Membrane permeabilization of colistin toward pan-drug resistant Gram-negative isolates

Prostaglandin F2α analogue, bimatoprost ameliorates colistin-induced nephrotoxicity

Colistin (polymyxin E) is an antibiotic that is effective against multidrug-resistant gram-negative bacteria. However, the high incidence of nephrotoxicity caused by colistin limits its clinical use. To identify compounds that might ameliorate colistin-induced nephrotoxicity, we obtained 1707 compounds from the Korea Chemical Bank and used a high-content screening (HCS) imaging-based assay. In this way, we found that bimatoprost (one of prostaglandin F2α analogue) ameliorated colistin-induced nephrotoxicity. To further assess the effects of bimatoprost on colistin-induced nephrotoxicity, we used in vitro and in vivo models. In cultured human proximal tubular cells (HK-2), colistin induced dose-dependent cytotoxicity. The number of terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL)-positive cells, indicative of apoptosis, was higher in colistin-treated cells, but this effect of colistin was ameliorated by cotreatment with bimatoprost. The generation of reactive oxygen species, assessed using 2,7-dichlorodihydrofluorescein diacetate, was less marked in cells treated with both colistin and bimatoprost than in those treated with colistin alone. Female C57BL/6 mice (n = 10 per group) that were intraperitoneally injected with colistin (10 mg/kg/12 hr) for 14 days showed high blood urea nitrogen and serum creatinine concentrations that were reduced by the coadministration of bimatoprost (0.5 mg/kg/12 hr). In addition, kidney injury molecule-1 (KIM1) and Neutrophil gelatinase-associated lipocalin (NGAL) expression also reduced by bimatoprost administration. Further investigation in tubuloid and kidney organoids also showed that bimatoprost attenuated the nephrotoxicity by colistin, showing dose-dependent reducing effect of KIM1 expression. In this study, we have identified bimatoprost, prostaglandin F2α analogue as a drug that ameliorates colistin-induced nephrotoxicity.

Structure-activity relationships of antibacterial peptides

Antimicrobial peptides play a crucial role in innate immunity, whose components are mainly peptide-based molecules with antibacterial properties. Indeed, the exploration of the immune system over the past 40 years has revealed a number of natural peptides playing a pivotal role in the defence mechanisms of vertebrates and invertebrates, including amphibians, insects, and mammalians. This review provides a discussion regarding the antibacterial mechanisms of peptide-based agents and their structure-activity relationships (SARs) with the aim of describing a topic that is not yet fully explored. Some growing evidence suggests that innate immunity should be strongly considered for the development of novel antibiotic peptide-based libraries. Also, due to the constantly rising concern of antibiotic resistance, the development of new antibiotic drugs is becoming a priority of global importance. Hence, the study and the understanding of defence phenomena occurring in the immune system may inspire the development of novel antibiotic compound libraries and set the stage to overcome drug-resistant pathogens. Here, we provide an overview of the importance of peptide-based antibacterial sources, focusing on accurately selected molecular structures, their SARs including recently introduced modifications, their latest biotechnology applications, and their potential against multi-drug resistant pathogens. Last, we provide cues to describe how antibacterial peptides show a better scope of action selectivity than several anti-infective agents, which are characterized by non-selective activities and non-targeted actions toward pathogens.

Efflux pump inhibitor combined with ofloxacin decreases MRSA biofilm formation by regulating the gene expression of NorA and quorum sensing

Carbonyl cyanide p-nitrophenylhydrazone (2e) displayed a lone or synergistic efficacy against MRSA (RSC Adv., 2020, 10, 17854). In this work, the synergistic mechanism of 2e with ofloxacin was studied. MRSA2858 had potential for biofilm formation, and the value of MBEC of 2e alone was 0.78-1.56 μM, while that of 2e + ofloxacin was 0.39-0.78 μM. 2e combined with ofloxacin showed a synergistic anti-biofilm effect against MRSA. Efflux pump inhibitor 2e can better bind to NorA protein. After MRSA2858 was treated with 2e of 1/2MIC (0.78 μM) and ofloxacin of 1/8MIC (0.097 μM), the transcript levels of efflux genes (norA) and quorum-sensing (QS) regulatory genes (agrA, sarA, icaA, hla) were substantially down-regulated, and alpha-hemolysin (Hla) was inhibited by 99.15%. 2e combined with ofloxacin was more effective than 2e alone in reducing bacterial load in vivo. All in all, efflux pump inhibitor 2e enhanced the bactericidal activities of antibiotics through regulating the gene expression of NorA and QS system.

