Coupling quaternary ammonium surfactants to the surface of liposomes improves both antibacterial efficacy and host cell biocompatibility

By functionalizing the surface of PEG-liposomes with linkers bearing quaternary ammonium compounds (QACs), we generated novel bacteria disruptors with anti-adhesive properties and reduced cytotoxicity compared to free QACs. Furthermore, QAC-functionalized liposomes are a promising platform for future drug encapsulation. The QAC (11-mercaptoundecyl)-N,N,N-trimethylammonium bromide (MTAB) was attached to maleimide-functionalized liposomes (DSPE-PEG) via thiol linker. The MTAB-functionalized liposomes were physicochemically characterized and their biological activity, in terms of anti-adherence activity and biofilm prevention in Escherichia coli were assessed. The results showed that MTAB-functionalized liposomes inhibit bacterial adherence and biofilm formation while reducing MTAB toxicity.

Dual Corona Vesicles with Intrinsic Antibacterial and Enhanced Antibiotic Delivery Capabilities for Effective Treatment of Biofilm-Induced Periodontitis

Periodontitis is a common disease caused by plaque biofilms, which are important pathogenic factors of many diseases and may be eradicated by antibiotic therapy. However, low-dose antibiotic therapy is a complicated challenge for eradicating biofilms as hundreds (even thousands) of times higher concentrations of antibiotics are needed than killing planktonic bacteria. Polymer vesicles may solve these problems via effective antibiotic delivery into biofilms, but traditional single corona vesicles lack the multifunctionalities essential for biofilm eradication. In this paper, we aim to effectively treat biofilm-induced periodontitis using much lower concentrations of antibiotics than traditional antibiotic therapy by designing a multifunctional dual corona vesicle with intrinsic antibacterial and enhanced antibiotic delivery capabilities. This vesicle is co-assembled from two block copolymers, poly(ε-caprolactone)-block-poly(lysine-stat-phenylalanine) [PCL-b-P(Lys-stat-Phe)] and poly(ethylene oxide)-block-poly(ε-caprolactone) [PEO-b-PCL]. Both PEO and P(Lys-stat-Phe) coronas have their specific functions: PEO endows vesicles with protein repelling ability to penetrate extracellular polymeric substances in biofilms ("stealthy" coronas), whereas P(Lys-stat-Phe) provides vesicles with positive charges and broad spectrum intrinsic antibacterial activity. As a result, the dosage of antibiotics can be reduced by 50% when encapsulated in the dual corona vesicles to eradicate Escherichia coli or Staphylococcus aureus biofilms. Furthermore, effective in vivo treatment has been achieved from a rat periodontitis model, as confirmed by significantly reduced dental plaque, and alleviated inflammation. Overall, this "stealthy" and antibacterial dual corona vesicle demonstrates a fresh insight for improving the antibiofilm efficiency of antibiotics and combating the serious threat of biofilm-associated diseases.

Nitroxide Functionalized Antibiotics Are Promising Eradication Agents against Staphylococcus aureus Biofilms

Treatment of biofilm-related Staphylococcus aureus infections represents an important medical challenge worldwide, as biofilms, even those involving drug-susceptible S. aureus strains, are highly refractory to conventional antibiotic therapy. Nitroxides were recently shown to induce the dispersal of Gram-negative biofilms in vitro, but their action against Gram-positive bacterial biofilms remains unknown. Here, we demonstrate that the biofilm dispersal activity of nitroxides extends to S. aureus, a clinically important Gram-positive pathogen. Coadministration of the nitroxide CTEMPO (4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxyl) with ciprofloxacin significantly improved the biofilm eradication activity of the antibiotic against S. aureus Moreover, covalently linking the nitroxide to the antibiotic moiety further reduced the ciprofloxacin minimal biofilm eradication concentration. Microscopy analysis revealed that fluorescent nitroxide-antibiotic hybrids could penetrate S. aureus biofilms and enter cells localized at the surface and base of the biofilm structure. No toxicity to human cells was observed for the nitroxide CTEMPO or the nitroxide-antibiotic hybrids. Taken together, our results show that nitroxides can mediate the dispersal of Gram-positive biofilms and that dual-acting biofilm eradication antibiotics may provide broad-spectrum therapies for the treatment of biofilm-related infections.

