Isosteric substitution in cationic-amphiphilic polymers reveals an important role for hydrogen bonding in bacterial membrane interactions

First In Vitro-In Silico Analysis for the Determination of Antimicrobial and Antioxidant Properties of 2-(4-Methoxyphenylamino)-2-oxoethyl Methacrylate and p-Acetamide

The antibacterial, antifungal, and antioxidant activities of 2-chloro-N-(4-methoxyphenyl)acetamide (p-acetamide) and 2-(4-methoxyphenylamino)-2-oxoethyl methacrylate (MPAEMA) were investigated by in vitro experiments and in silico analyses. MPAEMA has an antibacterial effect only against Gram-positive Staphylococcus aureus. It was determined that this did not affect any other bacteria and Candida glabrata yeast. On the other hand, p-acetamide showed antimicrobial activity against S. aureus ATCC 25923, C. glabrata ATCC 90030, Bacillus subtilis NRRL 744, Enterococcus faecalis ATCC 551289, Escherichia coli ATCC 25922, Klebsiella pneumoniae NRLLB4420, Pseudomonas aeruginosa ATCC 27853, and Listeria monocytogenes ATCC 1911. p-Acetamide showed the greatest antifungal effect by inhibiting the colony growth of Trichoderma longibrachiatum (98%). This was followed by Mucor plumbeus with 83% and Fusarium solani with 21%. MPAEMA inhibited colony growth of T. longibrachiatum by 95% and that of M. plumbeus by 91%. Also, p-acetamide and MPAEMA had a scavenging effect on free radicals. According to results of the in silico analysis, the antimicrobial effect of these compounds is due to their effect on DNA ligase. Based on drug-likeness analysis, they were found to be consistent with the Lipinski, Veber, or Ghose rule. p-Acetamide and MPAEMA may be used as drugs.

Dual functional therapeutics: mitigating bacterial infection and associated inflammation

The emergence of antimicrobial resistance, coupled with the occurrence of persistent systemic infections, has already complicated clinical therapy efforts. Moreover, infections are also accompanied by strong inflammatory responses, generated by the host's innate and adaptive immune systems. The closely intertwined relationship between bacterial infection and inflammation has multiple implications on the ability of antibacterial therapeutics to tackle infection and inflammation. Particularly, uncontrolled inflammatory responses to infection can lead to sepsis, a life-threatening physiological condition. In this review, we discuss dual-functional antibacterial therapeutics that have potential to be developed for treating inflammation associated with bacterial infections. Immense research is underway that aims to develop new therapeutic agents that, when administered, regulate the excess inflammatory response, i.e. they have immunomodulatory properties along with the desired antibacterial activity. The classes of antibiotics that have immunomodulatory function in addition to antibacterial activity have been reviewed. Host defense peptides and their synthetic mimics are amongst the most sought-after solutions to develop such dual-functional therapeutics. This review also highlights the important classes of peptidomimetics that exhibit both antibacterial and immunomodulatory properties.

Isoamphipathic antibacterial molecules regulating activity and toxicity through positional isomerism

Peptidomimetic antimicrobials exhibit a selective interaction with bacterial cells over mammalian cells once they have achieved an optimum amphiphilic balance (hydrophobicity/hydrophilicity) in the molecular architecture. To date, hydrophobicity and cationic charge have been considered the crucial parameters to attain such amphiphilic balance. However, optimization of these properties is not enough to circumvent unwanted toxicity towards mammalian cells. Hence, herein, we report new isoamphipathic antibacterial molecules (IAMs: 1-3) where positional isomerism was introduced as one of the guiding factors for molecular design. This class of molecules displayed good (MIC = 1-8 μg mL^-1 or μM) to moderate [MIC = 32-64 μg mL^-1 (32.2-64.4 μM)] antibacterial activity against multiple Gram-positive and Gram-negative bacteria. Positional isomerism showed a strong influence on regulating antibacterial activity and toxicity for ortho [IAM-1: MIC = 1-32 μg mL^-1 (1-32.2 μM), HC₅₀ = 650 μg mL^-1 (654.6 μM)], meta [IAM-2: MIC = 1-16 μg mL^-1 (1-16.1 μM), HC₅₀ = 98 μg mL^-1 (98.7 μM)] and para [IAM-3: MIC = 1-16 μg mL^-1 (1-16.1 μM), HC₅₀ = 160 μg mL^-1 (161.1 μM)] isomers. Co-culture studies and investigation of membrane dynamics indicated that ortho isomer, IAM-1 exerted more selective activity towards bacterial over mammalian membranes, compared to meta and para isomers. Furthermore, the mechanism of action of the lead molecule (IAM-1) has been characterized through detailed molecular dynamics simulations. In addition, the lead molecule displayed substantial efficacy against dormant bacteria and mature biofilms, unlike conventional antibiotics. Importantly, IAM-1 exhibited moderate in vivo activity against MRSA wound infection in a murine model with no detectable dermal toxicity. Altogether, the report explored the design and development of isoamphipathic antibacterial molecules to establish the role of positional isomerism in achieving selective and potential antibacterial agents.

Antibiotic Adjuvants: A Versatile Approach to Combat Antibiotic Resistance

The problem of antibiotic resistance is on the rise, with multidrug-resistant strains emerging even to the last resort antibiotics. The drug discovery process is often stalled by stringent cut-offs required for effective drug design. In such a scenario, it is prudent to delve into the varying mechanisms of resistance to existing antibiotics and target them to improve antibiotic efficacy. Nonantibiotic compounds called antibiotic adjuvants which target bacterial resistance can be used in combination with obsolete drugs for an improved therapeutic regime. The field of "antibiotic adjuvants" has gained significant traction in recent years where mechanisms other than β-lactamase inhibition have been explored. This review discusses the multitude of acquired and inherent resistance mechanisms employed by bacteria to resist antibiotic action. The major focus of this review is how to target these resistance mechanisms by the use of antibiotic adjuvants. Different types of direct acting and indirect resistance breakers are discussed including enzyme inhibitors, efflux pump inhibitors, inhibitors of teichoic acid synthesis, and other cellular processes. The multifaceted class of membrane-targeting compounds with poly pharmacological effects and the potential of host immune-modulating compounds have also been reviewed. We conclude with providing insights about the existing challenges preventing clinical translation of different classes of adjuvants, especially membrane-perturbing compounds, and a framework about the possible directions which can be pursued to fill this gap. Antibiotic-adjuvant combinatorial therapy indeed has immense potential to be used as an upcoming orthogonal strategy to conventional antibiotic discovery.

