Pharmacology of polymyxins: new insights into an 'old' class of antibiotics

Colistin potentiation in multidrug-resistant Acinetobacter baumannii by a non-cytotoxic guanidine derivative of silver

A novel nano-formulation (NF) that sensitizes Acinetobacter baumannii (AB) to otherwise ineffective colistin is described in the present study. Infections due to multidrug resistant (MDR) AB represent a major therapeutic challenge, especially in situations of pre-existing colistin resistance (colR). Subsequently, boosting the effectiveness of colistin would be a better alternative tactic to treat AB infections rather than discovering a new class of antibiotics. We have previously demonstrated an NF comprising self-assembled guanidinium and ionic silver nanoparticles [AD-L@Ag(0)] to have anti-biofilm and bactericidal activity. We report NF AD-L@Ag(0) for the very first time for the potentiation of colistin in Gram-negative colistin-resistant bacteria. Our results implied that a combination of clinically relevant concentrations of colistin and AD-L@Ag(0) significantly decreased colistin-resistant AB bacterial growth and viability, which otherwise was elevated in the presence of only colistin. In this study, we have described various combinations of minimum inhibitory concentration (MIC) of colistin (MICcol, ^1/2 MICcol, and ^1/4 MICcol) and that of AD-L@Ag(0) [MICAD-L@Ag(0), ^1/2 MICAD-L@Ag(0), and ^1/4 MICAD-L@Ag(0)] and tested them against MDR AB culture. The results (in broth as well as in solid media) signified that AD-L@Ag(0) was able to potentiate the anti-microbial activity of colistin at sub-MIC concentrations. Furthermore, the viability and metabolic activity of bacterial cells were also measured by CTC fluorescence assay and ATP bioluminescence assay. The results of these assays were in perfect concordance with the scores of cultures (colony forming unit and culture turbidity). In addition, quantitative real-time PCR (qRT-PCR) was performed to unveil the expression of selected genes, DNAgyrA, DNAgyrB, and dac. These genes introduce negative supercoiling in the DNA, and hence are important for basic cellular processes. These genes, due to mutation, modified the Lipid A of bacteria, further resisting the uptake of colistin. Therefore, the expression of these genes was upregulated when AB was treated with only colistin, substantiating that AB is resistant to colistin, whereas the combinations of MICcol + MICAD-L@Ag(0) downregulated the expression of these genes, implying that the developed formulation can potentiate the efficiency of colistin. In conclusion, AD-L@Ag(0) can potentiate the proficiency of colistin, further enhancing colistin-mediated death of AB by putatively disrupting the outer membrane (OM) and facilitating bacterial death.

Comparative polymyxin B pharmacokinetics in critically ill patients with renal insufficiency and in continuous veno-venous hemodialysis

PURPOSE

The aim of this study was to assess polymyxin B pharmacokinetics (PK) in patients with varying degrees of renal dysfunction and in patients who require continuous veno-venous hemodialysis (CVVHD).

METHODS

The study enrolled 37 patients with sepsis, including 13 patients with glomerular filtration rate (GFR) below 80 mL/min and 11 patients on CVVHD. Each patient received a loading dose of polymyxin B (200-300 mg) and at least 3 subsequent doses of 100-150 mg every 12 h. For every patient, 6-8 blood samples were collected between doses. Polymyxin B (PMB) serum concentration was determined using enzyme-linked immunosorbent assay.

RESULTS

In sepsis, patients with preserved renal function mean area under the curve over 24 h (AUC0-24 h) value reached 67.8 ± 9.8 mg*h/L, while in patients with GFR below 80 mL/min, mean AUC0-24 h was 87 ± 5.8 mg*h/L. PMB PK in patients with renal insufficiency was characterized by significantly lower clearance (CL) compared to the normal renal function group (2.1 ± 0.1 L/h vs 3.9 ± 0.4 L/h respectively). In patients on CVVHD, mean AUC0-24 h was 110.4 ± 10.3 mg*h/L, while CL reached 2 ± 0.23 L/h. The median recovery rate from dialysate constituted 22%. Simulation of different dosage regimens that indicate a fixed maintenance dose of 100 mg q12h with a loading dose of 200 mg is optimal for patients on CVVHD, and no dosage increase is required.

CONCLUSION

This study demonstrates decreased clearance of PMB in patients with renal insufficiency, which puts them at risk of toxicity. Therefore, patients with extremes of renal function might benefit from therapeutic drug monitoring. For patients with anuria, who require CVVHD, we suggest a fixed dose of 100 mg q12h.

Antimicrobial peptides against polymyxin-resistant Klebsiella pneumoniae: a patent review

The spread of polymyxin-resistant Klebsiella pneumoniae strains represents an emerging health challenge, limiting treatment options for the patients. Thus, the development of new antimicrobials is an urgent requirement. Antimicrobial peptides (AMPs) are a large class of compounds that are part of innate immune response; these peptides are promising compounds in the field of antimicrobial resistance and are present in all organisms. The present review evaluated patents on antimicrobial peptides tested against polymyxin-resistant K. pneumoniae, available on Espacenet as of September 2022. A total of 1313 patents were examined and 1197 excluded as they were out of focus for this review; 104 patents of peptides tested against K. pneumoniae were included; of which only 14 were tested against polymyxin-resistant K. pneumoniae strains. The results indicated that all AMPs evaluated were in the experimental or pre-clinical phase; the clinical phase is pending. Furthermore, a few peptides were tested effectively against polymyxin-resistant K. pneumoniae. Although, the research and patent filing alone are not enough to develop a suitable antimicrobial therapy, they can represent good starting point upon which to develop new antimicrobials. More investment is required to push these pharmaceuticals through the stages of development to introduce them into the market.