The Extent of Antimicrobial Resistance Due to Efflux Pump Regulation

A key mechanism driving antimicrobial resistance (AMR) stems from the ability of bacteria to up-regulate efflux pumps upon exposure to drugs. The resistance gained by this up-regulation is pliable because of the tight regulation of efflux pump levels. This leads to temporary enhancement in survivability of bacteria due to higher efflux pump levels in the presence of antibiotics, which can be reversed when the cells are no longer exposed to the drug. Knowledge of the extent of resistance thus gained would inform intervention strategies aimed at mitigating AMR. Here, we combine mathematical modeling and experiments to quantify the maximum extent of resistance that efflux pump up-regulation can confer via phenotypic induction in the presence of drugs and genotypic abrogation of regulation. Our model describes the dynamics of drug transport in and out of cells coupled with the associated regulation of efflux pump levels and predicts the increase in the minimum inhibitory concentration (MIC) of drugs due to such regulation. To test the model, we measured the uptake and efflux as well as the MIC of the compound ethidium bromide (EtBr), a substrate of the efflux pump LfrA, in wild-type Mycobacterium smegmatis mc²155, as well as in two laboratory-generated strains. Our model captured the observed EtBr levels and MIC fold-changes quantitatively. Further, the model identified key parameters associated with the resulting resistance, variations in which could underlie the extent to which such resistance arises across different drug-bacteria combinations, potentially offering tunable handles to optimize interventions aimed at minimizing AMR.

Neutrophil-mediated delivery of the combination of colistin and azithromycin for the treatment of bacterial infection

Novel treatment strategies are in urgent need to deal with the rapid development of antibiotic-resistant superbugs. Combination therapies and targeted drug delivery have been exploited to promote treatment efficacies. In this study, we loaded neutrophils with azithromycin and colistin to combine the advantages of antibiotic combinations, targeted delivery, and immunomodulatory effect of azithromycin to treat infections caused by Gram-negative pathogens. Delivery of colistin into neutrophils was mediated by fusogenic liposome, while azithromycin was directly taken up by neutrophils. Neutrophils loaded with the drugs maintained the abilitity to generate reactive oxygen species and migrate. In vitro assays demonstrated enhanced bactericidal activity against multidrug-resistant pathogens and reduced inflammatory cytokine production by the drug-loaded neutrophils. A single intravenous administration of the drug-loaded neutrophils effectively protected mice from Pseudomonas aeruginosa infection in an acute pneumonia model. This study provides a potential effective therapeutic approach for the treatment of bacterial infections.

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Neutrophils are loaded with colistin and azithromycin in vitro

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The loaded drugs enhance the bactericidal effect and reduce the inflammatory response

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Drug-loaded neutrophils conferred effective protection against bacterial infection

Enhancing Colistin Activity against Colistin-Resistant Escherichia coli through Combination with Alginate Nanoparticles and Small Molecules

Bacterial resistance to antibiotics has become a major public health problem worldwide, with the yearly number of deaths exceeding 700,000. To face this well-acknowledged threat, new molecules and therapeutic methods are considered. In this context, the application of nanotechnology to fight bacterial infection represents a viable approach and has experienced tremendous developments in the last decades. Escherichia coli (E. coli) is responsible for severe diarrhea, notably in the breeding sector, and especially in pig farming. The resulting infection (named colibacillosis) occurs in young piglets and could lead to important economic losses. Here, we report the design of several new formulations based on colistin loaded on alginate nanoparticles (Alg NPs) in the absence, but also in the presence, of small molecules, such as components of essential oils, polyamines, and lactic acid. These new formulations, which are made by concomitantly binding colistin and small molecules to Alg NPs, were successfully tested against E. coli 184, a strain resistant to colistin. When colistin was associated with Alg NPs, the minimal inhibition concentration (MIC) decreased from 8 to 1 µg/mL. It is notable that when menthol or lactic acid was co-loaded with colistin on Alg NPs, the MIC of colistin drastically decreased, reaching 0.31 or 0.62 µg/mL, respectively. These novel bactericidal formulations, whose innocuity towards eukaryotic HT-29 cells was established in vitro, are presumed to permeabilize the bacterial membrane and provoke the leakage of intracellular proteins. Our findings revealed the potentiating effect of the Alg NPs on colistin, but also of the small molecules mentioned above. Such ecological and economical formulations are easy to produce and could be proposed, after confirmation by in vivo and toxicology tests, as therapeutic strategies to replace fading antibiotics.