Mechanically robust and highly bactericidal macroporous polymeric gels based on quaternized N,N-(dimethylamino)ethyl methacrylate possessing varying alkyl chain lengths

In this paper, macroporous antimicrobial polymeric gels (MAPGs) functionalized with active quaternary ammonium cations attached to varying hydrocarbon chain lengths have been fabricated. Apart from the change in the alkyl chain length attached to the quaternary ammonium cation, the amount of crosslinker was also varied during the fabrication of the macroporous gels. The prepared gels were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy (FE-SEM) and swelling studies. In addition, the mechanical properties of the fabricated macroporous gels were studied using compression and tensile testing. The antimicrobial activity of the gels has been determined for Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) as well as Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus). Antimicrobial activity, as well as the mechanical properties of the macroporous gels, was found to be influenced by the alkyl chain length attached to the quaternary ammonium cations as well as by the amount of crosslinker used for the fabrication of the gel. In addition, on increasing the alkyl chain length from C4 (butyl) to C8 (octyl), the effectiveness of the polymeric gels increased. It was observed that the gels derived using a tertiary amine (NMe₂) containing monomer showed relatively low antimicrobial activity as compared to the gels obtained using quaternized monomers (C4 (butyl), C6 (hexyl), and C8 (octyl)). The gels based on the quaternized C8 monomer displayed the highest antimicrobial activity and mechanical stability as compared to the gels based on the C4 and C6 monomers.

Next-generation membrane-active glycopeptide antibiotics that also inhibit bacterial cell division

Resistance to vancomycin, a life-saving drug against Gram-positive bacterial infections necessitates developing alternative therapeutics. Herein, we report vancomycin derivatives that assimilate mechanisms beyond d-Ala-d-Ala binding. The role of hydrophobicity towards the structure and function of the membrane-active vancomycin showed that alkyl-cationic substitutions favored broad-spectrum activity. The lead molecule, VanQAmC₁₀ delocalized the cell division protein MinD in Bacillus subtilis, implying an impact on bacterial cell division. Further examination of wild-type, GFP-FtsZ, or GFP-FtsI producing- and ΔamiAC mutants of Escherichia coli revealed filamentous phenotypes and delocalization of the FtsI protein. The findings indicate that VanQAmC₁₀ also inhibits bacterial cell division, a property previously unknown for glycopeptide antibiotics. The conjunction of multiple mechanisms contributes to its superior efficacy against metabolically active and inactive bacteria, wherein vancomycin is ineffective. Additionally, VanQAmC₁₀ exhibits high efficacy against methicillin-resistant Staphylococcus aureus (MRSA) and Acinetobacter baumannii in mouse models of infection.

Designer co-beta-peptide copolymer selectively targets resistant and biofilm Gram-negative bacteria

New antimicrobials are urgently needed to combat Gram-negative bacteria, particularly multi-drug resistant (MDR) and phenotypically resistant biofilm species. At present, only sequence-defined alpha-peptides (e.g. polymyxin B) can selectively target Gram-negative bacterial lipopolysaccharides. We show that a copolymer, without a defined sequence, shows good potency against MDR Gram-negative bacteria including its biofilm form. The tapered blocky co-beta-peptide with controlled N-terminal hydrophobicity (#4) has strong interaction with the Gram-negative bacterial lipopolysaccharides via its backbone through electrostatic and hydrogen bonding interactions but not the Gram-positive bacterial and mammalian cell membranes so that this copolymer is non-toxic to these two latter cell types. The new #4 co-beta-peptide selectively kills Gram-negative bacteria with low cytotoxicity both in vitro and in a mouse biofilm wound infection model. This strategy provides a new concept for the design of Gram-negative selective antimicrobial peptidomimetics against MDR and biofilm species.

Advancements in antimicrobial nanoscale materials and self-assembling systems

Antimicrobial resistance is directly responsible for more deaths per year than either HIV/AIDS or malaria and is predicted to incur a cumulative societal financial burden of at least $100 trillion between 2014 and 2050. Already heralded as one of the greatest threats to human health, the onset of the coronavirus pandemic has accelerated the prevalence of antimicrobial resistant bacterial infections due to factors including increased global antibiotic/antimicrobial use. Thus an urgent need for novel therapeutics to combat what some have termed the 'silent pandemic' is evident. This review acts as a repository of research and an overview of the novel therapeutic strategies being developed to overcome antimicrobial resistance, with a focus on self-assembling systems and nanoscale materials. The fundamental mechanisms of action, as well as the key advantages and disadvantages of each system are discussed, and attention is drawn to key examples within each field. As a result, this review provides a guide to the further design and development of antimicrobial systems, and outlines the interdisciplinary techniques required to translate this fundamental research towards the clinic.