Use of Ceftazidime-Avibactam in the Treatment of Clinical Syndromes With Limited Treatment Options: A Retrospective Study

Background With rising trends of multi-drug organism infections and the limited availability of new antimicrobials, management of such cases has become a hassle for the clinician. Ceftazidime-Avibactam (CEF-AVI) is evolving as an effective alternative to polymyxins in the management of Carbapenem-Resistant Organisms (CRO) infections. The Food and Drug Administration (FDA) has approved CEF-AVI in a restricted group of clinical syndromes where the drug could have potential use. Objective The goal of this study was to evaluate the clinical outcome in terms of 14-day all-cause mortality and clinical cure at seven days in patients on CEF-AVI. Methodology A retrospective study was conducted on patients who received CEF-AVI in a period of one year in our hospital. Patients were included in the study if they have received CEF-AVI for more than one day of therapy (DOT) and samples from relevant sites have been sent for culture and sensitivity. Variables and outcomes were collected from the hospital information system and medical records. Results A total of 78 patients were included, 52 (66.7%) were started empirically on CEF-AVI while 26 (33.3%) were on targeted therapy. Out of the 78 patients, 43 patients had positive cultures among which 32 patients had Carbapenem-Resistant Enterobacteriaceae (CRE)/Carbapenem-Resistant Pseudomonas aeruginosa (CRPA) infection. The most common clinical syndrome in which the drug was used was occult sepsis (27/78; 34.6%) followed by primary bacteremia (20/78; 25.6%) and neutropenic sepsis (11/78; 14.1%). The clinical efficacy which was primarily assessed in terms of clinical cure was met for 55 (70.5%) patients. The 14-day mortality for the studies group was found to be 18 (23%). Conclusion The analysis of results shows encouraging clinical cure rates and 14-day mortality rates in a subset of severe infections which has limited treatment options.

The Blood-Brain Barrier and Pharmacokinetic/Pharmacodynamic Optimization of Antibiotics for the Treatment of Central Nervous System Infections in Adults

Bacterial central nervous system (CNS) infections are serious and carry significant morbidity and mortality. They encompass many syndromes, the most common being meningitis, which may occur spontaneously or as a consequence of neurosurgical procedures. Many classes of antimicrobials are in clinical use for therapy of CNS infections, some with established roles and indications, others with experimental reporting based on case studies or small series. This review delves into the specifics of the commonly utilized antibacterial agents, updating their therapeutic use in CNS infections from the pharmacokinetic and pharmacodynamic perspectives, with a focus on the optimization of dosing and route of administration that have been described to achieve good clinical outcomes. We also provide a concise synopsis regarding the most focused, clinically relevant information as pertains to each class and subclass of antimicrobial therapeutics. CNS infection morbidity and mortality remain high, and aggressive management is critical in ensuring favorable patient outcomes while averting toxicity and upholding patient safety.

Synthesis of an amphiphilic vancomycin aglycone derivative inspired by polymyxins: overcoming glycopeptide resistance in Gram-positive and Gram-negative bacteria in synergy with teicoplanin in vitro

Gram-negative bacteria possess intrinsic resistance to glycopeptide antibiotics so these important antibacterial medications are only suitable for the treatment of Gram-positive bacterial infections. At the same time, polymyxins are peptide antibiotics, structurally related to glycopeptides, with remarkable activity against Gram-negative bacteria. With the aim of breaking the intrinsic resistance of Gram-negative bacteria against glycopeptides, a polycationic vancomycin aglycone derivative carrying an n-decanoyl side chain and five aminoethyl groups, which resembles the structure of polymyxins, was prepared. Although the compound by itself was not active against the Gram-negative bacteria tested, it synergized with teicoplanin against Escherichia coli, Pseudomonas aeruginosa and Acinetobacter baumannii, and it was able to potentiate vancomycin against these Gram-negative strains. Moreover, it proved to be active against vancomycin- and teicoplanin-resistant Gram-positive bacteria.

Molecular mechanisms and clonal lineages of colistin-resistant bacteria across the African continent: a scoping review

Colistin (also known as polymyxin E), a polymyxin antibiotic discovered in the late 1940s, has recently reemerged as a last-line treatment option for multidrug-resistant infections. However, in recent years, colistin-resistant pathogenic bacteria have been increasingly reported worldwide. Accordingly, the presented review was undertaken to identify, integrate and synthesize current information regarding the detection and transmission of colistin-resistant bacteria across the African continent, in addition to elucidating their molecular mechanisms of resistance. PubMed, Google Scholar and Science Direct were employed for study identification, screening and extraction. Overall, based on the developed literature review protocol and associated inclusion/exclusion criteria, 80 studies published between 2000 and 2021 were included comprising varying bacterial species and hosts. Numerous mechanisms of colistin resistance were reported, including chromosomal mutation(s) and transferable plasmid-mediated colistin resistance (encoded by mcr genes). Perhaps unexpectedly, mcr-variants have exhibited rapid emergence and spread across most African regions. The genetic variant mcr-1 is predominant in humans, animals and the natural environment, and is primarily carried by IncHI2- type plasmid. The highest number of studies reporting the dissemination of colistin-resistant Gram-negative bacteria were conducted in the North African region.

Drug Repurposing Approaches towards Defeating Multidrug-Resistant Gram-Negative Pathogens: Novel Polymyxin/Non-Antibiotic Combinations

Multidrug-resistant (MDR) Gram-negative pathogens remain an unmet public health threat. In recent times, increased rates of resistance have been reported not only to commonly used antibiotics, but also to the last-resort antibiotics, such as polymyxins. More worryingly, despite the current trends in resistance, there is a lack of new antibiotics in the drug-discovery pipeline. Hence, it is imperative that new strategies are developed to preserve the clinical efficacy of the current antibiotics, particularly the last-line agents. Combining conventional antibiotics such as polymyxins with non-antibiotics (or adjuvants), has emerged as a novel and effective strategy against otherwise untreatable MDR pathogens. This review explores the available literature detailing the latest polymyxin/non-antibiotic combinations, their mechanisms of action, and potential avenues to advance their clinical application.

