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

Alkylated Sulfonium Modification of Low Molecular Weight Polyethylenimine to Form Lipopolymers as Gene Vectors

Hydrophobic modification of low molecular weight polyethylenimine (PEI) is an efficient method to form ideal gene-transfer carriers. Sulfonium-a combination of three different functional groups, was conjugated onto PEI 1.8k at a conjugation ratio of 1:0.1 to form a series of sulfonium PEI (SPs). These SPs were hydrophobically modified and characterized by Fourier transform infrared and HNMR. DNA-condensing abilities of SPs were tested with gel retardation experiment, and their cytotoxicity was evaluated via the MTT assay. The particle size and zeta potential of SP/DNA nanoparticles were measured and evaluated for cellular uptake and transfection ability on HepG2 cell line. The results showed that the sulfonium moiety was attached to PEI 1.8k with a high yield at a conjugation ratio of 1:0.1. SPs containing longer alkyl chains condensed DNA completely at an SP/DNA weight ratio of 2:1. The formed nanoparticle size was in the range of 168-265 nm, and the zeta potential was +16-45 mV. The IC50 values of SPs were 6.5-43.2 μg/mL. The cytotoxicity of SPs increased as the hydrophobic chain got longer. SP/DNA showed much stronger cellular uptakes than PEI 25k; however, pure SPs presented almost no gene transfection on cells. Heparin release experiment showed that SP's strong binding of DNA resulted in low release of DNA and thus hindered the gene transfection process. By mixing SP with PEI 1.8k, the mixture presented adjustable DNA binding and releasing. The mixture formed by 67% SP and 33% PEI 1.8k showed strong gene transfection. In conclusion, sulfonium is an effective linkage to carry hydrophobic groups to adjust cell compatibilities and gene transfection capabilities of PEI.

Harvesting phosphorus-containing moieties for their antibacterial effects

Clinically manifested resistance of bacteria to antibiotics has emerged as a global threat to society and there is an urgent need for the development of novel classes of antibacterial agents. Recently, the use of phosphorus in antibacterial agents has been explored in quite an unprecedent manner. In this comprehensive review, we summarize the use of phosphorus-containing moieties (phosphonates, phosphonamidates, phosphonopeptides, phosphates, phosphoramidates, phosphinates, phosphine oxides, and phosphoniums) in compounds with antibacterial effect, including their use as β-lactamase inhibitors and antibacterial disinfectants. We show that phosphorus-containing moieties can serve as novel pharmacophores, bioisosteres, and prodrugs to modify pharmacodynamic and pharmacokinetic properties. We further discuss the mechanisms of action, biological activities, clinical use and highlight possible future prospects.

Further Study of the Polar Group's Influence on the Antibacterial Activity of the 3-Substituted Quinuclidine Salts with Long Alkyl Chains

Quaternary ammonium compounds (QACs) are among the most potent antimicrobial agents increasingly used by humans as disinfectants, antiseptics, surfactants, and biological dyes. As reports of bacterial co- and cross-resistance to QACs and their toxicity have emerged in recent years, new attempts are being made to develop soft QACs by introducing hydrolyzable groups that allow their controlled degradation. However, the development of such compounds has been hindered by the structural features that affect the bioactivity of QACs, one of them being polarity of the substituent near the quaternary center. To further investigate the influence of the polar group on the bioactivity of QACs, we synthesized 3-aminoquinuclidine salts for comparison with their structural analogues, 3-acetamidoquinuclidines. We found that the less polar amino-substituted compounds exhibited improved antibacterial activity over their more polar amide analogues. In addition to their better minimum inhibitory concentrations, the candidates were excellent at suppressing Staphylococcus aureus biofilm formation and killing bacteria almost immediately, as shown by the flow cytometry measurements. In addition, two candidates, namely QNH2-C14 and QNH2-C16, effectively suppressed bacterial growth even at concentrations below the MIC. QNH2-C14 was particularly effective at subinhibitory concentrations, inhibiting bacterial growth for up to 6 h. In addition, we found that the compounds targeted the bacterial membrane, leading to its perforation and subsequent cell death. Their low toxicity to human cells and low potential to develop bacterial resistance suggest that these compounds could serve as a basis for the development of new QACs.

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.

