prospects for the future of biofilm therapeutics for a large number of infections that are currently refractory to conventional antibiotics.

More QACs, more questions: Recent advances in structure activity relationships and hurdles in understanding resistance mechanisms

Quaternary ammonium compounds (QACs) are a class of antimicrobials that have been around for over a century; nevertheless, they have found continued renewal in the structures to which they can be appended. Ranging from antimicrobial polymers to adding novel modes of action to existing antibiotics, QACs have found ongoing use due to their potent properties. However, resistance against QACs has begun to emerge, and the mechanism of resistance is still only partially understood. In this review, we aim to summarize the current state of the field and what is known about the mechanisms of resistance so that the QACs of the future can be designed to be evermore efficacious and utilized to unearth the remaining mysteries that surround bacteria's resistance to them.

Synthesis and Antibacterial Evaluation of Bis-thiazolium, Bis-imidazolium, and Bis-triazolium Derivatives

Given the worldwide spread of bacterial drug resistance, there is an urgent need to develop new compounds that exhibit potent antibacterial activity and that are unimpaired by this phenomenon. Quaternary ammonium compounds have been used for many years as disinfectants, but recent advances have shown that polycationic derivatives exhibit much stronger activity and are less prone to bacterial resistance than commonly used monocationic compounds. In this sense, we prepared three series of new bis-cationic compounds: bis-thiazoliums, bis-imidazoliums, and bis-1,2,4-triazoliums. If some compounds of the first series showed fair antibacterial activity, most of those belonging to the two other series were highly potent, with minimum inhibitory concentrations close to 1 μg mL^-1 . Some of them also exhibited low toxicity toward eukaryotic MRC-5 lung fibroblasts, and we showed that this toxicity is clearly correlated with clogP. Finally, four selected compounds were found to exhibit a clear bactericidal effect.

Quaternary Ammonium Compounds: Simple in Structure, Complex in Application

Quaternary ammonium compounds, referred to as QACs, are cationic substances with a structure on the edge of organic and inorganic chemistry and unique physicochemical properties. The purpose of the present work is to introduce QACs and their wide application potential. Fundamental properties, methods of preparation, and utilization in organic synthesis are reviewed. Modern applications and the use of QACs as reactive substrates, reagents, phase-transfer catalysts, ionic liquids, electrolytes, frameworks, surfactants, herbicides, and antimicrobials are further covered. A brief discussion of the health and environmental impact of QACs is also provided. The emphasis is largely on tetraalkylammonium compounds bearing linear alkyl chains.

Rapid kill assessment of an N-arylated NH125 analogue against drug-resistant microorganisms

While a number of disinfection techniques are employed in healthcare units, the eradication of drug-resistant microorganisms remains a challenge. We recently reported N-arylated NH125 analogue 1, which demonstrated potent biofilm eradication and antibacterial activities against a panel of drug-resistant pathogens. The broad-spectrum activities observed for 1 along with its rapid eradication of MRSA persister cells suggested that this agent, and related analogues, can serve as disinfectants for antibiotic resistant pathogens in healthcare settings. Here, we report the rapid bactericidal activities of 1 against a panel of exponentially-growing, drug-resistant pathogens. Against MRSA, MRSE, VRE and MDR A. baumannii, 1 eradicated bacterial cells after five minutes when tested at 50 μM (3- to 6-log reduction of CFU per mL). We highlighted the rapid killing activities by demonstrating that 1 eradicates 99.99% of viable MRSA 1707 cells in one minute (50 μM, 4-log reduction of CFU per mL). In addition, 1 rapidly eradicated fungal pathogen C. neoformans in kill kinetic experiments. A solution of 1 demonstrated similar shelf stability to known disinfectant BAC-16 when tested up to 111 days after being stored. Collectively, our data highlights the potential of 1 to be used as a disinfecting agent to prevent healthcare-associated, drug-resistant infections.

An Investigation into Rigidity-Activity Relationships in BisQAC Amphiphilic Antiseptics

Twenty-one mono- and biscationic quaternary ammonium amphiphiles (monoQACs and bisQACs) were rapidly prepared in order to investigate the effects of rigidity of a diamine core structure on antiseptic activity. As anticipated, the bioactivity against a panel of six bacteria including methicillin-resistant Staphylococcus aureus (MRSA) strains was strong for bisQAC structures, and is clearly correlated with the length of non-polar side chains. Modest advantages were noted for amide-containing side chains, as compared with straight-chained alkyl substituents. Surprisingly, antiseptics with more rigidly disposed side chains, such as those in DABCO-12,12, showed the highest level of antimicrobial activity, with single-digit MIC values or better against the entire bacterial panel, including sub-micromolar activity against an MRSA strain.

Antimicrobial 1,3,4-trisubstituted-1,2,3-triazolium salts

A series of 1,3,4-trisubstituted-1,2,3-triazolium bromide salts were prepared by efficient two-step sequences of azide-alkyne cycloaddition and benzylic substitution. The antimicrobial activity of each triazolium salt and correlating triazole precursor was evaluated using a minimum inhibitory concentration (MIC) assay. MIC activities as low as 1 µM against Gram-positive bacteria, 8 µM against Gram-negative bacteria and 4 µM against fungi were observed for salt analogs, while neutral triazoles were inactive. Analogs representing selective and broad-spectrum antimicrobial activity were each identified. MIC structure-activity relationships observed within this motif indicate that the presence of cationic charge and balance of overall hydrophobicity are strongly impactful, while benzyl vs. aryl substituent identity and variation of substituent regiochemistry are not.