Cell-Wall-Targeting Antibiotics Cause Lag-Phase Bacteria to Form Surface-Mediated Filaments Promoting the Formation of Biofilms and Aggregates

Antibiotics are known to promote bacterial formation of enhanced biofilms, the mechanism of which is not well understood. Here, using biolayer interferometry, we have shown that bacterial cultures containing antibiotics that target cell walls cause biomass deposition on surfaces over time with a linear profile rather than the Langmuir-like profiles exhibited by bacterial adherence in the absence of antibiotics. We observed about three times the initial rate and 12 times the final biomass deposition on surfaces for cultures containing carbenicillin than without. Unexpectedly, in the presence of antibiotics, the rate of biomass deposition inversely correlated with bacterial densities from different stages of a culture. Detailed studies revealed that carbenicillin caused faster growth of filaments that were seeded on surfaces from young bacteria (from lag phase) than those from high-density fast-growing bacteria, with rates of filament elongation of about 0.58 and 0.13 μm min^-1 , respectively. With surfaces that do not support bacterial adherence, few filaments were observed even in solution. These filaments aggregated in solution and formed increased amounts of biofilms on surfaces. These results reveal the lifestyle of antibiotic-induced filamentous bacteria, as well as one way in which the antibiotics promote biofilm formation.

Phenazine Antibiotic-Inspired Discovery of Bacterial Biofilm-Eradicating Agents

Bacterial biofilms are surface-attached communities of slow-growing and non-replicating persister cells that demonstrate high levels of antibiotic tolerance. Biofilms occur in nearly 80 % of infections and present unique challenges to our current arsenal of antibiotic therapies, all of which were initially discovered for their abilities to target rapidly dividing, free-floating planktonic bacteria. Bacterial biofilms are credited as the underlying cause of chronic and recurring bacterial infections. Innovative approaches are required to identify new small molecules that operate through bacterial growth-independent mechanisms to effectively eradicate biofilms. One source of inspiration comes from within the lungs of young cystic fibrosis (CF) patients, who often endure persistent Staphylococcus aureus infections. As these CF patients age, Pseudomonas aeruginosa co-infects the lungs and utilizes phenazine antibiotics to eradicate the established S. aureus infection. Our group has taken a special interest in this microbial competition strategy and we are investigating the potential of phenazine antibiotic-inspired compounds and synthetic analogues thereof to eradicate persistent bacterial biofilms. To discover new biofilm-eradicating agents, we have established an interdisciplinary research program involving synthetic medicinal chemistry, microbiology and molecular biology. From these efforts, we have identified a series of halogenated phenazines (HPs) that potently eradicate bacterial biofilms, and future work aims to translate these preliminary findings into ground-breaking clinical advances for the treatment of persistent biofilm infections.

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.

Biomaterial surfaces self-defensive against bacteria by contact transfer of antimicrobials

Despite extensive engineering of tissue-contacting biomedical devices to control healing, these devices remain susceptible to bacterial colonization, biofilm formation, and chronic infection. The threat of selecting for resistance genes largely precludes sustained antimicrobial elution as a wide-spread clinical solution. In response, self-defensive surfaces have been developed where antimicrobial is released only when and where there is a bacterial challenge. We explore a new self-defensive approach using anionic microgels into which small-molecule cationic antimicrobials are loaded by complexation. We identify conditions where antimicrobial remains sequestered within the microgels for periods as long as weeks. However, bacterial contact triggers release and leads to local bacterial killing. We speculate that the close proximity of bacteria alters the local thermodynamic environment and interferes with the microgel-antimicrobial complexation. The contact-transfer approach does not require bacterial metabolism but instead appears to be driven by differences between the microgels and the bacterial cell envelope where there is a high concentration of negative charge and hydrophobicity. Contact with metabolizing macrophages or osteoblasts is, however, insufficient to trigger antimicrobial release, indicating that contact transfer can be specific to bacteria and suggesting an avenue to biomedical device surfaces that can simultaneously promote healing and resist infection.