The StkSR Two-Component System Influences Colistin Resistance in Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic human pathogen responsible for numerous severe nosocomial infections. Genome analysis on the A. baumannii clinical isolate 04117201 revealed the presence of 13 two-component signal transduction systems (TCS). Of these, we examined the putative TCS named here as StkSR. The stkR response regulator was deleted via homologous recombination and its progeny, ΔstkR, was phenotypically characterized. Antibiogram analyses of ΔstkR cells revealed a two-fold increase in resistance to the clinically relevant polymyxins, colistin and polymyxin B, compared to wildtype. PAGE-separation of silver stained purified lipooligosaccharide isolated from ΔstkR and wildtype cells ruled out the complete loss of lipooligosaccharide as the mechanism of colistin resistance identified for ΔstkR. Hydrophobicity analysis identified a phenotypical change of the bacterial cells when exposed to colistin. Transcriptional profiling revealed a significant up-regulation of the pmrCAB operon in ΔstkR compared to the parent, associating these two TCS and colistin resistance. These results reveal that there are multiple levels of regulation affecting colistin resistance; the suggested ‘cross-talk’ between the StkSR and PmrAB two-component systems highlights the complexity of these systems.

A C∧S-Cyclometallated Gold(III) Complex as a Novel Antibacterial Candidate Against Drug-Resistant Bacteria

The worldwide emergence and spread of infections caused by multidrug-resistant bacteria endangers the efficacy of current antibiotics in the clinical setting. The lack of new antibiotics in the pipeline points to the need of developing new strategies. Recently, gold-based drugs are being repurposed for antibacterial applications. Among them, gold(III) complexes have received increasing attention as metal-based anticancer agents. However, reports on their antibacterial activity are scarce due to stability issues. The present work demonstrates the antibacterial activity of the gold(III) complex 2 stabilized as C∧S-cycloaurated containing a diphenylphosphinothioic amide moiety, showing minimum inhibitory concentration (MIC) values that ranged from 4 to 8 and from 16 to 32 mg/L among Gram-positive and Gram-negative multidrug-resistant (MDR) pathogens, respectively. Complex 2 has a biofilm inhibitory activity of only two to four times than its MIC. We also describe for the first time a potent antibacterial synergistic effect of a gold(III) complex combined with colistin, showing a bactericidal effect in less than 2 h; confirming the role of the outer membrane as a permeability barrier. Complex 2 shows a low rate of internalization in Staphylococcus aureus and Acinetobacter baumannii; it does not interact with replication enzymes or efflux pumps, causes ultrastructural damages in both membrane and cytoplasmic levels, and permeabilizes the bacterial membrane. Unlike control antibiotics, complex 2 did not generate resistant mutants in 30-day sequential cultures. We detected lower cytotoxicity in a non-tumoral THLE-2 cell line (IC50 = 25.5 μM) and no acute toxicity signs in vivo after an i.v. 1-mg/kg dose. The characterization presented here reassures the potential of complex 2 as a new chemical class of antimicrobial agents.

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.

Fosfomycin Resistance Evolutionary Pathways of Stenotrophomonas maltophilia in Different Growing Conditions

The rise of multidrug-resistant Gram-negative pathogens and the lack of novel antibiotics to address this problem has led to the rescue of old antibiotics without a relevant use, such as fosfomycin. Stenotrophomonas maltophilia is a Gram-negative, non-fermenter opportunistic pathogen that presents a characteristic low susceptibility to several antibiotics of common use. Previous work has shown that while the so-far described mechanisms of fosfomycin resistance in most bacteria consist of the inactivation of the target or the transporters of this antibiotic, as well as the production of antibiotic-inactivating enzymes, these mechanisms are not selected in S. maltophilia fosfomycin-resistant mutants. In this microorganism, fosfomycin resistance is caused by the inactivation of enzymes belonging to its central carbon metabolism, hence linking metabolism with antibiotic resistance. Consequently, it is relevant to determine how different growing conditions, including urine and synthetic sputum medium that resemble infection, could impact the evolutionary pathways towards fosfomycin resistance in S. maltophilia. Our results show that S. maltophilia is able to acquire high-level fosfomycin resistance under all tested conditions. However, although some of the genetic changes leading to resistance are common, there are specific mutations that are selected under each of the tested conditions. These results indicate that the pathways of S. maltophilia evolution can vary depending on the infection point and provide information for understanding in more detail the routes of fosfomycin resistance evolution in S. maltophilia.