Synergistic antibacterial activity of baicalin and EDTA in combination with colistin against colistin-resistant Salmonella

The emergence and rapid spread of multidrug resistant (MDR) Gram-negative bacteria have posed a serious threat to global health and security. Because of the time-consuming, high cost and high risk of developing new antibiotics, a significant method is to use antibiotic adjuvants to revitalize the existing antibiotics. The purpose of the study is to research the traditional Chinese medicine baicalin with the function of inhibiting the efflux pump and EDTA whether their single or combination can increase the activity of colistin against colistin-resistant Salmonella in vitro and in vivo, and to explore its molecular mechanisms. In vitro antibacterial experiments, we have observed that baicalin and EDTA alone could enhance the antibacterial activity of colistin. At the same time, the combination of baicalin and EDTA also showed a stronger synergistic effect on colistin, reversing the colistin resistance of all Salmonella strains. Molecular docking and RT-PCR results showed that the combination of baicalin and EDTA not only affected the expression of mcr-1, but also was an effective inhibitor of MCR-1. In-depth synergistic mechanism analysis revealed that baicalin and EDTA enhanced colistin activity through multiple pathways, including accelerating the tricarboxylic acid cycle (TCA cycle), inhibiting the bacterial antioxidant system and lipopolysaccharide (LPS) modification, depriving multidrug efflux pump functions and attenuating bacterial virulence. In addition, the combinational therapy of colistin, baicalin and EDTA displayed an obvious reduction in bacterial loads cfus of liver and spleen compared with monotherapy and 2-drug combination therapy. In conclusion, our study indicates that the combination of baicalin and EDTA as a novel colistin adjuvant can provide a reliable basis for formulating the therapeutic regimen for colistin resistant bacterial infection.

Flow-Based Chemiluminescence Microarrays as Screening Platform for Affinity Binders to Capture and Elute Bacteria

Affinity describes the non-covalent but selective interaction between an affinity binder (e.g., proteins, antibiotics, or antibodies) and its counterpart (e.g., bacteria). These affinity binders can serve to detect bacteria and respond to the need for selective concentration via affinity chromatography for trace analysis. By changing the pH value or salt and protein contents, affinity bindings can be reversed, and bacteria can be recovered for characterisation. Analytical microarrays use multiple affinity binders immobilised on the surface in a distinct pattern, which immensely reduces screening time for the discovery of superior binding motifs. Here, flow-based microarray systems can inform not only about binding, but also about desorption. In this work, we pioneer a screening assay for affinity binders against both gram-positive and negative bacteria based on an automated flow-based chemiluminescence (CL) microarray. Biotinylation of model organisms E. coli and E. faecalis enabled labelling with horseradish-peroxidase-coupled streptavidin, and detection with CL. Polymyxin B, an antibiotic against gram-negative bacteria, was found to bind both E. coli and E. faecalis. Simultaneous screening for desorption methods unexpectedly revealed methyl alpha-D-mannopyranoside as a promising buffer for desorption from Polymyxin B. This proof-of-principle study shows that our new platform greatly facilitates the screening of new affinity binders against bacteria, with promise for future automation.

A comprehensive review of microorganism-derived cyclic peptides: Bioactive functions and food safety applications

Cyclic peptides possess advanced structural characteristics of stability and play a vital role in medical treatment and agriculture. However, the biological functions of microorganism-derived cyclic peptides (MDCPs) and their applications in food industry were relatively absent. MDCPs are derived from extensive fermented food or soil. In this review, the synthesis approaches and structural characteristics are overviewed, while the interrelationship between bioactivities and functions is emphasized. This review summarizes the bioactivities of MDCPs from in vitro to in vivo, including antimicrobial activities, immune regulation, and antiviral cell activation. Their multiple functions as well as applications during food product processing, packaging, and storage are also comprehensively reviewed. Remarkably, some potential risks and cytotoxicity of MDCPs are also critically discussed. Moreover, future applications of MDCPs in the development of novel food additives and bioengineering materials are organized. Based on this review of native MDCPs, it is noteworthy that expected improvements of synthetic cyclic peptides in bioactive properties present potential valuable applications in future food, including artificial meat.

Integrated metabolomic and transcriptomic analyses of the synergistic effect of polymyxin-rifampicin combination against Pseudomonas aeruginosa

BACKGROUND

Understanding the mechanism of antimicrobial action is critical for improving antibiotic therapy. For the first time, we integrated correlative metabolomics and transcriptomics of Pseudomonas aeruginosa to elucidate the mechanism of synergistic killing of polymyxin-rifampicin combination.

METHODS

Liquid chromatography-mass spectrometry and RNA-seq analyses were conducted to identify the significant changes in the metabolome and transcriptome of P. aeruginosa PAO1 after exposure to polymyxin B (1 mg/L) and rifampicin (2 mg/L) alone, or in combination over 24 h. A genome-scale metabolic network was employed for integrative analysis.

RESULTS

In the first 4-h treatment, polymyxin B monotherapy induced significant lipid perturbations, predominantly to fatty acids and glycerophospholipids, indicating a substantial disorganization of the bacterial outer membrane. Expression of ParRS, a two-component regulatory system involved in polymyxin resistance, was increased by polymyxin B alone. Rifampicin alone caused marginal metabolic perturbations but significantly affected gene expression at 24 h. The combination decreased the gene expression of quorum sensing regulated virulence factors at 1 h (e.g. key genes involved in phenazine biosynthesis, secretion system and biofilm formation); and increased the expression of peptidoglycan biosynthesis genes at 4 h. Notably, the combination caused substantial accumulation of nucleotides and amino acids that last at least 4 h, indicating that bacterial cells were in a state of metabolic arrest.