Eradicating uropathogenic Escherichia coli biofilms with a ciprofloxacin-dinitroxide conjugate

Urinary tract infections (UTIs) are amongst the most common and prevalent infectious diseases worldwide, with uropathogenic Escherichia coli (UPEC) reported as the main causative pathogen. Fluoroquinolone antibiotics are commonly used to treat UTIs but for infections involving UPEC biofilms, which are commonly associated with catheter use and recurrent episodes, ciprofloxacin is often ineffective. Here we report the development of a ciprofloxacin-dinitroxide (CDN) conjugate with potent UPEC biofilm-eradication activity. CDN 11 exhibited a 2-fold increase in potency over the parent antibiotic ciprofloxacin against UPEC biofilms. Moreover, CDN 11 resulted in almost complete UPEC biofilm cell eradication (99.7%) at concentrations as low as 12.5 μM, and significantly potentiated ciprofloxacin's biofilm-eradication activity against UPEC upon co-administration. The biofilm-eradication activity of CDN 11 highlights the potential of nitroxide functionalized antibiotics as a promising strategy for the treatment of biofilm-related UTIs.

Cationic Amphiphiles with Specificity against Gram-Positive and Gram-Negative Bacteria: Chemical Composition and Architecture Combat Bacterial Membranes

Small-molecule cationic amphiphiles (CAms) were designed to combat the rapid rise in drug-resistant bacteria. CAms were designed to target and compromise the structural integrity of bacteria membranes, leading to cell rupture and death. Discrete structural features of CAms were varied, and structure-activity relationship studies were performed to guide the rational design of potent antimicrobials with desirable selectivity and cytocompatibility profiles. In particular, the effects of cationic conformational flexibility, hydrophobic domain flexibility, and hydrophobic domain architecture were evaluated. Their influence on antimicrobial efficacy in Gram-positive and Gram-negative bacteria was determined, and their safety profiles were established by assessing their impact on mammalian cells. All CAms have a potent activity against bacteria, and hydrophobic domain rigidity and branched architecture contribute to specificity. The insights gained from this project will aid in the optimization of CAm structures.

Sepsis: mechanisms of bacterial injury to the patient

In bacteremia the majority of bacterial species are killed by oxidation on the surface of erythrocytes and digested by local phagocytes in the liver and the spleen. Sepsis-causing bacteria overcome this mechanism of human innate immunity by versatile respiration, production of antioxidant enzymes, hemolysins, exo- and endotoxins, exopolymers and other factors that suppress host defense and provide bacterial survival. Entering the bloodstream in different forms (planktonic, encapsulated, L-form, biofilm fragments), they cause different types of sepsis (fulminant, acute, subacute, chronic, etc.). Sepsis treatment includes antibacterial therapy, support of host vital functions and restore of homeostasis. A bacterium killing is only one of numerous aspects of antibacterial therapy. The latter should inhibit the production of bacterial antioxidant enzymes and hemolysins, neutralize bacterial toxins, modulate bacterial respiration, increase host tolerance to bacterial products, facilitate host bactericidal mechanism and disperse bacterial capsule and biofilm.

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.

Synthetic small molecules as anti-biofilm agents in the struggle against antibiotic resistance

Biofilm formation significantly contributes to microbial survival in hostile environments and it is currently considered a key virulence factor for pathogens responsible for serious chronic infections. In the last decade many efforts have been made to identify new agents able to modulate bacterial biofilm life cycle, and many compounds have shown interesting activities in inhibiting biofilm formation or in dispersing pre-formed biofilms. However, only a few of these compounds were tested using in vivo models for their clinical significance. Contrary to conventional antibiotics, most of the anti-biofilm compounds act as anti-virulence agents as they do not affect bacterial growth. In this review we selected the most relevant literature of the last decade, focusing on the development of synthetic small molecules able to prevent bacterial biofilm formation or to eradicate pre-existing biofilms of clinically relevant Gram-positive and Gram-negative pathogens. In addition, we provide a comprehensive list of the possible targets to counteract biofilm formation and development, as well as a detailed discussion the advantages and disadvantages of the different current biofilm-targeting strategies.

Matrix assisted antibacterial activity of polymer conjugates with pendant antibiotics, and bioactive and biopassive moieties

We report the comparative antibacterial activity of a polymer–antibiotic conjugate with coupled bioactive (cationic) and biopassive (zwitterionic) functionalities against several biofilm and nonbiofilm forming bacterial species in both liquid and solid matrices.