Antimicrobial peptide-inspired NH125 analogues: bacterial and fungal biofilm-eradicating agents and rapid killers of MRSA persisters

During microbial infection, antimicrobial peptides are utilized by the immune response to rapidly eradicate microbial pathogens through the destruction of cellular membranes. Inspired by antimicrobial peptides, quaternary ammonium cationic (QAC) compounds have emerged as agents capable of destroying bacterial membranes leading to rapid bacterial death, including the eradication of persistent, surface-attached bacterial biofilms. NH125, an imidazolium cation with a sixteen membered fatty tail, was recently reported to eradicate persister cells and was our starting point for the development of novel antimicrobial agents. Here, we describe the design, chemical synthesis and biological investigations of a collection of 30 diverse NH125 analogues which provided critical insights into structural features that are important for antimicrobial activities in this class. From these studies, multiple NH125 analogues were identified to possess potent antibacterial and antifungal activities, eradicate both bacterial and fungal biofilms and rapidly eradicate MRSA persister cells in stationary phase. NH125 analogues also demonstrated more rapid persister cell killing activities against MRSA when tested alongside a panel of diverse membrane-active agents, including BAC-16 and daptomycin. NH125 analogues could have a significant impact on persister- and biofilm-related problems in numerous biomedical applications.

Hybrid BisQACs: Potent Biscationic Quaternary Ammonium Compounds Merging the Structures of Two Commercial Antiseptics

Benzalkonium chloride (BAC) and cetyl pyridinium chloride (CPC) are two of the most common household antiseptics, but show weaker efficacy against Gram-negative bacteria as well as against methicillin-resistant Staphylococcus aureus (MRSA) strains, relative to other S. aureus strains. We prepared 28 novel quaternary ammonium compounds (QACs) that represent a hybrid of these two structures, using 1- to 2-step synthetic sequences. The biscationic (bisQAC) species prepared show uniformly potent activity against six bacterial strains tested, with nine novel antiseptics displaying single-digit micromolar activity across the board. Effects of unequal chain lengths of two installed side chains had less impact than the overall number of side chain carbon atoms present, which was optimal at 22-25 carbons. This is further indication that simple refinements to multiQAC architectures can show improvement over current household antiseptics.

Thiabicyclononane-Based Antimicrobial Polycations

Bicyclo[3.3.1]nonane (BCN) polycations were synthesized by the reaction of the bivalent electrophile thiabicyclo[3.3.1]nonane dinitrate with a series of simple bis(pyridine) nucleophiles. Oligomers of moderate chain length were formed in a modular approach that tolerated the inclusion of functionalized and variable-length linkers between the pyridine units. Post-polymerization modification via copper-catalyzed azide-alkyne cyloaddition was enabled by the inclusion of terminal alkyne groups in these monomers. Most of the resulting polymers, new members of the polyionene class, inhibited the growth of bacteria at the μg/mL level and killed static bacterial cells at polymer concentrations of tens of ng/mL, with moderate to good selectivity with respect to lysis of red blood cells. While resistance to the BCN polymers was developed only very slowly over multiple passages, a degradable version of the polycation was observed to make E. coli cells more susceptible to other quaternary ammonium based antimicrobials. Solid substrates (glass and crystalline silicon) covalently functionalized with a representative BCN polycation were also able to repetitively kill bacteria in solution at high rates and with cleaning by simple sonication between exposures.

Ester- and amide-containing multiQACs: Exploring multicationic soft antimicrobial agents

Quaternary ammonium compounds (QACs) are ubiquitous antiseptics whose chemical stability is both an aid to prolonged antibacterial activity and a liability to the environment. Soft antimicrobials, such as QACs designed to decompose in relatively short times, show the promise to kill bacteria effectively but not leave a lasting footprint. We have designed and prepared 40 soft QAC compounds based on both ester and amide linkages, in a systematic study of mono-, bis-, and tris-cationic QAC species. Antimicrobial activity, red blood cell lysis, and chemical stability were assessed. Antiseptic activity was strong against a panel of six bacteria including two MRSA strains, with low micromolar activity seen in many compounds; amide analogs showed superior activity over ester analogs, with one bisQAC displaying average MIC activity of ∼1μM. For a small subset of highly bioactive compounds, hydrolysis rates in pure water as well as buffers of pH =4, 7, and 10 were tracked by LCMS, and indicated good stability for amides while rapid hydrolysis was observed for all compounds in acidic conditions.

The Development of Next-Generation Pyridinium-Based multiQAC Antiseptics

A series of 18 bis- and tris-pyridinium amphiphiles were prepared and tested for both antimicrobial activity and lytic capability, in comparison with the commercially available pyridinium antiseptic cetylpyridinium chloride (CPC). Assessments were made against Gram-positive and Gram-negative bacteria, including two methicillin-resistant Staphylococcus aureus (MRSA) strains. While 2Pyr-11,11 was identified as one of the most potent antimicrobial quaternary ammonium compounds (QACs) reported to date, boasting nanomolar inhibition against five of six bacteria tested, no significant improvement in bioactivity of tris-pyridinium amphiphiles over their bis-pyridinium counterparts was observed. However, the multicationic QACs (multiQACs) presented herein did display significant advantages over the monocationic CPC; while similar red blood cell lysis was observed, superior activity against both Gram-negative bacteria and resistant S. aureus strains led to the discovery of four pyridinium-based multiQACs with advantageous therapeutic indices.