Evaluation of isoeugenol in inhibition of Staphylococcus aureus efflux pumps and their toxicity using Drosophila melanogaster model

The Staphylococcus aureus bacteria is a pathogen considered opportunistic and that has been acquiring resistance to several classes of antibiotics, mainly due to the synthesis of efflux pumps, which are proteins that expel these drugs intracellularly, reducing their effectiveness. The objective of this study was to evaluate the ability of isoeugenol to inhibit S. aureus efflux pumps and to determine its toxicity against a eukaryotic model (Drosophila melanogaster). IS-58, K2068 and K4414 S. aureus strains were used in the study. Isoeugenol minimum inhibitory concentration (MIC) and antibiotic modulation were evaluated in efflux pump inhibitory tests as well as in ethidium bromide (EtBr) assays. Toxicity tests against D. melanogaster assessed mortality and negative geotaxis. Isoeugenol obtained a relevant MIC result and a synergism was observed when isoeugenol was associated with the antibiotics, mainly with ciprofloxacin. Isoeugenol was able to affect all three efflux pumps tested, especially in strain K4414. The mortality of D. melanogaster caused by isoeugenol administration started after 12 h of exposure, being volume dependent and having an LC50 of 81.69 μL/L. In the negative geotaxis test, a statistical difference was observed after 24h of exposure compared to the control, demonstrating that damage to the locomotor apparatus had occurred. Based on the results, isoeugenol is a putative efflux pump inhibitor, becoming an alternative in blocking these proteins, and demonstrated acute toxicity against D. melanogaster.

Investigation of the bacterial cell envelope nanomechanical properties after long-term exposure to nitrofurans

Antibiotic residues in the environment may negatively affect biological communities in the natural ecosystems. However, their influence on environmental bacterial strains has not been thoroughly investigated. In this study, two representatives of 5-nitrofuran antibiotics (nitrofurantoin and furaltadone) were investigated in terms of their long-term influence on the cell envelopes of newly isolated environmental bacterial strains (Sphingobacterium caeni FTD2, Achromobacter xylosoxidans NFZ2 and Pseudomonas hibiscicola FZD2). A 12-month exposure of bacterial cells to nitrofurans at a concentration of 20 mg L^-1 induced changes in the cell structure and texture (bacteria under stress conditions showed a loss of their original shape and seemed to be vastly inflated, the cells increased average surface roughness after exposure to NFT and FTD, respectively). AFM observations allowed the calculation of the bacterial cell nanomechanical properties. Significant increase in adhesion energy of bacteria after prolonged contact with nitrofurantoin was demonstrated. Changes in the permeability of bacterial membrane, fatty acids' composition and bacterial cell surface hydrophobicity were determined. Despite visible bacterial adaptation to nitrofurans, prolonged presence of pharmaceuticals in the environment has led to significant alterations in the cells' structures which was particularly visible in P. hibiscicola.

Repurposing celecoxib analogues as leads for antibiotics

There is an urgent need for new antibiotics and alternative strategies to combat bacterial pathogens. Molecular docking, antibacterial evaluation in vitro and in vivo, cytotoxicity assessment and enzyme inhibition analyses were performed. Compound 12 exhibited antimicrobial activity against Staphylococcus aureus (MIC: 4 μg/ml), various clinically isolated strains of MRSA (MIC: 4-16 μg/ml) and Acinetobacter baumannii (MIC: 4 μg/ml) when combined with subinhibitory concentrations of colistin B. Compound 12 (20 mg/kg) yielded mild improvement in survival of methicillin-resistant Staphylococcus aureus (MRSA)-infected mice. Additionally, enzyme inhibition tests showed that compound 12 exhibited inhibitory effects against S. aureus dihydrofolate reductase (105.1 μg/ml) and DNA gyrase (122.8 μg/ml). Compound 12 is a promising antibacterial candidate for further development.

Enhanced Antimicrobial and Antibiofilm Effect of New Colistin-Loaded Human Albumin Nanoparticles

Multidrug-resistant (MDR) Gram-negative bacteria (GNB), such as Acinetobacter and Klebsiella, are responsible for severe hospital-acquired infections. Colistin, despite its toxicity and low tissue penetration, is considered the last resort antibiotic against these microorganisms. Of concern, the use of Colistin has recently been compromised by the emergence of Colistin resistance. Herein, we developed a new formulation consisting of multifunctional chitosan-coated human albumin nanoparticles for the delivery of Colistin (Col/haNPs). Col/haNPs were in vitro characterized for encapsulation efficiency, drug release, stability and cytotoxicity and were evaluated for antibacterial activity against MDR GNB (Acinetobacter baumannii and Klebsiella pneumoniae). Col/haNPs showed sizes lower than 200 nm, high encapsulation efficiency (98.65%) and prolonged in vitro release of Colistin. The safety of the nanoformulation was demonstrated by a negligible cytotoxicity on human fibroblasts and hemolytic activity. Col/haNPs evidenced a high antibacterial effect with a significant decrease in MIC values compared to free Colistin, in particular against Col-resistant strains with a pronounced decline of bacterial growth over time. Moreover, Col/haNPs exhibited an inhibitory effect on biofilm formation that was 4 and 60 fold higher compared to free Colistin, respectively for Colistin susceptible and resistant A. baumannii. Our findings suggest that Col/haNPs could represent a promising Colistin nanocarrier with high antimicrobial activity on MDR GNB.