Double BAC and Triple BAC: A Systematic Analysis of the Disinfectant Properties of Multicationic Derivatives of Benzalkonium Chloride (BAC)

Over the past decades, the shortcomings of established quaternary ammonium disinfectants have become increasingly clear. Although benzalkonium chloride (BAC) has enjoyed nearly a century of significantly protecting human health through surgical preparation, home use, and industrial applications, increasing levels of bacterial resistance have rendered it decreasingly effective. In light of more recent efforts that have informed us that multicationic amphiphilic disinfectants show both higher activity as well as diminished susceptibility to resistance, we embarked on the preparation of 27 multicationic QACs in an attempt to clearly document structure-activity relationships of next-generation BAC structures. Select biscationic BAC derivatives demonstrate single-digit micromolar activity against all seven bacteria tested and MIC values of 2- to 32-fold better than BAC. Particularly notable is the improvement against the more concerning bacteria like Acinetobacter baumannii and Pseudomonas aeruginosa, which pose a modern threat to legacy disinfectants like BAC. With simple synthetic paths, consistently high yields (averaging ∼80 %), and strong biological activity, potent structures with clear SAR trends and strong therapeutic indices have been established.

Synthesis and Biological Evaluation of 3-Amidoquinuclidine Quaternary Ammonium Compounds as New Soft Antibacterial Agents

Quaternary ammonium compounds (QACs) are among the most effective antimicrobial agents that have been used for more than a century. However, due to the growing trend of bacterial resistance and high toxicity of QACs, research in this field remains a pressing matter. Recent studies of the structure-activity relationship suggest that the introduction of the amide functional group into QAC structures results in soft variants that retain their antimicrobial properties while opening the possibility of fine-tuned activity regulation. Here, we report the synthesis and structure-function study of three structurally distinct series of naturally derived soft QACs. The obtained 3-amidoquinuclidine QACs showed a broad range of antibacterial activities related to the hydrophobic-hydrophilic balance of the QAC structures. All three series yielded candidates with minimal inhibitory concentrations (MIC) in the single-digit μM range. Time-resolved growth analysis revealed subtle differences in the antibacterial activity of the selected candidates. The versatile MIC values were recorded in different nutrient media, suggesting that the media composition may have a dramatic impact on the antibacterial potential. The new QACs were found to have excellent potential to suppress bacterial biofilm formation while exhibiting low ability to induce bacterial resistance. In addition, the selected candidates were found to be less toxic than commercially available QACs and proved to be potential substrates for protease degradation. These data suggest that 3-amidoquinuclidine QACs could be considered as novel antimicrobial agents that pose a low threat to ecosystems and human health.

Antibacterial Theranostic Agents with Negligible Living Cell Invasiveness: AIE-Active Cationic Amphiphiles Regulated by Alkyl Chain Engineering

To address the threat of bacterial infection in the following post-antibiotic era, developing effective antibacterial approaches is of utmost urgency. Theranostic medicine integrating diagnosis and therapy is a promising protocol to fight against pathogenic bacteria. But numerous reported antibacterial theranostic materials are disclosed to be trapped in the excessive invasiveness to living mammal cells, leading to false positives and possible biosafety risks. Herein, a series of cationic pyridinium-substituted phosphindole oxide derivatives featuring aggregation-induced emission are designed, and alkyl chain engineering is conducted to finely tune their hydrophobicity and investigate their bioaffinity preference for living mammal cells and pathogenic bacteria. Most importantly, an efficient theranostic agent (PyBu-PIO) is acquired that is free from living cell invasiveness with negligible cytotoxicity and yet holds a good affinity for Gram-positive bacteria, including drug-resistant strains, with a superior inactivating effect. Externally applying PyBu-PIO onto Gram-positive bacteria-infected skin wounds can achieve creditable imaging effects and successfully accelerate the healing processes with reliable biosafety. This work proposes living cell invasiveness as a criterion for antibacterial theranostic materials and provides important enlightenment for the design of antibacterial theranostic materials.

Rigidity-Activity Relationships of bisQPC Scaffolds against Pathogenic Bacteria

Biscationic quaternary phosphonium compounds (bisQPCs) represent a promising class of antimicrobials, displaying potent activity against both Gram-negative and Gram-positive bacteria. In this study, we explored the effects of structural rigidity on the antimicrobial activity of QPC structures bearing a two-carbon linker between phosphonium groups, testing against a panel of six bacteria, including multiple strains harboring known disinfectant resistance mechanisms. Using simple alkylation reactions, 21 novel compounds were prepared, although alkene isomerization as well as an alkyne reduction were observed during the respective syntheses. The resulting bisQPC compounds showed strong biological activity, but were hampered by diminished solubility of their iodide salts. One compound (P2P-10,10 I) showed single-digit micromolar activity against the entire panel of bacteria. Overall, intriguing biological activity was observed, with less rigid structures displaying better efficacy against Gram-negative strains and more rigid structures demonstrating slightly increased efficacy against S. aureus strains.