CONCLUSION

This study underscores the substantial potential of integrative systems pharmacology to determine mechanisms of synergistic bacterial killing by antibiotic combinations, which will help optimize their use in patients.

Evaluation and Validation of the Limited Sampling Strategy of Polymyxin B in Patients with Multidrug-Resistant Gram-Negative Infection

Polymyxin B (PMB) is the final option for treating multidrug-resistant Gram-negative bacterial infections. The acceptable pharmacokinetic/pharmacodynamic target is an area under the concentration–time curve across 24 h at a steady state (AUC_ss,24h) of 50–100 mg·h/L. The limited sampling strategy (LSS) is useful for predicting AUC values. However, establishing an LSS is a time-consuming process requiring a relatively dense sampling of patients. Further, given the variability among different centers, the predictability of LSSs is frequently questioned when it is extrapolated to other clinical centers. Currently, limited data are available on a reliable PMB LSS for estimating AUC_ss,24h. This study assessed and validated the practicability of LSSs established in the literature based on data from our center to provide reliable and ready-made PMB LSSs for laboratories performing therapeutic drug monitoring (TDM) of PMB. The influence of infusion and sampling time errors on predictability was also explored to obtain the optimal time points for routine PMB TDM. Using multiple regression analysis, PMB LSSs were generated from a model group of 20 patients. A validation group (10 patients) was used to validate the established LSSs. PMB LSSs from two published studies were validated using a dataset of 30 patients from our center. A population pharmacokinetic model was established to simulate the individual plasma concentration profiles for each infusion and sampling time error regimen. Pharmacokinetic data obtained from the 30 patients were fitted to a two-compartment model. Infusion and sampling time errors observed in real-world clinical practice could considerably affect the predictability of PMB LSSs. Moreover, we identified specific LSSs to be superior in predicting PMB AUC_ss,24h based on different infusion times. We also discovered that sampling time error should be controlled within −10 to 15 min to obtain better predictability. The present study provides validated PMB LSSs that can more accurately predict PMB AUC_ss,24h in routine clinical practice, facilitating PMB TDM in other laboratories and pharmacokinetics/pharmacodynamics-based clinical studies in the future.

Antibiotic polymyxin arranges lipopolysaccharide into crystalline structures to solidify the bacterial membrane

Polymyxins are last-resort antibiotics with potent activity against multi-drug resistant pathogens. They interact with lipopolysaccharide (LPS) in bacterial membranes, but mechanistic details at the molecular level remain unclear. Here, we characterize the interaction of polymyxins with native, LPS-containing outer membrane patches of Escherichia coli by high-resolution atomic force microscopy imaging, along with structural and biochemical assays. We find that polymyxins arrange LPS into hexagonal assemblies to form crystalline structures. Formation of the crystalline structures is correlated with the antibiotic activity, and absent in polymyxin-resistant strains. Crystal lattice parameters alter with variations of the LPS and polymyxin molecules. Quantitative measurements show that the crystalline structures decrease membrane thickness and increase membrane area as well as stiffness. Together, these findings suggest the formation of rigid LPS-polymyxin crystals and subsequent membrane disruption as the mechanism of polymyxin action and provide a benchmark for optimization and de novo design of LPS-targeting antimicrobials.

Self-Uptake Mechanism of Polymyxin-Based Lipopeptide against Gram-Negative Bacterial Membrane: Role of the First Adsorbed Lipopeptide

Research in the continuously increasing threat of polymyxin-resistant multidrug-resistant Pseudomonas aeruginosa, which causes severe infection in immunocompromised patients, has resulted in the development of several polymyxin-derived cyclic lipopeptides containing l-α-γ- diamino butyric acid-like FADDI-019 (F19). In this work, F19's insertion into a minimal model of the asymmetric outer membrane of the bacterium, which contained only penta-acylated lipid A (LipA) and lacked keto-d-octulosonic acid and O-antigens, in the top leaflet and phospholipids in the bottom leaflet, was studied. F19 exhibited all of the hallmarks of the self-uptake mechanism into the asymmetric bilayer. While a single monomer of the lipopeptide did not get partitioned into the inside of the bilayer, it competitively displaced Ca²⁺ from the membrane surface, observed as a decrease in Ca²⁺ coordination number with phosphate groups (1.89 vs 1.718), resulting in membrane destabilization. This resulted in an increment of the average defect size and the probability of interplay between lipid tails and hydrophobic residues of another F19. When more than one monomer was present in the system, the first monomer remained docked on the surface, while other monomers intercalated into the bilayer interior with their hydrophobic moieties "sleeved" by lipid acyl chains. The free energy barrier for partial insertion of the lipopeptide into a bilayer in the presence of surface-docked second F19 was recorded at ∼1.3 kcal/mol using two-dimensional (2D) well-tempered metadynamics, making it a low barrier process at 300 K. This study is an attempt to demonstrate the self-uptake mechanism of F19 during intercalation process into the bilayer interior, which may help in the design of better alternates for polymyxins to work against polymyxin resistance.

Lipid Microenvironment Modulates the Pore-Forming Ability of Polymyxin B

The ability of polymyxin B, an antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria as a last-line therapeutic option, to form ion pores in model membranes composed of various phospholipids and lipopolysaccharides was studied. Our data demonstrate that polymyxin B predominantly interacts with negatively charged lipids. Susceptibility decreases as follows: Kdo2-Lipid A >> DOPG ≈ DOPS >> DPhPG ≈ TOCL ≈ Lipid A. The dimer and hexamer of polymyxin B are involved in the pore formation in DOPG(DOPS)- and Kdo2-Lipid A-enriched bilayers, respectively. The pore-forming ability of polymyxin B significantly depends on the shape of membrane lipids, which indicates that the antibiotic produces toroidal lipopeptide-lipid pores. Small amphiphilic molecules diminishing the membrane dipole potential and inducing positive curvature stress were shown to be agonists of pore formation by polymyxin B and might be used to develop innovative lipopeptide-based formulations.