Activity of carbonyl cyanide-3-chlorophenylhydrazone on biofilm formation and antimicrobial resistance in Pseudomonas aeruginosa using quantum dots-meropenem conjugates as nanotools

Hospital infections associated with multidrug-resistant (MDR) Pseudomonas aeruginosa are a worldwide public health problem. Efflux systems and biofilm formation are mechanisms related to resistance to carbapenemics. In this study, quantum dots (QDs) were used to evaluate the effect of carbonyl cyanide-3-chlorophenylhydrazone (CCCP), an efflux pump system inhibitor, on biofilm formation and antimicrobial resistance profile of P. aeruginosa strains. For this, QDs were covalently conjugated to meropenem (MPM) and incubated with a P. aeruginosa resistant isolate (P118) or a control sensitive strain (ATCC Pa27853). P118 was also analyzed with conjugates after previous CCCP efflux inhibitor incubation. Fluorescence microscopy images showed that both sensitive and resistant bacteria were efficiently labeled. Nevertheless, P118 isolates presented fluorescent cell agglomerates, suggesting biofilm formation. The addition of the CCCP changed the labeling profile of the resistant isolate, and the absence of agglomerates was observed, indicating no biofilm formation. Genetic assays revealed the presence of MexA and MexE genes encoding channel proteins from efflux pump systems in both resistant and sensitive strains. Disk-diffusion and broth microdilution tests determined drug susceptibility profiles in the presence and absence of CCCP for P118 isolates. We verified that the CCCP efflux system inhibitor may contribute to P. aeruginosa resistant phenotype reduction for some antimicrobials. This study verified the efficiency of QD-MPM conjugates to trigger and study biofilm formation, or its inhibition, before and after CCCP addition. QDs conjugated to antimicrobials can be used as nanotools to investigate multidrug-resistant bacterial strains on biofilm formation.

A Novel Peptide Antibiotic, Pro10-1D, Designed from Insect Defensin Shows Antibacterial and Anti-Inflammatory Activities in Sepsis Models

Owing to the challenges faced by conventional therapeutics, novel peptide antibiotics against multidrug-resistant (MDR) gram-negative bacteria need to be urgently developed. We had previously designed Pro9-3 and Pro9-3D from the defensin of beetle Protaetia brevitarsis; they showed high antimicrobial activity with cytotoxicity. Here, we aimed to develop peptide antibiotics with bacterial cell selectivity and potent antibacterial activity against gram-negative bacteria. We designed 10-meric peptides with increased cationicity by adding Arg to the N-terminus of Pro9-3 (Pro10-1) and its D-enantiomeric alteration (Pro10-1D). Among all tested peptides, the newly designed Pro10-1D showed the strongest antibacterial activity against Escherichia coli, Acinetobacter baumannii, and MDR strains with resistance against protease digestion. Pro10-1D can act as a novel potent peptide antibiotic owing to its outstanding inhibitory activities against bacterial film formation with high bacterial cell selectivity. Dye leakage and scanning electron microscopy revealed that Pro10-1D targets the bacterial membrane. Pro10-1D inhibited inflammation via Toll Like Receptor 4 (TLR4)/Nuclear factor-κB (NF-κB) signaling pathways in lipopolysaccharide (LPS)-stimulated RAW264.7 cells. Furthermore, Pro10-1D ameliorated multiple-organ damage and attenuated systemic infection-associated inflammation in an E. coli K1-induced sepsis mouse model. Overall, our results suggest that Pro10-1D can potentially serve as a novel peptide antibiotic for the treatment of gram-negative sepsis.