Synthesis, biological evaluation and structure-activity relationship of 2-(2-hydroxyaryl)alkenylphosphonium salts with potency as anti-MRSA agents

Here we report the synthesis, in vitro antimicrobial activity, preliminary toxicity and mechanism study of a new series of 2-(2-hydroxyaryl)alkenylphosphonium salts with the variation of phosphonium moiety obtained by a two-step synthetic method from phosphine oxides. The salts showed pronounced activity against Gram-positive bacteria, including MRSA strains, and some fungi. Mechanism of action against S. aureus was studied by CV test, TEM and proteomic assay. No cell wall integrity loss was observed while proteomic assay results suggested interference in different metabolic processes of S. aureus. For this series, lipophilicity was determined as a key factor for the inhibition of Gram-positive bacteria growth and S. aureus killing. Biological properties of methylated derivatives were notably different with manifested action against Gram-negative bacteria.

Merging antimicrobial and visible emission properties within 1,3,4-trisubstituted-1,2,3-triazolium salts

Bioactive molecules displaying visible wavelength emission can be useful for bioimaging, chemosensing and photodynamic therapy applications. Reported herein are 1,3,4-trisubsituted-1,2,3-triazolium salts displaying both antimicrobial and visible emission properties. Using a click chemistry approach, 2-fluorenyl, 1-naphthyl, 2-naphthyl, 2-anthracenyl and 1-pyrenyl units were incorporated at the N1 position, imparting visible emission properties to their triazolium bromide salts with Stokes shifts greater than 100 nm relative to the emission of their triazole precursors. The increasing size of such hydrophobic aryl units impacts minimum inhibitory concentration (MIC) values against Gram-positive bacteria, Gram-negative bacteria and yeast, and can be counterbalanced by hydrophobic substituent variation at other positions of the molecule in order to preserve bioactivity. Among the series of compounds studied are analogs displaying blue, green and yellow colored emission and MIC values as low as 0.4 μM (Gram-positive bacteria), 8 μM (Gram-negative bacteria) and 2 μM (yeast). XRD analysis validates the regioselective benzylation at the N3 position of the 1,2,3-triazole ring and the ability of such compounds to associate through dimeric intermolecular π-stacking interactions.

Quaternary Phosphonium Compounds: An Examination of Non-Nitrogenous Cationic Amphiphiles That Evade Disinfectant Resistance

Quaternary ammonium compounds (QACs) serve as mainstays in the formulation of disinfectants and antiseptics. However, an over-reliance and misuse of our limited QAC arsenal has driven the development and spread of resistance to these compounds, as well as co-resistance to common antibiotics. Extensive use of these compounds throughout the COVID-19 pandemic thus raises concern for the accelerated proliferation of antimicrobial resistance and demands for next-generation antimicrobials with divergent architectures that may evade resistance. To this end, we endeavored to expand beyond canonical ammonium scaffolds and examine quaternary phosphonium compounds (QPCs). Accordingly, a synthetic and biological investigation into a library of novel QPCs unveiled biscationic QPCs to be effective antimicrobial scaffolds with improved broad-spectrum activities compared to commercial QACs. Notably, a subset of these compounds was found to be less effective against a known QAC-resistant strain of MRSA. Bioinformatic analysis revealed the unique presence of a family of small multiresistant transporter proteins, hypothesized to enable efflux-mediated resistance to QACs and QPCs. Further investigation of this resistance mechanism through efflux-pump inhibition and membrane depolarization assays illustrated the superior ability of P6P-10,10 to perturb the cell membrane and exert the observed broad-spectrum potency compared to its commercial counterparts. Collectively, this work highlights the promise of biscationic phosphonium compounds as next-generation disinfectant molecules with potent bioactivities, thereby laying the foundation for future studies into the synthesis and biological investigation of this nascent antimicrobial class.

Medical gloves modified by a one-minute spraying process with blood-repellent, antibacterial and wound-healing abilities

During clinical surgery, bleeding that occurs in the operative region is inevitable. Due to the blood adhesion on ordinary medical gloves, it reduces surgery quality to a certain extent and even prolongs operation time. Herein, we show that medical blood-repellent gloves (MBRG) can be obtained by spraying the blood-repellent mist spray (MS) on the surface of ordinary medical gloves, which are available for immediate use in around one minute. After the modification, MBRG not only have a significantly higher blood repellent rate than that of ordinary medical gloves, but also can effectively inhibit the growth of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), and even promote the healing of infected wounds. MS is easy to prepare, low-toxic, and can be widely used on the surface of various medical gloves, such as rubber gloves, polyethylene film gloves, and nitrile gloves, which may have an impact on the development of future medical gloves.