An area under the concentration-time curve threshold as a predictor of efficacy and nephrotoxicity for individualizing polymyxin B dosing in patients with carbapenem-resistant gram-negative bacteria

BACKGROUND

Evidence supports therapeutic drug monitoring of polymyxin B, but clinical data for establishing an area under the concentration-time curve across 24 h at steady state (AUC_ss,24 h) threshold are still limited. This study aimed to examine exposure-response/toxicity relationship for polymyxin B to establish an AUC_ss,24 h threshold in a real-world cohort of patients.

METHODS

Using a validated Bayesian approach to estimate AUC_ss,24 h from two samples, AUC_ss,24 h threshold that impacted the risk of polymyxin B-related nephrotoxicity and clinical response were derived by classification and regression tree (CART) analysis and validated by Cox regression analysis and logical regression analysis.

RESULTS

A total of 393 patients were included; acute kidney injury (AKI) was 29.0%, clinical response was 63.4%, and 30-day all-cause mortality was 35.4%. AUC_ss,24 h thresholds for AKI of > 99.4 mg h/L and clinical response of > 45.7 mg h/L were derived by CART analysis. Cox and logical regression analyses showed that AUC_ss,24 h of > 100 mg h/L was a significant predictor of AKI (HR 16.29, 95% CI 8.16-30.25, P < 0.001) and AUC_ss,24 h of ≥ 50 mg h/L (OR 4.39, 95% CI 2.56-7.47, P < 0.001) was independently associated with clinical response. However, these exposures were not associated with mortality. In addition, the correlation between trough concentration (1.2-2.8 mg/L) with outcomes was similar to AUC_ss,24 h.

CONCLUSIONS

For critically ill patients, AUC_ss,24 h threshold of 50-100 mg h/L was associated with decreased nephrotoxicity while assuring clinical efficacy. Therapeutic drug monitoring is recommended for individualizing polymyxin B dosing.

Important Roles and Potential Uses of Natural and Synthetic Antimicrobial Peptides (AMPs) in Oral Diseases: Cavity, Periodontal Disease, and Thrush

Numerous epithelial cells and sometimes leukocytes release AMPs as their first line of defense. AMPs encompass cationic histatins, defensins, and cathelicidin to encounter oral pathogens with minimal resistance. However, their concentrations are significantly below the effective levels and AMPs are unstable under physiological conditions due to proteolysis, acid hydrolysis, and salt effects. In parallel to a search for more effective AMPs from natural sources, considerable efforts have focused on synthetic stable and low-cytotoxicy AMPs with significant activities against microorganisms. Using natural AMP templates, various attempts have been used to synthesize sAMPs with different charges, hydrophobicity, chain length, amino acid sequence, and amphipathicity. Thus far, sAMPs have been designed to target Streptococcus mutans and other common oral pathogens. Apart from sAMPs with antifungal activities against Candida albicans, future endeavors should focus on sAMPs with capabilities to promote remineralization and antibacterial adhesion. Delivery systems using nanomaterials and biomolecules are promising to stabilize, reduce cytotoxicity, and improve the antimicrobial activities of AMPs against oral pathogens. Nanostructured AMPs will soon become a viable alternative to antibiotics due to their antimicrobial mechanisms, broad-spectrum antimicrobial activity, low drug residue, and ease of synthesis and modification.

Harnessing the Role of Bacterial Plasma Membrane Modifications for the Development of Sustainable Membranotropic Phytotherapeutics

Membrane-targeted molecules such as cationic antimicrobial peptides (CAMPs) are amongst the most advanced group of antibiotics used against drug-resistant bacteria due to their conserved and accessible targets. However, multi-drug-resistant bacteria alter their plasma membrane (PM) lipids, such as lipopolysaccharides (LPS) and phospholipids (PLs), to evade membrane-targeted antibiotics. Investigations reveal that in addition to LPS, the varying composition and spatiotemporal organization of PLs in the bacterial PM are currently being explored as novel drug targets. Additionally, PM proteins such as Mla complex, MPRF, Lpts, lipid II flippase, PL synthases, and PL flippases that maintain PM integrity are the most sought-after targets for development of new-generation drugs. However, most of their structural details and mechanism of action remains elusive. Exploration of the role of bacterial membrane lipidome and proteome in addition to their organization is the key to developing novel membrane-targeted antibiotics. In addition, membranotropic phytochemicals and their synthetic derivatives have gained attractiveness as popular herbal alternatives against bacterial multi-drug resistance. This review provides the current understanding on the role of bacterial PM components on multidrug resistance and their targeting with membranotropic phytochemicals.

Effects of Medicinal Leech-Related Cationic Antimicrobial Peptides on Human Blood Cells and Plasma

Cationic antimicrobial peptides (CAMPs) are considered as next-generation antibiotics with a lower probability of developing bacterial resistance. In view of potential clinical use, studies on CAMP biocompatibility are important. This work aimed to evaluate the behavior of synthetic short CAMPs (designed using bioinformatic analysis of the medicinal leech genome and microbiome) in direct contact with blood cells and plasma. Eight CAMPs were included in the study. Hemolysis and lactate dehydrogenase assays showed that the potency to disrupt erythrocyte, neutrophil and mononuclear cell membranes descended in the order pept_1 > pept_3 ~ pept_5 > pept_2 ~ pept_4. Pept_3 caused both cell lysis and aggregation. Blood plasma and albumin inhibited the CAMP-induced hemolysis. The chemiluminescence method allowed the detection of pept_3-mediated neutrophil activation. In plasma coagulation assays, pept_3 prolonged the activated partial thromboplastin time (APTT) and prothrombin time (at 50 μM by 75% and 320%, respectively). Pept_3 was also capable of causing fibrinogen aggregation. Pept_6 prolonged APTT (at 50 μM by 115%). Pept_2 was found to combine higher bactericidal activity with lower effects on cells and coagulation. Our data emphasize the necessity of investigating CAMP interaction with plasma.