Antibiotic resistance breakers: current approaches and future directions

Infections of antibiotic-resistant pathogens pose an ever-increasing threat to mankind. The investigation of novel approaches for tackling the antimicrobial resistance crisis must be part of any global response to this problem if an untimely reversion to the pre-penicillin era of medicine is to be avoided. One such promising avenue of research involves so-called antibiotic resistance breakers (ARBs), capable of re-sensitising resistant bacteria to antibiotics. Although some ARBs have previously been employed in the clinical setting, such as the β-lactam inhibitors, we posit that the broader field of ARB research can yet yield a greater diversity of more effective therapeutic agents than have been previously achieved. This review introduces the area of ARB research, summarises the current state of ARB development with emphasis on the various major classes of ARBs currently being investigated and their modes of action, and offers a perspective on the future direction of the field.

Proteomic interrogation of antibiotic resistance and persistence in Escherichia coli - progress and potential for medical research

Introduction Escherichia coli strains possess two survival strategies to endure lethal antibiotic exposure including antibiotic resistance and persistence, in which persistence can contribute to the emergence of antibiotic resistance and increasing the risk of multidrug resistance. Using high-throughput proteomics for the comprehensive understanding of mechanisms of antibiotic resistance and persistence is an effective strategy for development of target-based anti-bacterial therapies. Areas covered In this review, we summarize a comprehensive proteomic perspective of antibiotic resistance and persistence in E. coli, and overview of anti-antibiotic resistance and anti-persister molecules and strategies for the development of potential therapies. Expert opinion Proteomics allows us to globally identify the critical proteins and pathways involved in antibiotic resistance and persistence. Advancements in methodologies of proteomics and multi-omic strategies are required to overcome the limitations of proteomics and better understand mechanisms of antibiotic resistance and persistence in E. coli, and to open the possibility for identification of new targets for alternative strategies in therapeutics.

Trends, Epidemiology, and Management of Multi-Drug Resistant Gram-Negative Bacterial Infections in the Hospitalized Setting

The increasing prevalence of antibiotic resistance is a threat to human health, particularly within vulnerable populations in the hospital and acute care settings. This leads to increasing healthcare costs, morbidity, and mortality. Bacteria rapidly evolve novel mechanisms of resistance and methods of antimicrobial evasion. Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Acinetobacter baumannii have all been identified as pathogens with particularly high rates of resistance to antibiotics, resulting in a reducing pool of available treatments for these organisms. Effectively combating this issue requires both preventative and reactive measures. Reducing the spread of resistant pathogens, as well as reducing the rate of evolution of resistance is complex. Such a task requires a more judicious use of antibiotics through a better understanding of infection epidemiology, resistance patterns, and guidelines for treatment. These goals can best be achieved through the implementation of antimicrobial stewardship programs and the development and introduction of new drugs capable of eradicating multi-drug resistant Gram-negative pathogens (MDR GNB). The purpose of this article is to review current trends in MDR Gram-negative bacterial infections in the hospitalized setting, as well as current guidelines for management. Finally, new and emerging antimicrobials, as well as future considerations for combating antibiotic resistance on a global scale are discussed.

Development and Challenges of Antimicrobial Peptides for Therapeutic Applications

More than 3000 antimicrobial peptides (AMPs) have been discovered, seven of which have been approved by the U.S. Food and Drug Administration (FDA). Now commercialized, these seven peptides have mostly been utilized for topical medications, though some have been injected into the body to treat severe bacterial infections. To understand the translational potential for AMPs, we analyzed FDA-approved drugs in the FDA drug database. We examined their physicochemical properties, secondary structures, and mechanisms of action, and compared them with the peptides in the AMP database. All FDA-approved AMPs were discovered in Gram-positive soil bacteria, and 98% of known AMPs also come from natural sources (skin secretions of frogs and toxins from different species). However, AMPs can have undesirable properties as drugs, including instability and toxicity. Thus, the design and construction of effective AMPs require an understanding of the mechanisms of known peptides and their effects on the human body. This review provides an overview to guide the development of AMPs that can potentially be used as antimicrobial drugs.

Bioinspired Liposomes for Oral Delivery of Colistin to Combat Intracellular Infections by Salmonella enterica

Bacterial invasion into eukaryotic cells and the establishment of intracellular infection has proven to be an effective means of resisting antibiotic action, as anti-infective agents commonly exhibit a poor permeability across the host cell membrane. Encapsulation of anti-infectives into nanoscaled delivery systems, such as liposomes, is shown to result in an enhancement of intracellular delivery. The aim of the current work is, therefore, to formulate colistin, a poorly permeable anti-infective, into liposomes suitable for oral delivery, and to functionalize these carriers with a bacteria-derived invasive moiety to enhance their intracellular delivery. Different combinations of phospholipids and cholesterol are explored to optimize liposomal drug encapsulation and stability in biorelevant media. These liposomes are then surface-functionalized with extracellular adherence protein (Eap), derived from Staphylococcus aureus. Treatment of HEp-2 and Caco-2 cells infected with Salmonella enterica using colistin-containing, Eap-functionalized liposomes resulted in a significant reduction of intracellular bacteria, in comparison to treatment with nonfunctionalized liposomes as well as colistin alone. This indicates that such bio-invasive carriers are able to facilitate intracellular delivery of colistin, as necessary for intracellular anti-infective activity. The developed Eap-functionalized liposomes, therefore, present a promising strategy for improving the therapy of intracellular infections.