An Optimally Designed Engineering Exosome-Reductive COF Integrated Nanoagent for Synergistically Enhanced Diabetic Fester Wound Healing

Oxidative stress and local overactive inflammation have been considered major obstacles in diabetic wound treatment. Although antiphlogistic tactics have been reported widely, they are also challenged by pathogen contamination and compromised angiogenesis. Herein, a versatile integrated nanoagent based on 2D reductive covalent organic frameworks coated with antibacterial immuno-engineered exosome (PCOF@E-Exo) is reported to achieve efficient and comprehensive combination therapy for diabetic wounds. The E-Exo is collected from TNF-α-treated mesenchymal stem cells (MSCs) under hypoxia and encapsulated cationic antimicrobial carbon dots (CDs). This integrated nanoagent not only significantly scavenges reactive oxygen species and induces anti-inflammatory M2 macrophage polarization, but also stabilizes hypoxia-inducible factor-1α (HIF-1α). More importantly, the PCOF@E-Exo exhibits intriguing bactericide capabilities toward Gram-negative, Gram-positive, and drug-resistant bacteria, showing favorable intracellular bacterial destruction and biofilm permeation. In vivo results demonstrate that the synergetic impact of suppressing oxidative injury and tissue inflammation, promoting angiogenesis and eradicating bacterial infection, could significantly accelerate the infected diabetic fester wound healing with better therapeutic benefits than monotherapy or individual antibiotics. The proposed strategy can inspire further research to design more delicate platforms using the combination of immunotherapy with other therapeutic methods for more efficient ulcerated diabetic wounds treatments.

Tunable AIE-Active Assemblies Inducing Bacterial Agglutination toward Noninvasive Photodynamic Antiseptic

Aggregation-induced emission (AIE) photosensitizer assemblies capable of inducing bacterial agglutination were fabricated to achieve a high PDT efficacy in a noninvasive manner. An AIE-active molecule CTRA was designed to bear the donor and acceptor subunits for singlet oxygen (¹O₂) generation. An amphiphile DGal with a β-d-galactose headgroup was introduced to coassemble for the photosensitizer assemblies bearing glycosyls for bacteria capture as well as to tune the ¹O₂ generation of CTRA. At the optimal combination of two components, a highly efficient noninvasive PDT activity with nearly 100% effectiveness against S. aureus was realized.

Antimicrobial Polymers of Linear and Bottlebrush Architecture: Probing the Membrane Interaction and Physicochemical Properties

Polymeric antimicrobial peptide mimics are a promising alternative for the future management of the daunting problems associated with antimicrobial resistance. However, the development of successful antimicrobial polymers (APs) requires careful control of factors such as amphiphilic balance, molecular weight, dispersity, sequence, and architecture. While most of the earlier developed APs focus on random linear copolymers, the development of APs with advanced architectures proves to be more potent. It is recently developed multivalent bottlebrush APs with improved antibacterial and hemocompatibility profiles, outperforming their linear counterparts. Understanding the rationale behind the outstanding biological activity of these newly developed antimicrobials is vital to further improving their performance. This work investigates the physicochemical properties governing the differences in activity between linear and bottlebrush architectures using various spectroscopic and microscopic techniques. Linear copolymers are more solvated, thermo-responsive, and possess facial amphiphilicity resulting in random aggregations when interacting with liposomes mimicking Escheria coli membranes. The bottlebrush copolymers adopt a more stable secondary conformation in aqueous solution in comparison to linear copolymers, conferring rapid and more specific binding mechanism to membranes. The advantageous physicochemical properties of the bottlebrush topology seem to be a determinant factor in the activity of these promising APs.

Influence of PEG Subunit on the Biological Activity of Ionenes: Synthesis of Novel Polycations, Antimicrobial and Toxicity Studies

An alarming increase of antibiotic resistance among pathogens creates an urgent need to develop new antimicrobial agents. Many reported polycations show high antimicrobial activity along with low hemolytic activity. Unfortunately, most of those molecules remain highly cytotoxic against various mammalian cells. In this work, we present a systematic study on the impact of triethylene glycol monomethyl ether side groups (short PEG analog) on antimicrobial, hemolytic, and cytotoxic properties of novel amphiphilic ionenes. A detailed description of synthesis, leading to well-defined alternating polymers, which differ in structural elements responsible for hydrophilicity (PEG) and hydrophobicity (alkyl chain), is presented. Obtained results show that the PEG moiety and fine-tuned hydrophilic-lipophilic balance of ionenes synergistically lead to low-cytotoxic, low-hemolytic molecules with high activity against S. aureus, including methicillin-resistant strains (MRSA). Additionally, the results of mechanistic studies on bacterial cells and fluorescently labeled liposomes are also discussed. This article is protected by copyright. All rights reserved.

Polymeric Biomaterials for Prevention and Therapeutic Intervention of Microbial Infections

The escalating emergence of multidrug-resistant (MDR) pathogens and their ability to colonize into biofilms on a multitude of surfaces have struck global health as a nightmare. The stagnation in the development of antibiotics and the deterioration of clinical pipelines have incited an invigorating search for smart and innovative alternatives in the scientific community. Further, a steep rise in the usage of biomedical devices and implants has resulted in an accelerated occurrence of infections. Toward the goal of mitigation of the aforementioned challenges, antimicrobial polymers have stood out as an astounding option. In this perspective, we highlight our contribution to the field of polymeric biomaterials for tackling antimicrobial resistance (AMR) and infections. Polymers inspired from antimicrobial peptides (AMPs) have been utilized as therapeutic interventions to curb MDR infections and also to rejuvenate obsolete antibiotics. Further, cationic polymers have been used to impart antimicrobial properties to different biomedical surfaces. These cationic polymer-coated surfaces can inactivate pathogens upon contact as well as prevent their biofilm formation. In addition, antimicrobial hydrogels, which are prepared from either inherently antimicrobial polymers or biocide-loaded polymeric hydrogel matrices, have also been engineered. With a brief overview of the progress in the field, detailed elaboration of the polymeric biomaterials for prevention and therapeutic intervention of microbial infections developed by our group is presented. Finally, the challenges in the field of antimicrobial polymers with discussion on the proceedings of polymeric research to alleviate these challenges are discussed.