Epoxy Isonitriles, A Unique Class of Antibiotics: Synthesis of Their Metabolites and Biological Investigations

Epoxy isonitrile containing natural products often possess specific and potent antibacterial activity against Gram-positive pathogens. This scaffold, however, is extremely labile under acidic and basic conditions, undergoing a Payne rearrangement to produce a stable epoxy ketone metabolite and releasing hydrogen cyanide. We synthesized and performed biological assays with epoxy ketone containing metabolites and identified that the epoxy isonitrile moiety is pertinent for biological activity. Serendipitously, we discovered an α,β-unsaturated epoxy ketone analogue that exhibited moderate activity against Staphylococcus aureus.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.

Halogenated quinolines bearing polar functionality at the 2-position: Identification of new antibacterial agents with enhanced activity against Staphylococcus epidermidis

Antibiotic-resistant bacteria and surface-attached biofilms continue to play a significant role in human health and disease. Innovative strategies are needed to identify new therapeutic leads to tackle infections of drug-resistant and tolerant bacteria. We synthesized a focused library of 14 new halogenated quinolines to investigate the impact of ClogP values on antibacterial and biofilm-eradication activities. During these investigations, we found select polar appendages at the 2-position of the HQ scaffold were more well-tolerated than others. We were delighted to see multiple compounds display enhanced activities against the major human pathogen S. epidermidis. In particular, HQ 2 (ClogP = 3.44) demonstrated enhanced activities against MRSE 35984 planktonic cells (MIC = 0.59 μM) compared to MRSA and VRE strains in addition to potent MRSE biofilm eradication activities (MBEC = 2.35 μM). Several of the halogenated quinolines identified here reported low cytotoxicity against HeLa cells with minimal hemolytic activity against red blood cells. We believe that halogenated quinoline small molecules could play an important role in the development of next-generation antibacterial therapeutics capable of targeting and eradicating biofilm-associated infections.

Bis-Cyclic Guanidines as a Novel Class of Compounds Potent against Clostridium difficile

Clostridium difficile infection (CDI) symptoms range from diarrhea to severe toxic megacolon and even death. Due to its rapid acquisition of resistance, C. difficile is listed as an urgent antibiotic-resistant threat, and has surpassed methicillin-resistant Staphylococcus aureus (MRSA) as the most common hospital-acquired infection in the USA. To combat this pathogen, a new structural class of pseudo-peptides that exhibit antimicrobial activities could play an important role. Herein we report a set of bis-cyclic guanidine compounds that show potent antibacterial activity against C. difficile with decent selectivity. Eight compounds showed high in vitro potency against C. difficile UK6 with MIC values of 1.0 μg mL^-1 , and cytotoxic selectivity index (SI) values up to 37. Moreover, the most selective compound is also effective in the treatment of C. difficile-induced disease in a mouse model of CDI, and appears to be a very promising new candidate for the treatment of CDI.

Viability, Enzymatic and Protein Profiles of Pseudomonas aeruginosa Biofilm and Planktonic Cells after Monomeric/Gemini Surfactant Treatment

This study set out to investigate the biological activity of monomeric surfactants dodecyltrimethylammonium bromide (DTAB) and the next generation gemini surfactant hexamethylene-1,6-bis-(N,N-dimethyl-N-dodecylammonium bromide) (C6) against the environmental strain Pseudomonas aeruginosa PB_1. Minimal inhibitory concentrations (MIC) were determined using the dilution method. The viability of the planktonic cells and biofilm was assessed using the plate count method. Enzymatic profile was determined using the API-ZYM system. Proteins were extracted from the biofilm and planktonic cells and analysed using SDS-PAGE. The MIC of the gemini surfactants was 70 times lower than that of its monomeric analogue. After 4 h of treatment at MIC (0.0145 mM for C6 and 1.013 mM for DTAB), the number of viable planktonic cells was reduce by less than 3 logarithm units. At the concentration ≥MIC, a reduction in the number of viable cells was observed in mature biofilms (p < 0.05). Treatment for 4 h with gemini surfactant at 20 MIC caused complete biofilm eradication. At sub-MIC, the concentration of some enzymes reduced and their protein profiles changed. The results of this study show that due to its superior antibacterial activity, gemini compound C6 can be applied as an effective microbiocide against P. aeruginosa in both planktonic and biofilm forms.