Molecular Mechanisms of Colistin-Induced Nephrotoxicity

The emergence of multidrug resistant (MDR) infections and the shortage of new therapeutic options have made colistin, a polymyxin antibiotic, the main option for the treatment of MDR Gram-negative bacterial infections in the last decade. However, the rapid onset of renal damage often prevents the achievement of optimal therapeutic doses and/or forces the physicians to interrupt the therapy, increasing the risk of drug resistance. The proper management of colistin-induced nephrotoxicity remains challenging, mostly because the investigation of the cellular and molecular pharmacology of this drug, off the market for decades, has been largely neglected. For years, the renal damage induced by colistin was considered a mere consequence of the detergent activity of this drug on the cell membrane of proximal tubule cells. Lately, it has been proposed that the intracellular accumulation is a precondition for colistin-mediated renal damage, and that mitochondria might be a primary site of damage. Antioxidant approaches (e.g., ascorbic acid) have shown promising results in protecting the kidney of rodents exposed to colistin, yet none of these strategies have yet reached the bedside. Here we provide a critical overview of the possible mechanisms that may contribute to colistin-induced renal damage and the potential protective strategies under investigation.

Antimicrobial and Antibiofilm Activity of Synergistic Combinations of a Commercially Available Small Compound Library With Colistin Against Pseudomonas aeruginosa

Biofilm-associated Pseudomonas aeruginosa infections remain a significant clinical challenge since the conventional antibiotic treatment or combination therapies are largely ineffective; and new approaches are needed. To circumvent the major challenges associated with discovery of new antimicrobials, we have screened a library of compounds that are commercially available and approved by the FDA (Prestwick Chemical Library) against P. aeruginosa for effective antimicrobial and anti-biofilm activity. A preliminary screen of the Prestwick Chemical Library alone did not yield any repositionable candidates, but in a screen of combinations with a fixed sub-inhibitory concentration of the antibiotic colistin we observed 10 drugs whose bacterial inhibiting activity was reproducibly enhanced, seven of which were enhanced by more than 50%. We performed checkerboard assays of these seven drugs in combination with colistin against planktonic cells, and analysis of their interactions over the complete combination matrix using the Zero Interaction Potency (ZIP) model revealed interactions that varied from highly synergistic to completely antagonistic. Of these, five combinations that showed synergism were down-selected and tested against preformed biofilms of P. aeruginosa. Two of the five combinations were active against preformed biofilms of both laboratory and clinical strain of P. aeruginosa, resulting in a 2-log reduction in culturable cells. In summary, we have identified synergistic combinations of five commercially available, FDA-approved drugs and colistin that show antimicrobial activity against planktonic P. aeruginosa (Clomiphene Citrate, Mitoxantrone Dihydrochloride, Methyl Benzethonium Chloride, Benzethonium Chloride, and Auranofin) as well as two combinations (Auranofin and Clomiphene Citrate) with colistin that show antibiofilm activity.

Curcumin alleviates persistence of Acinetobacter baumannii against colistin

Persisters are phenotypic variants of normal susceptible bacterial populations that survive prolonged exposure to high doses of antibiotics and are responsible for pertinacious infections and post-treatment relapses. Out of the three antibiotics, Acinetobacter baumannii formed the highest percentage of persister cells against rifampicin followed by amikacin and the least against colistin. Colistin-treated cells formed the high levels of reactive oxygen species (ROS) whose quenching with bipyridyl and thiourea led to an increased persister population. Curcumin, a polyphenolic pro-oxidant, significantly decreased persistence against colistin. The quenching of ROS generated by curcumin-colistin combination and the use of resveratrol, an anti-oxidant, with colistin increased the persister population, supporting the significance of ROS in decreased persistence against this combination. The down-regulation of repair genes by this combination in comparison to colistin alone supported the modulation of gene expression in response to ROS and their importance in decreased persistence. Increased membrane permeability by colistin, facilitating the penetration of curcumin into cells and resulting in increased ROS and compromised repair compounded by the decreased efflux of colistin by the inhibition of efflux pumps, may be responsible for enhanced lethality and low persistence. Hence, the curcumin-colistin combination can be another option with anti-persister potential for the control of chronic A. baumannii infections.