Amide Moieties Modulate the Antimicrobial Activities of Conjugated Oligoelectrolytes against Gram-negative Bacteria

Cationic conjugated oligoelectrolytes (COEs) are a class of compounds that can be tailored to achieve relevant in vitro antimicrobial properties with relatively low cytotoxicity against mammalian cells. Three distyrylbenzene‐based COEs were designed containing amide functional groups on the side chains. Their properties were compared to two representative COEs with only quaternary ammonium groups. The optimal compound, COE2−3C−C3‐Apropyl, has an antimicrobial efficacy against Escherichia coli with an MIC=2 μg mL⁻¹, even in the presence of human serum albumin low cytotoxicity (IC₅₀=740 μg mL⁻¹) and minimal hemolytic activity. Moreover, we find that amide groups increase interactions between COEs and a bacterial lipid mimic based on calcein leakage assay and allow COEs to readily permeabilize the cytoplasmic membrane of E. coli. These findings suggest that hydrogen bond forming moieties can be further applied in the molecular design of antimicrobial COEs to further improve their selectivity towards bacteria.

Pursuit of next-generation glycopeptides: a journey with vancomycin

Vancomycin, a blockbuster antibiotic of the glycopeptide class, has been a life-saving therapeutic against multidrug-resistant Gram-positive infections. The emergence of glycopeptide resistance has however enunciated the need to develop credible alternatives with potent activity against vancomycin-resistant bacteria. Medicinal chemistry has responded to this challenge through various strategies, one of them being the development of semisynthetic analogues. Many groups, including ours, have been contributing towards the development of semisynthetic vancomycin analogues to tackle vancomycin-resistant bacteria. In this feature article, we have discussed our research contribution to the field of glycopeptides, which includes our strategies and designs of vancomycin analogues incorporating multimodal mechanisms of action. The strategies discussed here, such as conferring membrane activity, enhanced binding to target, multivalency, etc. involve semisynthetic modifications to vancomycin at the carboxy terminal and the amino group of the vancosamine sugar of vancomycin, to develop novel analogues. These analogues have demonstrated their superior efficacy in tackling the inherited forms of vancomycin resistance in Gram-positive and Gram-negative bacteria, including highly drug-resistant strains. More importantly, these analogues also possess the ability to tackle various non-inherited forms of bacterial resistance, such as metabolically dormant stationary-phase and persister cells, bacterial biofilms, and intracellular pathogens. Our derivatives also display superior pharmacokinetics, and less propensity for resistance development, owing to their different modes of action. Through this feature article, we present to the reader a concise picture of the multitude of approaches that can be used to tackle different types of resistance through semisynthetic modifications to vancomycin. We have also highlighted the challenges and lacunae in the field, and potential directions which future research can explore.

Antimicrobial Peptides and Macromolecules for Combating Microbial Infections: From Agents to Interfaces

Bacterial resistance caused by the overuse of antibiotics and the shelter of biofilms has evolved into a global health crisis, which drives researchers to continuously explore antimicrobial molecules and strategies to fight against drug-resistant bacteria and biofilm-associated infections. Cationic antimicrobial peptides (AMPs) are considered to be a category of potential alternative for antibiotics owing to their excellent bactericidal potency and lesser likelihood of inducing drug resistance through their distinctive antimicrobial mechanisms. In this review, the hitherto reported plentiful action modes of AMPs are systematically classified into 15 types and three categories (membrane destructive, nondestructive membrane disturbance, and intracellular targeting mechanisms). Besides natural AMPs, cationic polypeptides, synthetic polymers, and biopolymers enable to achieve tunable antimicrobial properties by optimizing their structures. Subsequently, the applications of these cationic antimicrobial agents at the biointerface as contact-active surface coatings and multifunctional wound dressings are also emphasized here. At last, we provide our perspectives on the development of clinically significant cationic antimicrobials and related challenges in the translation of these materials.

Emerging Roles of Glycopeptide Antibiotics: Moving beyond Gram-Positive Bacteria

Glycopeptides, a class of cell wall biosynthesis inhibitors, have been the antibiotics of choice against drug-resistant Gram-positive bacterial infections. Their unique mechanism of action involving binding to the substrate of cell wall biosynthesis and substantial longevity in clinics makes this class of antibiotics an attractive choice for drug repurposing and reprofiling. However, resistance to glycopeptides has been observed due to alterations in the substrate, cell wall thickening, or both. The emergence of glycopeptide resistance has resulted in the development of synthetic and semisynthetic glycopeptide analogues to target acquired resistance. Recent findings demonstrate that these derivatives, along with some of the FDA approved glycopeptides have been shown to have antimicrobial activity against Gram-negative bacteria, Mycobacteria, and viruses thus expanding their spectrum of activity across the microbial kingdom. Additional mechanisms of action and identification of novel targets have proven to be critical in broadening the spectrum of activity of glycopeptides. This review focuses on the applications of glycopeptides beyond their traditional target group of Gram-positive bacteria. This will aid in making the scientific community aware about the nontraditional activity profiles of glycopeptides, identify the existing loopholes, and further explore this antibiotic class as a potential broad-spectrum antimicrobial agent.

Host Defense Peptide-Mimicking Polymers and Polymeric-Brush-Tethered Host Defense Peptides: Recent Developments, Limitations, and Potential Success

Amphiphilic antimicrobial polymers have attracted considerable interest as structural mimics of host defense peptides (HDPs) that provide a broad spectrum of activity and do not induce bacterial-drug resistance. Likewise, surface engineered polymeric-brush-tethered HDP is considered a promising coating strategy that prevents infections and endows implantable materials and medical devices with antifouling and antibacterial properties. While each strategy takes a different approach, both aim to circumvent limitations of HDPs, enhance physicochemical properties, therapeutic performance, and enable solutions to unmet therapeutic needs. In this review, we discuss the recent advances in each approach, spotlight the fundamental principles, describe current developments with examples, discuss benefits and limitations, and highlight potential success. The review intends to summarize our knowledge in this research area and stimulate further work on antimicrobial polymers and functionalized polymeric biomaterials as strategies to fight infectious diseases.