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.

Imidazolium and benzimidazolium-containing compounds: from simple toxic salts to highly bioactive drugs

The toxicity of simple imidazolium and benzimidazolium salts started to be more and more investigated in the last few years and was taken in consideration in the context of microorganisms, plants and more evolved organisms' exposure. However, the toxicity of these salts can be exploited in the development of different biological applications by incorporating them in the structure of compounds that specifically target microorganisms and cancer cells. We highlight in this minireview the way researchers became aware of the inherent problem of the stability and bioaccumulation of imidazolium and benzimidazolium salts and how they found inspiration to exploit their toxicity by incorporating them into new highly potent drugs.

Dual Inhibitor of Staphylococcus aureus Virulence and Biofilm Attenuates Expression of Major Toxins and Adhesins

Staphylococcus aureus is a major bacterial pathogen that invades and damages host tissue by the expression of devastating toxins. We here performed a phenotypic screen of 35 molecules that were structurally inspired by previous hydroxyamide-based S. aureus virulence inhibitors compiled from commercial sources or designed and synthesized de novo. One of the most potent compounds, AV73, not only reduced hemolytic alpha-hemolysin production in S. aureus but also impeded in vitro biofilm formation. The effect of AV73 on bacterial proteomes and extracellular protein levels was analyzed by quantitative proteomics and revealed a significant down-regulation of major virulence and biofilm promoting proteins. To elucidate the mode of action of AV73, target identification was performed using affinity-based protein profiling (AfBPP), where among others YidC was identified as a target.

Diverted Total Synthesis of the Baulamycins and Analogues Reveals an Alternate Mechanism of Action

The baulamycins were identified as in vitro siderophore biosynthesis inhibitors. Diverted total synthesis was used to construct the natural products and eight strategic analogues, three of which had improved inhibitory activity. Biological testing then revealed that membrane damage is the predominant mode of action in Staphylococcus aureus cells.

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.

Synthesis and bioactivities study of new antibacterial peptide mimics: The dialkyl cationic amphiphiles

The emergence of infectious diseases caused by pathogenic bacteria is widespread. Therefore, it is urgently required to enhance the development of novel antimicrobial agents with high antibacterial activity and low cytotoxicity. A series of novel dialkyl cationic amphiphiles bearing two identical length lipophilic alkyl chains and one non-peptidic amide bond were synthesized and tested for antimicrobial activities against both Gram-positive and Gram-negative bacteria. Particular compounds synthesized showed excellent antibacterial activity toward drug-sensitive bacteria such as S. aureus, E. faecalis, E. coli and S. enterica, and clinical isolates of drug-resistant species such as methicillin-resistant S. aureus (MRSA), KPC-producing and NDM-1-producing carbapenem-resistant Enterobacteriaceae (CRE). For example, the MIC values of the best compound 4g ranged from 0.5 to 2 μg/mL against all these strains. Moreover, these small molecules acted rapidly as bactericidal agents, and functioned primarily by permeabilization and depolarization of bacterial membranes. Importantly, these compounds were difficult to induce bacterial resistance and can potentially combat drug-resistant bacteria. Thus, these compounds can be developed into a new class of antibacterial peptide mimics against Gram-positive and Gram-negative bacteria, including drug-resistant bacterial strains.

Membrane-active macromolecules kill antibiotic-tolerant bacteria and potentiate antibiotics towards Gram-negative bacteria