Synergy between conventional antibiotics and anti-biofilm peptides in a murine, sub-cutaneous abscess model caused by recalcitrant ESKAPE pathogens

With the antibiotic development pipeline running dry, many fear that we might soon run out of treatment options. High-density infections are particularly difficult to treat due to their adaptive multidrug-resistance and currently there are no therapies that adequately address this important issue. Here, a large-scale in vivo study was performed to enhance the activity of antibiotics to treat high-density infections caused by multidrug-resistant Gram-positive and Gram-negative bacteria. It was shown that synthetic peptides can be used in conjunction with the antibiotics ciprofloxacin, meropenem, erythromycin, gentamicin, and vancomycin to improve the treatment outcome of murine cutaneous abscesses caused by clinical hard-to-treat pathogens including all ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter cloacae) pathogens and Escherichia coli. Promisingly, combination treatment often showed synergistic effects that significantly reduced abscess sizes and/or improved clearance of bacterial isolates from the infection site, regardless of the antibiotic mode of action. In vitro data suggest that the mechanisms of peptide action in vivo include enhancement of antibiotic penetration and potential disruption of the stringent stress response.

There has been enormous publicity about the inexorable rise of resistance and the dearth of new therapies. However less attention has been placed on adaptively multidrug-resistant high density bacterial infections for which antibiotics are highly used but no effective therapies currently exist. Here we have provided new hope for this previously intractable class of infections as typified by abscess infections that are responsible for 3.2 million annual emergency room visits in the US alone. We show how to enhance the activity of antibiotics to treat multidrug-resistant Gram-positive and Gram-negative bacteria, using peptides that target the bacterial stress response, persister-based resistance and the outer membrane permeability barrier. In particular we have employed a new bacterial subcutaneous abscess mouse model to demonstrate that: (a) 7 of the society’s most recalcitrant pathogens formed cutaneous abscesses and even when antibiotics were directly delivered into abscess tissues, they showed poor efficacy; (b) By combining antibiotics with the local administration of anti-biofilm peptides that target cellular (stringent) stress responses, we could pharmacologically treat the infection and reduce the severity of cutaneous abscesses; (c) This synergy was due to increased outer membrane permeability as well as the disruption of the conserved stringent stress response that controls virulence and antibiotic resistance, particularly due to so-called persisters. These peptides have therefore the potential to broaden our limited antibiotic arsenal for a group of extremely difficult to treat infections.

Carbonyl Cyanide m-Chlorophenylhydrazine (CCCP) Reverses Resistance to Colistin, but Not to Carbapenems and Tigecycline in Multidrug-Resistant Enterobacteriaceae

Background: Carbapenems (CAR), colistin (CST), and tigecycline (TGC) alone or in combination therapy has become the last-resort antibiotics for treating infections caused by multidrug resistant (MDR) bacteria. However, resistance to these reserve antibiotics are increasingly being reported worldwide. Hence, the quest to find other agents that will synergistically restore the efficacy of these antibiotics have increased. Methods: Sixty-three clinical Enterobacteriaceae isolates comprising of Klebsiella pneumoniae (n = 24), Enterobacter spp. (n = 15), Serratia marcescens (n = 12), Citrobacter freundii (n = 8), Escherichia coli (n = 2), and K. oxytoca/michiganensis (n = 2) with known carbapenem resistance mechanisms and undescribed CST and TGC resistance mechanisms were subjected to broth microdilution and meropenem (MEM) disc synergy test in the presence and absence of carbonyl cyanide m-chlorophenylhydrazine (CCCP), a H⁺ conductor (protonophore). Results and conclusions: Susceptibility to MEM, imipenem (IMP), CST, and TGC was found in only 2, 0, 17, and 9 isolates respectively. Addition of CCCP reversed resistance to CST, TGC, IMP, and MEM in 44, 3, 0, and 0 isolates respectively; CST had the highest mean minimum inhibitory concentration (MIC) fold change (193.12; p < 0.0001) post CCCP compared to that of MEM (1.70), IMP (1.49) and TGC (1.16). Eight isolates tested positive for the MEM-CCCP disc synergy test. We concluded that CCCP reverse CST resistance in CST-resistant Enterobacteriaceae. Although CCCP is an experimental agent with no therapeutic value clinically, further studies are necessary to decipher the mechanisms underlying the CST-CCCP synergy to inform the development of adjuvants that could be therapeutically effective in CST-resistant infections.