A novel antibiotic combination of linezolid and polymyxin B octapeptide PBOP against clinical Pseudomonas aeruginosa strains

Background

Antibiotic-resistant Gram-negative bacteria are becoming a major public health threat such as the important opportunistic pathogen Pseudomonas aeruginosa (P. aeruginosa). The present study investigated enhancement of the linezolid spectrum, which is normally used to treat Gram-positive bacteria, at inhibiting P. aeruginosa growth.

Methods

The checkerboard test or time-kill assay were carried out to determine the antibacterial effects of linezolid in cooperation with polymyxin B octapeptide PBOP (LP) against P. aeruginosa based on in vitro model. The protective effect of LP against P. aeruginosa infection was assessed based on a Caenorhabditis elegans (C. elegans) model.

Results

The synergistic activity and antibacterial effects were significantly increased against P. aeruginosa by LP treatment, while linezolid and PBOP as monotherapies exhibited no remarkably bactericidal activity against the clinical strains. Additionally, LP treatment modified biofilm production, morphology, swimming motility of P. aeruginosa, and protected C. elegans from P. aeruginosa infection.

Conclusions

This research demonstrates that LP combination has significant synergistic activity against P. aeruginosa, and PBOP is potential to be an activity enhancer. Notably, this strategy improved the antibacterial activity spectrum of linezolid and other anti-Gram-positive agents and represents an effective choice to surmount the antibiotic resistance of bacteria in the long term.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12941-022-00531-5.

Chelation in Antibacterial Drugs: From Nitroxoline to Cefiderocol and Beyond

In the era of escalating antimicrobial resistance, the need for antibacterial drugs with novel or improved modes of action (MOAs) is a health concern of utmost importance. Adding or improving the chelating abilities of existing drugs or finding new, nature-inspired chelating agents seems to be one of the major ways to ensure progress. This review article provides insight into the modes of action of antibacterial agents, class by class, through the perspective of chelation. We covered a wide scope of antibacterials, from a century-old quintessential chelating agent nitroxoline, currently unearthed due to its newly discovered anticancer and antibiofilm activities, over the commonly used antibacterial classes, to new cephalosporin cefiderocol and a potential future class of tetramates. We show the impressive spectrum of roles that chelation plays in antibacterial MOAs. This, by itself, demonstrates the importance of understanding the fundamental chemistry behind such complex processes.

A novel family of non-secreted tridecaptin lipopeptide produced by Paenibacillus elgii

Bacteria from the genus Paenibacillus make a variety of antimicrobial compounds, including lipopeptides produced by a non-ribosomal synthesis mechanism (NRPS). In the present study, we show the genomic and phenotypical characterization of Paenibacillus elgii AC13 which makes three groups of small molecules: the antimicrobial pelgipeptins and two other families of peptides that have not been described in P. elgii. A family of lipopeptides with [M + H]⁺ 1664, 1678, 1702, and 1717 m/z was purified from the culture cell fraction. Partial characterization revealed that they are similar to tridecaptin from P. terrae. However, they present amino acid chain modifications in positions 3, 7, and 10. These new variants were named tridecaptin G1, G2, G3, and G4. Furthermore, a gene cluster was identified in P. elgii AC13 genome, revealing high similarity to the tridecaptin-NRPS gene cluster from P. terrae. Tridecaptin G1 and G2 showed in vitro antimicrobial activity against Escherichia coli, Klebsiella pneumonia (including a multidrug-resistant strain), Staphylococcus aureus, and Candida albicans. Tri G3 did not show antimicrobial activity against S. aureus and C. albicans at all tested concentrations. An intriguing feature of this family of lipopeptides is that it was only observed in the cell fraction of the P. elgii AC13 culture, which could be a result of the amino acid sequence modifications presented in these variants.

Total Synthesis and Structural Elucidation of Ogipeptins

The first total synthesis of ogipeptin A was achieved. Recently, using the advanced Marfey's method, we determined the absolute configuration patterns of three β-hydroxy-α,γ-diaminobutyric acids (β-OH Dabs) composing ogipeptins. On the basis of this result, we conducted solid-phase total synthesis of three diastereomers of ogipeptin A. The analytical data of one diastereomer exactly corresponded with those of natural ogipeptin A. Therefore, the absolute configurations of ogipeptins have been elucidated.

Nanostructure Control of an Antibiotic-based Polyion Complex Using a Series of Polycations with Different Side-chain Modification Rates

Developing nanovehicles for delivering antibiotics is a promising approach to overcome the issue of antibiotic resistance. This study aims to utilize a polyion complex (PICs) system for developing novel nanovehicles for polymyxin-type antibiotics, which are known as last resort drugs. The formation of antibiotic-based PIC nanostructures was investigated using colistimethate sodium (CMS), an anionic cyclic short peptide, and a series of block catiomers bearing different amounts of guanidinium moieties on their side chains. In addition, only the modified catiomer, and not the unmodified catiomer, self-assembles with CMS, implying the importance of the guanidine moieties for enhancing the interaction between the catiomer and CMS via the formation of multivalent hydrogen bonding. Moreover, micellar and vesicular PIC nanostructures are selectively formed depending on the ratio of the guanidine residues. Size-exclusion chromatography revealed that the encapsulation efficiency of CMS is dependent on the guanidinium modification ratio. The antimicrobial activity of the PIC nanostructures is also confirmed, indicating that the complexation of CMS in the PICs and further release from the PICs successfully occurs. This article is protected by copyright. All rights reserved.