Chronic bacterial biofilms place a massive burden on healthcare due to the presence of antibiotic-tolerant dormant bacteria. Some of the conventional antibiotics such as erythromycin, vancomycin, linezolid, rifampicin etc. are inherently ineffective against Gram-negative bacteria, particularly in their biofilms. Here, we report membrane-active macromolecules that kill slow dividing stationary-phase and antibiotic tolerant cells of Gram-negative bacteria. More importantly, these molecules potentiate antibiotics (erythromycin and rifampicin) to biofilms of Gram-negative bacteria. These molecules eliminate planktonic bacteria that are liberated after dispersion of biofilms (dispersed cells). The membrane-active mechanism of these molecules forms the key for potentiating the established antibiotics. Further, we demonstrate that the combination of macromolecules and antibiotics significantly reduces bacterial burden in mouse burn and surgical wound infection models caused by Acinetobacter baumannii and Carbapenemase producing Klebsiella pneumoniae (KPC) clinical isolate respectively. Colistin, a well-known antibiotic targeting the lipopolysaccharide (LPS) of Gram-negative bacteria fails to kill antibiotic tolerant cells and dispersed cells (from biofilms) and bacteria develop resistance to it. On the contrary, these macromolecules prevent or delay the development of bacterial resistance to known antibiotics. Our findings emphasize the potential of targeting the bacterial membrane in antibiotic potentiation for disruption of biofilms and suggest a promising strategy towards developing therapies for topical treatment of Gram-negative infections.

Design and Solution-Phase Synthesis of Membrane-Targeting Lipopeptides with Selective Antibacterial Activity

Designing selective antibacterial molecules remains an unmet goal in the field of membrane-targeting agents. Herein, we report the rational design and synthesis of a new class of lipopeptides, which possess highly selective bacterial killing over mammalian cells. The selective interaction with bacterial over mammalian membranes was established through various spectroscopic, as well as microscopic experiments, including biophysical studies with the model membranes. A detailed antibacterial structure-activity relationship was delineated after preparing a series of molecules consisting of the peptide moieties with varied sequence of amino acids, such as d-phenylalanine, d-leucine, and d-lysine. Antibacterial activity was found to vary with the nature and positioning of hydrophobicity in the molecules, as well as number of positive charges. Optimized lipopeptide 9 did not show any hemolytic activity even at 1000 μg mL^-1 and displayed >200-fold and >100-fold selectivity towards S. aureus and E. coli, respectively. More importantly, compound 9 was found to display good antibacterial activity (MIC 6.3-12.5 μg mL^-1 ) against the five top most critical bacteria according to World Health Organization (WHO) priority pathogens list. Therefore, the results suggested that this new class of lipopeptides bear real promises for the development as future antibacterial agents.

Hydra amphiphiles: Using three heads and one tail to influence aggregate formation and to kill pathogenic bacteria

Hydra amphiphiles mimic the morphology of the mythical multi-headed creatures for which they are named. Likewise, when faced with a pathogenic bacterium, some hydra derivatives are as destructive as their fabled counterparts were to their adversaries. This report focuses on eight new tricephalic (triple-headed), single-tailed amphiphiles. Each amphiphile has a mesitylene (1,3,5-trimethylbenzene) core, two benzylic trimethylammonium groups and one dimethylalkylammonium group with a linear hydrophobe ranging from short (C₈H₁₇) to ultralong (C₂₂H₄₅). The logarithm of the critical aggregation concentration, log(CAC), decreases linearly with increasing tail length, but with a smaller dependence than that of ionic amphiphiles with fewer head groups. Tail length also affects antibacterial activity; amphiphiles with a linear 18 or 20 carbon atom hydrophobic chain are more effective at killing bacteria than those with shorter or longer chains. Comparison to a recently reported amphiphilic series with three heads and two tails allows for the development of an understanding of the relationship between number of tails and both colloidal and antibacterial properties.

Direct Synthesis of Dextran-Based Antibacterial Hydrogels for Extended Release of Biocides and Eradication of Topical Biofilms