Membrane-disruptive peptides/peptidomimetics-based therapeutics: Promising systems to combat bacteria and cancer in the drug-resistant era

Membrane-disruptive peptides/peptidomimetics (MDPs) are antimicrobials or anticarcinogens that present a general killing mechanism through the physical disruption of cell membranes, in contrast to conventional chemotherapeutic drugs, which act on precise targets such as DNA or specific enzymes. Owing to their rapid action, broad-spectrum activity, and mechanisms of action that potentially hinder the development of resistance, MDPs have been increasingly considered as future therapeutics in the drug-resistant era. Recently, growing experimental evidence has demonstrated that MDPs can also be utilized as adjuvants to enhance the therapeutic effects of other agents. In this review, we evaluate the literature around the broad-spectrum antimicrobial properties and anticancer activity of MDPs, and summarize the current development and mechanisms of MDPs alone or in combination with other agents. Notably, this review highlights recent advances in the design of various MDP-based drug delivery systems that can improve the therapeutic effect of MDPs, minimize side effects, and promote the co-delivery of multiple chemotherapeutics, for more efficient antimicrobial and anticancer therapy.

Macromolecular Nanotherapeutics and Antibiotic Adjuvants to Tackle Bacterial and Fungal Infections

The escalating rise in the population of multidrug-resistant (MDR) pathogens coupled with their biofilm forming ability has struck the global health as nightmare. Alongwith the threat of aforementioned menace, the sluggish development of new antibiotics and the continuous deterioration of the antibiotic pipeline has stimulated the scientific community toward the search of smart and innovative alternatives. In near future, membrane targeting antimicrobial polymers, inspired from antimicrobial peptides, can stand out significantly to combat against the MDR superbugs. Many of these amphiphilic polymers can form nanoaggregates through self-assembly with superior and selective antimicrobial efficacy. Additionally, these macromolecular nanoaggregrates can be utilized to engineer smart antibiotic-delivery system for on-demand drug-release, exploiting the infection site's micoenvironment. This strategy substantially increases the local concentration of antibiotics and reduces the associated off-target toxicity. Furthermore, amphiphilc macromolecules can be utilized to rejuvinate obsolete antibiotics to tackle the drug-resistant infections. This review article highlights the recent developments in macromolecular architecture to design numerous nanostructures with broad-spectrum antimicrobial activity, their application in fabricating smart drug delivery systems and their efficacy as antibiotic adjuvants to circumvent antimicrobial resistance. Finally, the current challenges and future prospects are briefly discussed for further exploration and their practical application in clinical settings.

Macromolecular Crowding Effect on the Activity of Liposome-Bound Alkaline Phosphatase: A Paradoxical Inhibitory Action

The cytoplasm of a cell is extremely crowded, with 20-30% being large biomolecules. This crowding enforces a significant amount of the physical and chemical barrier around biomolecules, so understanding any biomolecular event within the cellular system is challenging. Unsurprisingly, enzymes show a diverse kind of catalytic behavior inside a crowded environment and thus have remained an area of active interest in the last few decades. The situation can become even more complex and exciting in the case of understanding the behavior of a membrane-bound enzyme (almost 25-30% of enzymes are membrane-bound) in such a crowded environment that until now has remained unexplored. Herein, we have particularly investigated how a membrane-bound enzyme (using liposome-bound alkaline phosphatase) can behave in a crowded environment comprising polymer molecule-like poly(ethylene glycol) (PEG) of different weights (PEG400, PEG4000, and PEG9000) and Ficoll 400. We have compared the activity using a polymer microbead conjugated enzyme and have found that liposome-bound alkaline phosphatase had much higher activity in crowded environments, showing the importance and superiority of soft-deformable particles (i.e., vesicles) over hard spheres in macro-molecularly crowded media. Interstingly, we have found a paradoxical behavior of inhibitors in terms of both their extent and pathway of inhibitory action. For instance, phosphates, known as competitive inhibitors in buffer, behave as uncompetitive inhibitors in liposome-bound enzymes in crowded media with an ∼5-fold less inhibitory effect, whereas phenyl alanine (an uncompetitive inhibitor in buffer) did not show any inhibitory potential when the enzyme was membrane-bound and in crowded media containing PEG9000 (30 wt %). Overall, this demonstration elucidates aspects of membrane-bound enzymes in crowded media in terms of both catalytic behavior and inhibitory actions and can lead to further studies of the understanding of enzymatic behavior in such complex crowded environments having a dampening effect in regular diffusive transport.

The Best Peptidomimetic Strategies to Undercover Antibacterial Peptides

Health-care systems that develop rapidly and efficiently may increase the lifespan of humans. Nevertheless, the older population is more fragile, and is at an increased risk of disease development. A concurrently growing number of surgeries and transplantations have caused antibiotics to be used much more frequently, and for much longer periods of time, which in turn increases microbial resistance. In 1945, Fleming warned against the abuse of antibiotics in his Nobel lecture: "The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant". After 70 years, we are witnessing the fulfilment of Fleming's prophecy, as more than 700,000 people die each year due to drug-resistant diseases. Naturally occurring antimicrobial peptides protect all living matter against bacteria, and now different peptidomimetic strategies to engineer innovative antibiotics are being developed to defend humans against bacterial infections.