Identification of metabolite extraction method for targeted exploration of antimicrobial resistance associated metabolites of Klebsiella pneumoniae

Antimicrobial resistant Klebsiella pneumoniae (K. pneumoniae), as being a pathogen of critical clinical concern, urgently demands effective therapeutic options. However, the discovery of novel antibiotics over the last three decades has declined drastically and necessitates exploring novel strategies. Metabolomic modulation has been the promising approach for the development of effective therapeutics to deal with AMR; however, only limited efforts have been made to-date, possibly due to the unavailability of suitable metabolites extraction protocols. Therefore, in order to establish a detailed metabolome of K. pneumoniae and identify a method for targeted exploration of metabolites that are involved in the regulation of AMR associated processes, metabolites were extracted using multiple methods of metabolites extraction (freeze-thaw cycle (FTC) and sonication cycle (SC) method alone or in combination (FTC followed by SC; FTC + SC)) from K. pneumoniae cells and then identified using an orbitrap mass analyzer (ESI-LC-MS/MS). A total of 151 metabolites were identified by using FTC, 132 metabolites by using FTC+SC, 103 metabolites by using SC and 69 metabolites common among all the methods used which altogether enabled the identification of 199 unique metabolites. Of these 199, 70 metabolites were known to have an association with AMR phenotype and among these, the FTC + SC method yielded better (identified 55 metabolites), quantitatively and qualitatively compared to FTC and SC alone (identified 51 and 41 metabolites respectively). Each method of metabolite extraction showed a definite degree of biasness and specificity towards chemical classes of metabolites and jointly contributed to the development of a detailed metabolome of the pathogen. FTC method was observed to give higher metabolomic coverage as compared to SC alone and FTC + SC. However, FTC + SC resulted in the identification of a higher number of AMR associated metabolites of K. pneumoniae compared to FTC and SC alone.

A Potent Antibiotic Combination of Linezolid and Polymycin B Nonapeptide Against Klebsiella pneumoniae Infection In Vitro and In Vivo

The emergence of antibiotic resistant Gram-negative bacteria such as Klebsiella pneumoniae (KP) is becoming a major public health threat and imposing a financial burden worldwide. A serious lack of new drugs under development is undermining efforts to fight them. In this study, we report a potent combination of linezolid and polymyxin B nonapeptide PBNP (LP) against KP infection in vitro and in vivo. The checkerboard test and the time-kill assay were performed to detect the antibacterial activity of LP against KP in vitro. And the Caenorhabditis elegans (C. elegans) was used as infection model to evaluate the protective effect of LP against KP infection in vivo. The LP combination showed significantly synergistic activity and antibacterial effects against KP, while linezolid and PBNP as monotherapies revealed no dramatically antibacterial activity against the KP strains. Additionally, we found that the LP treatment altered the biofilm production and morphology of KP. Furthermore, the LP treatments significantly protected C. elegans from KP infection. In conclusion, this study indicated that the LP combination exhibited significantly synergistic activity against KP and PBNP can be used as a potential activity enhancer. More importantly, this strategy provided the improvement of antibacterial activity spectrum of agents like linezolid and represented a potent alternative to overcome antibiotic resistance in the future.

Colistin Resistance and Molecular Characterization of the Genomes of mcr-1-Positive Escherichia coli Clinical Isolates

Research on resistance against polymyxins induced by the mcr-1 gene is gaining interest. In this study, using agar dilution method, polymerase chain reaction, and comparative genomic analysis, we investigated the colistin resistance mechanism of clinical E. coli isolates. The minimum inhibitory concentration (MIC) analysis results revealed that of the 515 isolates tested, bacteria with significantly increased MIC levels against colistin were isolated in 2019. Approximately one-fifth (17.14% to 19.65%) of the isolates showed MIC values ≥1 mg/L against colistin in 2015, 2016, and 2017. However, in 2019, up to three-quarters (74.11%, 146/197) of the isolates showed MIC values ≥1 mg/L against colistin indicating an increase in colistin resistance. Six isolates (EC7518, EC4968, EC3769, EC16, EC117, EC195, 1.13%, 6/515) were found to carry the mcr-1 gene and a novel mcr-1 variant with Met2Ile mutation was identified in EC3769. All six strains showed higher MIC levels (MIC=4 mg/L) than any mcr-1-negative strains (MIC ≤ 2 mg/L). Whole-genome sequencing of the six mcr-1-positive isolates revealed that EC195 carried the highest number of resistance genes (n = 28), nearly a half more than those of the following EC117 (n = 19). Thus, EC195 showed a wider resistance spectrum and higher MIC levels against the antimicrobials tested than the other five isolates. Multi-locus sequence typing demonstrated that these mcr-1-positive strains belonged to six different sequence types. The six mcr-1 genes were located in three different incompatibility group plasmids (IncI2, IncHI2 and IncX4). The genetic context of mcr-1 was related to a sequence derived from Tn6330 (ISApl1-mcr-1-pap2-ISApl1). Investigations into the colistin resistance mechanism and characterization of the molecular background of the mcr genes may help trace the development and spread of colistin resistance in clinical settings.