Cationic small molecular biocides have been developed as promising antibiofilm agents because of their tunability in chemical structures and their ability to disrupt established biofilms. However, the impact of biocides in antibiofilm treatment is largely limited due to the lack of an effective delivery system that can ensure sustained release of biocides at the target site. Herein we report a biocide-encapsulated antibacterial and antibiofilm hydrogel that acts as an efficient delivery vehicle for the biocide and eradicates matured bacterial biofilm. The hydrogels are prepared using dextran methacrylate (Dex-MA), a biocompatible and photopolymerizable polymer, and a nontoxic cationic biocide with two cationic charges, two nonpeptidic amide bonds, and optimized amphiphilicity, which is capable of eradicating established bacterial biofilms. The gels, prepared via direct loading of the biocide and with highly controllable amounts, display 100% activity against both drug-sensitive and drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA). Importantly, the gels are shown to release the biocide and kill bacteria for an extended period of time (until day 5). When being treated with the established bacterial biofilms, the released biocide from the gel is shown to completely eradicate establishedS. aureus, Escherichia coli, and MRSA biofilms, the most common biofilm forming bacteria that cause severe infections (e.g., skin infections, urinary tract infections, etc.) in humans. Moreover, the gels were shown to annihilate preformed MRSA biofilm with >99.99% bacterial reduction under in vitro and in vivo conditions in a superficial MRSA infection model in mice. Notably, when tested, excellent skin compatibility is observed for these materials in various animal models such as a rat model of acute dermal toxicity, guinea pig model of skin sensitization, and rabbit model of skin irritation. The biocompatible antibacterial and antibiofilm hydrogels developed herein thus might be useful in treating bacterial biofilm associated infections, especially topical infections.

A Highly Potent Class of Halogenated Phenazine Antibacterial and Biofilm-Eradicating Agents Accessed Through a Modular Wohl-Aue Synthesis

Unlike individual, free-floating planktonic bacteria, biofilms are surface-attached communities of slow- or non-replicating bacteria encased within a protective extracellular polymeric matrix enabling persistent bacterial populations to tolerate high concentrations of antimicrobials. Our current antibacterial arsenal is composed of growth-inhibiting agents that target rapidly-dividing planktonic bacteria but not metabolically dormant biofilm cells. We report the first modular synthesis of a library of 20 halogenated phenazines (HP), utilizing the Wohl-Aue reaction, that targets both planktonic and biofilm cells. New HPs, including 6-substituted analogues, demonstrate potent antibacterial activities against MRSA, MRSE and VRE (MIC = 0.003-0.78 µM). HPs bind metal(II) cations and demonstrate interesting activity profiles when co-treated in a panel of metal(II) cations in MIC assays. HP 1 inhibited RNA and protein biosynthesis while not inhibiting DNA biosynthesis using ³H-radiolabeled precursors in macromolecular synthesis inhibition assays against MRSA. New HPs reported here demonstrate potent eradication activities (MBEC = 0.59-9.38 µM) against MRSA, MRSE and VRE biofilms while showing minimal red blood cell lysis or cytotoxicity against HeLa cells. PEG-carbonate HPs 24 and 25 were found to have potent antibacterial activities with significantly improved water solubility. HP small molecules could have a dramatic impact on persistent, biofilm-associated bacterial infection treatments.

Sepsis and septic shock: Pathogenesis and treatment perspectives

The majority of bacteremias do not develop to sepsis: bacteria are cleared from the bloodstream. Oxygen released from erythrocytes and humoral immunity kill bacteria in the bloodstream. Sepsis develops if bacteria are resistant to oxidation and proliferate in erythrocytes. Bacteria provoke oxygen release from erythrocytes to arterial blood. Abundant release of oxygen to the plasma triggers a cascade of events that cause: 1. oxygen delivery failure to cells; 2. oxidation of plasma components that impairs humoral regulation and inactivates immune complexes; 3. disseminated intravascular coagulation and multiple organs' failure. Bacterial reservoir inside erythrocytes provides the long-term survival of bacteria and is the cause of ineffectiveness of antibiotics and host immune reactions. Treatment perspectives that include different aspects of sepsis development are discussed.

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.

Synthetic analogs of rhamnolipids modulate structured biofilms formed by rhamnolipid-nonproducing mutant of Pseudomonas aeruginosa