Vancomycin Derivative Inactivates Carbapenem-Resistant Acinetobacter baumannii and Induces Autophagy

Vancomycin is a standard drug for the treatment of multidrug-resistant Gram-positive bacterial infections. Albeit, development of resistance (VRE, VRSA) and its inefficacy against persistent infections is a demerit. It is also intrinsically inactive against Gram-negative bacteria. Herein, we report a vancomycin derivative, VanQAmC₁₀, that addresses these challenges. VanQAmC₁₀ was rapidly bactericidal against carbapenem-resistant A. baumannii (6 log₁₀ CFU/mL reduction in 6 h), disrupted A. baumannii biofilms, and eradicated their stationary phase cells. In MRSA infected macrophages, the compound reduced the bacterial burden by 1.3 log₁₀ CFU/mL while vancomycin exhibited a static effect. Further investigation indicated that the compound, unlike vancomycin, promoted the intracellular degradative mechanism, autophagy, in mammalian cells, which may have contributed to its intracellular activity. The findings of the work provide new perspectives on the field of glycopeptide antibiotics.

Amphiphilic Cationic Macromolecules Highly Effective Against Multi-Drug Resistant Gram-Positive Bacteria and Fungi With No Detectable Resistance

The ever increasing threats of Gram-positive superbugs such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Staphylococcus aureus (VRSA), and vancomycin-resistant Enterococccus faecium (VRE) are serious matter of concern worldwide toward public health. Such pathogens cause repeated recurrence of infections through the formation of biofilms which consist of metabolically inactive or slow growing dormant bacterial population in vast majority. Concurrently, dispersal of biofilms originates even more virulent dispersed cells responsible for pathogenesis. Along with this, fungal infections most commonly associated with Candida albicans also created a major complicacy in human healthcare. Moreover, concomitant survival of C. albicans and MRSA in a multispecies biofilms created extremely complicated polymicrobial infections. Surprisingly, infections associated with single species biofilm as well as multiple species biofilm (co-existence of MRSA and C. albicans) are almost untreatable with conventional antibiotics. Therefore, the situation demands an urgent development of antimicrobial agent which would tackle persistent infections associated with bacteria, fungi and their biofilms. Toward this goal, herein we developed a new class of branched polyethyleneimine based amphiphilic cationic macromolecules (ACMs) bearing normal alkyl, alkyl ester and alkyl amide moieties. An optimized compound with dual activity against drug-resistant bacteria (MIC = 2-4 μg/mL) and fungi (MIC = 4-8 μg/mL) was identified with minimal toxicity toward human erythrocytes (HC₅₀ = 270 μg/mL). The lead compound, ACM-A_Hex (12) displayed rapid bactericidal and fungicidal kinetics (>5 log CFU/mL reduction within 1-4 h). It also killed metabolically dormant stationary (MRSA and VRE) and persister (S. aureus) cells. Moreover, this compound was able to disrupt the preformed biofilm of MRSA and reduced the bacterial burden related to the dispersed cells. It showed significant proficiencies to eliminate polymicrobial biofilms of MRSA and C. albicans. Bacteria also could not develop any resistant against this class of membrane active molecules even after 15 days of successive passages. Taken together this class of macromolecule can be developed further as a dual therapeutic agent to combat infections associated with bacterial and fungal co-existence.

Structural design and antimicrobial properties of polypeptides and saccharide–polypeptide conjugates

The development and progress of antimicrobial polypeptides and saccharide–polypeptide conjugates in regards to their structural design, biological functions and antimicrobial mechanism.

Tunable nanostructures by directional assembly of donor–acceptor supramolecular copolymers and antibacterial activity

This manuscript reports supramolecular copolymerization of amphiphilic donor (D) and acceptor (A) units and their antibacterial activity.

Electrochemically Enabled Double C-H Activation of Amides: Chemoselective Synthesis of Polycyclic Isoquinolinones

We developed an electrochemically enabled dehydrogenative annulation reaction of amides and alkynes for the synthesis of antitumor polycyclic isoquinolinones through a double C-H activation route. No external oxidant is required in this reaction, and electricity is used for Ru catalyst circulation. The most remarkable feature of this reaction is the effective improvement of product regioselectivity under mild electrolytic conditions in comparison with previously set strong oxidant conditions.

Small antibacterial molecules highly active against drug-resistant Staphylococcus aureus

The rapid growth of antibiotic resistance in Staphylococcus aureus coupled with their biofilm forming ability has made the infections difficult to treat with conventional antibiotics. This has created a massive threat towards public health and is a huge concern worldwide. Aiming to address this challenging issue, herein we report a new class of small antibacterial molecules (SAMs) with high antibacterial activity against multidrug-resistant S. aureus. The design principle of the molecules was based on the variation of hydrophobic/hydrophilic balance through incorporation of two quaternary ammonium groups, ethanol moieties, non-peptidic amide bonds and aliphatic chains. The lead compound, identified through a comprehensive analysis of structure-activity relationships, displayed high activity against clinical isolates of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant S. aureus (VRSA) with MIC values in the range of 1-4 μg mL-1. More importantly, this compound was capable of killing stationary phase bacteria and disrupting established biofilms of MRSA. Additionally, the compound revealed minimum toxicity towards human erythrocytes (HC50 = 577 μg mL-1) and did not show significant toxicity towards mammalian cells (MDCK and A549) up to 128 μg mL-1. Remarkably, the incorporation of non-peptidic amide bonds made the compounds less susceptible to degradation in human plasma, serum and mouse liver homogenate. Taken together, the results therefore indicate great promise for this class of molecules to be developed as potent antibacterial agents in treating infections caused by drug-resistant S. aureus.