Broad-Spectrum Bactericidal Activity and Remarkable Selectivity of Main-Chain Sulfonium-Containing Polymers with Alternating Sequences

Incorporation of cationic groups into polymers represents one of the most widely used strategies to prepare antibacterial materials. Sulfonium, as a typical cationic moiety, displays potent antibacterial efficacy in the form of small molecules, however, has long underperformed in polymeric systems. Herein, we developed a series of alternating polysulfoniums, where the hydrophobicity of each alternating unit can be accurately tuned by altering the monomer precursors. Excellent antibacterial activity against a broad spectrum of clinically relevant bacteria, including Methicillin-resistant Staphylococcus aureus, can be obtained in the optimal compositions with minimum bactericidal concentrations in the range of 1.25-10 μg/mL, as well as negligible hemolytic effect at polymer concentrations even up to 10000 μg/mL. Bacteria do not readily develop resistance to polysulfoniums due to the antibacterial action is possibly the membrane disrupting mechanism. This work demonstrates sulfonium-based polymers with well-defined sequences can function as a promising candidate to combat drug-resistant bacterial infection.

Main-Chain Polysulfonium Salts: Development of Non-Ammonium Antibacterial Polymers Similar in Their Activity to Antibiotic Drugs Vancomycin and Kanamycin

Typically, quaternary ammonium polymers are employed for antibacterial purposes. However, a century of use has led bacteria to develop resistance to such materials. Therefore, attention is now turning toward other cationic moieties. In this context, the present work explores sulfur-based main-chain cationic polymers. The results indicate that sulfonium polymers with a β-hydroxy motif do not suffer from structural instability issues as is commonly observed in cationic polythioethers. Furthermore, they can be highly effective toward important Gram-positive bacterial strains such as Mycobacterium smegmatis, a model organism to develop drugs against rapidly spreading tuberculosis infections. More importantly, however, more challenging Gram-negative strains such as Escherichia coli can also be targeted by the polysulfoniums with equal effectiveness. Interestingly, side-chain sulfonium polyelectrolytes are observed to be devoid of any significant antibacterial activity. Finally, a comparison with kanamycin and vancomycin suggests the present polymers to be similarly effective as the bactericidal antibiotic drugs. Overall, these results indicate the effectiveness of the main-chain trivalent β-hydroxy sulfonium motif for the development of novel antibacterial polymers with a non-ammonium structure.

Sulfonium-based liposome-encapsulated antibiotics deliver a synergistic antibacterial activity

The devastating antibacterial infections, coupled with their antibiotic resistance abilities, emphasize the need for effective antibacterial therapeutics. In this prospect, liposomal delivery systems have been employed in improving the efficacy of the antibacterial agents. The liposome-based antibiotics enhance the therapeutic potential of the new or existing antibiotics and reduce their adverse effects. The current study describes the development of sulfonium-based antibacterial lipids that demonstrate the delivery of existing antibiotics. The presence of cationic sulfonium moieties and inherent membrane targeting abilities of the lipids could help reduce the antibiotic resistance abilities of the bacteria and deliver the antibiotics to remove the infectious pathogens electively. The transmission electron microscopic images and dynamic light scattering analyses revealed the liposome formation abilities of the sulfonium-based amphiphilic compounds in the aqueous medium. The effectiveness of the compounds was tested against the Gram-negative and Gram-positive bacterial strains. The viability of the bacterial cells was remarkably reduced in the presence of the compounds. The sulfonium-based compounds with pyridinium moiety and long hydrocarbon chains showed the most potent antibacterial activities among the tested compounds. Mechanistic studies revealed the membrane-targeted bactericidal activities of the compounds. The potent compound also showed tetracycline and amoxicillin encapsulation and sustained release profiles in the physiologically relevant medium. The tetracycline and amoxicillin-encapsulated lipid showed much higher antibacterial activities than the free antibiotics at similar concentrations, emphasizing the usefulness of the synergistic effect of sulfonium-based lipid and the antibiotics, signifying that the sulfonium lipid penetrated the bacterial membrane and increased the cellular uptake of the antibiotics. The potent lipid also showed therapeutic potential, as it is less toxic to mammalian cells (like HeLa and HaCaT cells) at concentrations higher than their minimum inhibitory concentration values against S. aureus, E. coli, and MRSA. Hence, the sulfonium-based lipid exemplifies a promising framework for assimilating various warheads, and provides a potent antibacterial material.

Trivalent sulfonium compounds (TSCs): Tetrahydrothiophene-based amphiphiles exhibit similar antimicrobial activity to analogous ammonium-based amphiphiles

Recent advances in the development of quaternary ammonium compounds (QACs) have focused on new structural motifs to increase bioactivity, but significantly less studied has been the change from ammonium- to sulfonium-based disinfectants. Herein, we report the synthesis of structurally analogous series of quaternary ammonium and trivalent sulfonium compounds (TSCs). The bioactivity profiles of these compounds generally mirror each other, and the antibacterial activity of sulfonium-based THT-18 was found to be comparable to the commercial disinfectant, BAC. The development of these compounds presents a new avenue for further study of disinfectants to combat the growing threat of bacterial resistance.