Rhamnolipids secreted by Pseudomonas aeruginosa are required for the bacteria to form biofilm efficiently and form biofilm with internal structures including pores and channels. In this work, we explore the effect of a class of synthetic analogs of rhamnolipids at controlling (promoting and inhibiting) the biofilm formation activities of a non-rhamnolipid-producing strain - rhlA - of P. aeruginosa. This class of rhamnolipid analogs is known to modulate the swarming motilities of wild-type PAO1 and rhlA mutant, but its effect on biofilm formation of rhlA mutant is unknown. We show that small structural details of these molecules are important for the bioactivities, but do not affect the general physical properties of the molecules. The bioactive synthetic analogs of rhamnolipids promote biofilm formation by rhlA mutant at low concentrations, but inhibit the biofilm formation at high concentrations. To explore the internal structures formed by the biofilms, we first demonstrate that wild-type biofilms are formed with substantial topography (hills and valleys) when the sample is under shaking conditions. Using this observation as a comparison, we found that synthetic analogs of rhamnolipids promoted structured (porous) biofilm of rhlA mutant, at intermediate concentrations between the low ones that promoted biofilm formation and the high ones that inhibited biofilm formation. This study suggests a potential chemical signaling approach to control multiple bacterial activities.

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.

Identification of N-Arylated NH125 Analogues as Rapid Eradicating Agents against MRSA Persister Cells and Potent Biofilm Killers of Gram-Positive Pathogens

Bacterial biofilms housing dormant persister cells are innately tolerant to antibiotics and disinfectants, yet several membrane-active agents are known to eradicate tolerant bacterial cells. NH125, a membrane-active persister killer and starting point for development, led to the identification of two N-arylated analogues (1 and 2) that displayed improved biofilm eradication potencies compared to the parent compound and rapid persister-cell-killing activities in stationary cultures of methicillin-resistant Staphylococcus aureus (MRSA). We found 1 and 2 to be superior to other membrane-active agents in biofilm eradication assays, with 1 demonstrating minimum biofilm eradication concentrations (MBEC) of 23.5, 11.7, and 2.35 μm against MRSA, methicillin-resistant Staphylococcus epidermidis (MRSE), and vancomycin-resistant Enterococcus faecium (VRE) biofilms, respectively. We tested our panel of membrane-active agents against MRSA stationary cultures and found 1 to rapidly eradicate MRSA stationary cells by 4 log units (99.99 %) in 30 min. The potent biofilm eradication and rapid persister-cell-killing activities exhibited by N-arylated NH125 analogues could have significant impact in addressing biofilm-associated problems.

Recent advances in thromboresistant and antimicrobial polymers for biomedical applications: just say yes to nitric oxide (NO)

Biomedical devices are essential for patient diagnosis and treatment; however, when blood comes in contact with foreign surfaces or homeostasis is disrupted, complications including thrombus formation and bacterial infections can interrupt device functionality, causing false readings and/or shorten device lifetime. Here, we review some of the current approaches for developing antithrombotic and antibacterial materials for biomedical applications. Special emphasis is given to materials that release or generate low levels of nitric oxide (NO). Nitric oxide is an endogenous gas molecule that can inhibit platelet activation as well as bacterial proliferation and adhesion. Various NO delivery vehicles have been developed to improve NO's therapeutic potential. In this review, we provide a summary of the NO releasing and NO generating polymeric materials developed to date, with a focus on the chemistry of different NO donors, the polymer preparation processes, and in vitro and in vivo applications of the two most promising types of NO donors studied thus far, N-diazeniumdiolates (NONOates) and S-nitrosothiols (RSNOs).

Halogenated quinolines discovered through reductive amination with potent eradication activities against MRSA, MRSE and VRE biofilms

Small molecules capable of eradicating non-replicating bacterial biofilms are of great importance to human health as conventional antibiotics are ineffective against these surface-attached bacterial communities. Here, we report the discovery of several halogenated quinolines (HQs) identified through a reductive amination reaction that demonstrated potent eradication of MRSA (HQ-6; MBEC = 125 μM), MRSE (HQ-3; MBEC = 3.0 μM) and VRE (HQ-4, HQ-5 and HQ-6; MBEC = 1.0 μM) biofilms. HQs were evaluated using the Calgary Biofilm Device (CBD) and demonstrated near equipotent killing activities against planktonic and biofilm cells based on MBC and MBEC values. When tested against red blood cells, these HQ analogues demonstrated low haemolytic activity (3 to 21% at 200 μM) thus we conclude that these HQ analogues do not operate primarily through the destruction of bacterial membranes, typical of other biofilm-eradicating agents (i.e., antimicrobial peptides). HQ antibacterial agents are potent biofilm-eradicating compounds and could lead to useful treatments for biofilm-associated bacterial infections.