Amino Acid Conjugated Polymers: Antibacterial Agents Effective against Drug-Resistant Acinetobacter baumannii with No Detectable Resistance

An optimum hydrophilic/hydrophobic balance has been recognized as a crucial parameter in designing cationic polymers that mimic antimicrobial peptides (AMPs). To date, this balance was achieved either by hydrophilicity variation through altering the nature and the number of cationic charges or by hydrophobicity modulation through incorporation of alkyl groups of different chain lengths. However, how the hydrophobicity variation through AMPs' building blocks-amino acids-influences the antibacterial efficacy of AMP-mimicking cationic polymers has rarely been explored. Toward this goal, herein we report a class of amino acid conjugated polymers (ACPs) with tunable antibacterial activity through a simple post-polymer-functionalization strategy. Our polymeric design comprised a permanent cationic charge in every repeating unit, whereby the hydrophobicity was tuned through incorporation of different amino acids. Our results revealed that the amino acid alteration has a strong influence on antibacterial efficacy. Upon increasing the amino acid side-chain hydrophobicity, both the antibacterial activity (against broad spectrum of bacteria) and toxicity increased. However, the distinct feature of this class of polymers was their good activity against Acinetobacter baumannii-the top most critical pathogen according to WHO, which has created an alarming situation worldwide, causing the majority of infections in humans. A nontoxic (no hemolysis even at 1000 μg/mL) ACP including a glycine residue (ACP-1 (Gly)) showed very good activity (MIC = 8-16 μg/mL) against both drug-sensitive and drug-resistant strains of A. baumannii, including clinical isolates. This polymer not only was rapidly bactericidal against growing planktonic A. baumannii but also killed nondividing stationary-phase cells instantaneously (<2 min). Moreover, it eradicated the established biofilm formed by drug-resistant A. baumannii clinical isolates. No propensity for bacterial resistance development against this polymer was seen even after 14 continuous passages. Taken together, the results highlight that hydrophobicity modulation through incorporation of amino acids in cationic polymers will provide a significant opportunity in designing new ACPs with potent antibacterial activity and minimum toxicity toward mammalian cells. More importantly, the excellent anti-A. baumannii efficacy of the optimized lead polymer indicates its immense potential for being developed as therapeutic agent.

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.

Structure-function relationships of nonviral gene vectors: Lessons from antimicrobial polymers

In recent years, substantial advances have been achieved in the design and synthesis of nonviral gene vectors. However, lack of effective and biocompatible vectors still remains a major challenge that hinders their application in clinical settings. In the past decade, there has been a rapid expansion of cationic antimicrobial polymers, due to their potent, rapid, and broad-spectrum biocidal activity against resistant microbes, and biocompatible features. Given that antimicrobial polymers share common features with nonviral gene vectors in various aspects, such as membrane affinity, functional groups, physicochemical characteristics, and unique macromolecular architectures, these polymers may provide us with inspirations to overcome challenges in the design of novel vectors toward more safe and efficient gene delivery in clinic. Building off these observations, we provide here an overview of the structure-function relationships of polymers for both antimicrobial applications and gene delivery by elaborating some key structural parameters, including functional groups, charge density, hydrophobic/hydrophilic balance, MW, and macromolecular architectures. By borrowing a leaf from antimicrobial agents, great advancement in the development of newer nonviral gene vectors with high transfection efficiency and biocompatibility will be more promising. STATEMENT OF SIGNIFICANCE: The development of gene delivery is still in the preclinical stage for the lack of effective and biocompatible vectors. Given that antimicrobial polymers share common features with gene vectors in various aspects, such as membrane affinity, functional groups, physicochemical characteristics, and unique macromolecular architectures, these polymers may provide us with inspirations to overcome challenges in the design of novel vectors toward more safe and efficient gene delivery in clinic. In this review, we systematically summarized the structure-function relationships of antimicrobial polymers and gene vectors, with which the design of more advanced nonviral gene vectors is anticipated to be further boosted in the future.

Strategies from nature: polycaprolactone-based mimetic antimicrobial peptide block copolymers with low cytotoxicity and excellent antibacterial efficiency

PCL₁₆-b-K_n diblock copolymers exhibit excellent antibacterial activities and low cytotoxicity. Meanwhile, they act by the pore-forming bactericidal mechanism without inducement of drug resistance.

Radical Cation Initiated Surface Polymerization on Photothermal Rubber for Smart Antifouling Coatings

Biofouling on surfaces of various materials has attracted considerable attention in biomedical and marine industries. Surface grafting based on covalent surface-initiated polymerization offers a popular route to address this problem by providing diverse robust polymer coatings capable of preventing the biofouling in complex environments. However, the existing methods for synthesizing polymer coatings are complicated and rigorous, or require special catalysts, greatly limiting their practical applications. In this work, a radical-cation-based surface-initiated polymerization protocol to graft the surface of darkened trans-polyisoprene (TPI) rubber with a thermo-responsive smart polymer, poly(N-isopropylacrylamide) (PNIPAM), through a simple iodine doping process is reported. A series of characterizations were performed to provide adequate evidence to confirm the successful grafting. Combining the thermal sensitivity of PNIPAM with the photothermal conversion ability of the darkened rubber, efficient bacteria-killing and antifouling capabilities were successfully achieved as a result of temperature-controlled iodine release and switchable amphiphilicity of PNIPAM.

Modulation of S. aureus and P. aeruginosa biofilm: an in vitro study with new coumarin derivatives

Coumarin is an important heterocyclic molecular framework of bioactive molecules against broad spectrum pathological manifestations. In the present study 18 new coumarin derivatives (CDs) were synthesized and characterized for antibiofilm activity against two model bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa. It was observed that all the CDs executed significant effect in moderating activities against both planktonic and biofilm forms of these selected bacteria. Hence, to interpret the underlying probable reason of such antibiofilm effect, in-silico binding study of CDs with biofilm and motility associated proteins of these organisms were performed. All CDs have shown their propensity for occupying the native substrate binding pocket of each protein with moderate to strong binding affinities. One of the CDs such as CAMN1 showed highest binding affinity with these proteins. Interestingly, the findings of in-silico studies coincides the experimental results of antibiofilm and motility affect of CDs against both S. aureus and P. aeruginosa. Moreover, in-silico studies suggested that the antibiofilm activity of test CDs may be due to the interference of biofilm and motility associated proteins of the selected model organisms (PilT from P. aeruginosa and TarK, TarO from S. aureus). The detailed synthesis, characterization, methodology and results of biological screening along with computational studies have been reported. This study could be of greater interest in the context of the development of new anti-bacterial agent in the future.

Designing of membrane-active nano-antimicrobials based on cationic copolymer functionalized nanodiamond: Influence of hydrophilic segment on antimicrobial activity and selectivity

Designing cationic nano-antimicrobial is a promising solution for combating drug resistant microbes. In this work, hydrophilic cationic copolymer was applied for the surface functionalization of nanodiamonds (NDs) aiming at developing a highly membrane-active nano-antibacterial agent with satisfactory selectivity. As a result, after functionalization, the increased repulsive forces within NDs and interaction with solvent molecular network made the heavily aggregated pristine NDs break down into tiny nanoparticles with particle size ranging from 10 to 100 nm. The improved hydrophilicity and enlarged surface area endowed QND-H₅ and QND-H₁₀ a powerful bactericidal capability toward both of Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). In the further bactericidal assessment, it was also demonstrated that the formation of hydrogen bonding between the 2-hydroxyethyl methacrylate (HEMA) side chains and lipid head groups of bacterial membrane also contributed to the enhanced bactericidal ability. Field emission scanning electron microscopy analysis confirmed that as-prepared nano-hybrid acted bactericidal ability via physical nature of outer membrane and cytoplasmic membrane-separating destruction mechanism toward E. coli, which may derive from the hydrogen bonding ability, making them more effective toward bacterial. More importantly, it was found that with just 10% of HEMA, QND-H₁₀ displayed good selectivity toward bacteria over mammalian cells as shown by the high HC₅₀ values with relatively low MIC values, suggesting the great potential application in medical fields. These results indicate that hydrogen bonding is an important element to achieve the desired high antibacterial activity and selectivity, particularly when cationic nano-antibacterial agents are required for medical application.

Recent Progress in Polymer Research to Tackle Infections and Antimicrobial Resistance

Global health is increasingly being threatened by the rapid emergence of drug-resistant microbes. The ability of these microbes to form biofilms has further exacerbated the scenario leading to notorious infections that are almost impossible to treat. For addressing this clinical threat, various antimicrobial polymers, polymer-based antimicrobial hydrogels and polymer-coated antimicrobial surfaces have been developed in the recent past. This review aims to discuss such polymer-based antimicrobial strategies with a focus on their current advancement in the field. Antimicrobial polymers, whose designs are inspired from antimicrobial peptides (AMPs), are described with an emphasis on structure-activity analysis. Additionally, antibiofilm activity and in vivo efficacy are delineated to elucidate the real potential of these antimicrobial polymers as possible therapeutics. Antimicrobial hydrogels, prepared from either inherently antimicrobial polymers or biocide-loaded into polymer-derived hydrogel matrix, are elaborated followed by various strategies to engineer polymer-coated antimicrobial surfaces. In the end, the current challenges are accentuated along with future directions for further expansion of the field toward tackling infections and antimicrobial resistance.

Antibacterial cotton fabrics based on hydrophilic amino-containing scaffolds

A hydrophilic amino compound, 4,7,10-trioxatridecane-1,13-diamine, has been utilized in several chemical and biochemical studies. Among previous applications is its use as a flexible and economical spacer molecule to increase the length between two moieties of interest, one of which may be a solid-phase interface. In this study, we immobilized this molecule on cotton fabrics and showed that this modified surface (DA) exhibited significant antibacterial activities in both Gram-negative bacteria and a Gram-positive bacterium. Studies on the structure-activity relationship revealed that additional chemical modifications on DA usually led to lowered antibacterial activities, emphasizing an importance of having free amino groups. Further investigation by fluorescence microscope indicated that this modified surface likely interfered with the membrane integrity of bacteria, leading to cell lysis. In addition, this scaffold was also tested for its biocompatibility with mouse fibroblast cells, and exerted no detrimental effect to the cell growth, highlighting its potential as a practical antibacterial surface modifier.

One-pot transition-metal free transamidation to sterically hindered amides

A highly efficient one-pot transamidation of primary amides has been developed under transition-metal free conditions, generating a variety of amides including hindered amides in good yield (up to 86%) catalyzed by CsF.

Selectively targeting bacteria by tuning the molecular design of membrane-active peptidomimetic amphiphiles

Here we report the design of membrane-active peptidomimetic molecules with a tunable arrangement of hydrophobic and polar groups.

Influence of lipid composition of model membranes on methacrylate antimicrobial polymer-membrane interactions

Using atomistic molecular dynamics simulations, the role of lipid composition in the interactions of multiple methacrylate antimicrobial polymer agents with model membranes, and the consequent response of the membranes is studied. In our earlier study, methacrylate polymers were observed to induce phase demixing and associated thickness mismatch in a POPE-POPG model microbial membrane. In this work, we probe (1) the role of varying the degree of saturation in lipid acyl chains in the membrane interactions of methacrylate polymers, and (2) whether electrostatics (addition of anionic lipids) can influence the interactions of the polymers with model mammalian membranes. Lipid composition is observed to significantly modify membrane-polymer interactions, leading to differences in both the mode of partitioning and the conformations adopted by the polymers, in addition to impacting membrane properties differently. The results strongly suggest that the oft-cited electrostatic interactions between the antimicrobial agents and the microbial membranes do not fully account for the recognition and subsequent partitioning of the antimicrobial agents. The ability of the methacrylate polymers to sense interfacial lipid packing defects, determined by the PE/PC head groups of lipids, is also found to be influential in their membrane partitioning. Deliberate inclusion of charged anionic lipids into a model mammalian membrane, leading to additional favorable electrostatics, does not reproduce a similar polymer partitioning mechanism to that in its microbial counterpart. The differences observed in the interactions of methacrylate polymers with the various model membranes can be instrumental in extending our understanding of underlying modes of membrane disruption by general antimicrobial agents as well.