Polymyxin B-Associated Nephrotoxicity and Its Predictors: A Retrospective Study in Carbapenem-Resistant Gram-Negative Bacterial Infections

Polymyxin B (PMB), a kind of polymyxin, was widely used in carbapenem-resistant Gram-negative bacterial (CR-GNB) infections. However, adverse reactions such as nephrotoxicity and neurotoxicity limit its use in clinical practice. The aim of this study was to explore PMB associated with nephrotoxicity and its predictors. Patients who received PMB intravenous drip for more than 72 h were eligible for the study. Characteristics of patients, concomitant nephrotoxic agents, underlying disease, and antimicrobial susceptibility were submitted for descriptive analysis. Univariate analysis and binary logistic regression were used to assess the factors leading to acute kidney injury (AKI). AKI was assessed with serum creatinine variations according to the classification of risk (stage R), injury (stage I), failure (stage F), loss, and end-stage of kidney disease. Among 234 patients with CR-GNB infections who used PMB in our study, 67 (28.63%) patients developed AKI, including 31 (14.25%) patients in stage R, 15 (6.41%) patients in stage I, and 21 (8.97%) patients in stage F. The incident rate of PMB-related nephrotoxicity in patients with normal renal function was 32.82% (43/131). The higher risk factors of AKI include males [odds ratio (OR) = 3.237; 95% confidence interval (95%CI) = 1.426–7.350], digestive system diseases [OR = 2.481 (1.127–5.463)], using furosemide (&gt;20 mg/day) [OR = 2.473 (1.102–5.551)], and baseline serum creatinine [OR = 0.994 (0.990–0.999)]. Nonparametric tests of K-independent samples showed that baseline serum creatinine and the PMB maintenance dose were associated with the severity of nephrotoxicity (both p &lt; 0.05). Male, digestive system diseases, using furosemide (&gt;20 mg/day), and high baseline serum creatinine were the independent risk factors of PMB-associated AKI development. The maintenance dose of PMB may be related to the severity of AKI. These risk factors should be taken into consideration when initiating PMB-based therapy. The serum creatinine value should be closely monitored when using PMB.

Antibiotic-loaded lipid-based nanocarrier: a promising strategy to overcome bacterial infection

According to the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC), bacterial infections are one of the greatest threats to global health, food production, and life expectancy. In this sense, the development of innovative formulations aiming at greater therapeutic efficacy, safety, and shorter treatment duration compared to conventional products is urgently needed. Lipid-based nanocarriers (LBNs) have demonstrated the potential to enhance the effectiveness of available antibiotics. Among them, liposome, nanoemulsion, solid lipid nanoparticle (SLN), and nanostructured lipid carrier (NLC) are the most promising due to their solid technical background for laboratory and industrial production. This review describes recent advances in developing antibiotic-loaded LBNs against susceptible and resistant bacterial strains and biofilm. LBNs revealed to be a promising alternative to deliver antibiotics due to their superior characteristics compared to conventional preparations, including their modified drug release, improved bioavailability, drug protection against chemical or enzymatic degradation, greater drug loading capacity, and biocompatibility. Antibiotic-loaded LBNs can improve current clinical drug therapy, bring innovative products and rescue discarded antibiotics. Thus, antibiotic-loaded LBNs have potential to open a window of opportunities to continue saving millions of lives and prevent the devastating impact of bacterial infection.

Nordihydroguaiaretic acid reverses the antibacterial activity of colistin against MCR-1-positive bacteria in vivo/in vitro by inhibiting MCR-1 activity and injuring the bacterial cell membrane

BACKGROUND

Colistin (polymyxin E) is an effective antibiotic for the treatment of most multidrug-resistant Gram-negative bacteria. However, some bacteria, including bacterial spp. belonging to the Enterobacteriaceae family, have an acquired resistance against polymyxins, which is attributed to they possess plasmid-carried resistance genes (mcr-1 and its variants). So, there is an urgent need to develop new therapeutic strategies to target broad spectrum resistant spp. from Enterobacteriaceae family in response to the loss of the protective barrier of last-line antibiotics. Here, we report the adjuvant capacity of nordihydroguaiaretic acid (NDGA) for restoring the antibacterial activity of colistin against MCR-1-positive E. coli ZJ487 in vivo/in vitro.

METHODS

A checkerboard assay, time-killing analysis, isobolograms, growth curves and inducible resistance test showed the effect of NDGA combined with colistin in vitro. TLC was used to detect the inhibitory effect of NDGA on MCR-1. Colony determination and hematoxylin and eosin (HE) staining were used to assess the synergistic effect of NDGA and colistin in mice.

RESULTS

Our results showed that NDGA in combination with colistin showed a synergistic bactericidal action without inducing resistance. NDGA directly inhibited MCR-1 activity and resulted in measurable injury to the bacterial cell membrane to recover the antibacterial effect of colistin. Most importantly, NDGA in combination with colistin exhibited an in vivo synergistic effect in murine peritonitis infection models, as evidenced by the survival rate of MCR-1-positive E. coli ZJ487-infected mice which increased from 6.67 to 50.0%.

CONCLUSION

Our study demonstrated that NDGA effectively rescues the efficiency of colistin against MCR-positive E. coli ZJ487 by simultaneously inhibiting both, the MCR activity and the injury to the cell membrane of bacteria.

Intraventricular Drug Delivery and Sampling for Pharmacokinetics and Pharmacodynamics Study

Although the blood-brain barrier (BBB) protects the brain from foreign entities, it also prevents some therapeutics from crossing into the central nervous system (CNS) to ameliorate diseases or infections. Drugs are administered directly into the CNS in animals and humans to circumvent the BBB. The present protocol describes a unique way of treating brain infections through intraventricular delivery of antibiotics, i.e., polymyxins, the last-line antibiotics to treat multi-drug resistant Gram-negative bacteria. A straightforward stereotaxic surgery protocol was developed to implant a guide cannula reaching into the lateral ventricle in rats. After a recovery period of 24 h, rats can be injected consciously and repeatedly through a cannula that is fitted to the guide. Injections can be delivered manually as a bolus or infusion using a microinjection pump to obtain a slow and controlled flow rate. The intraventricular injection was successfully confirmed with Evans Blue dye. Cerebrospinal fluid (CSF) can be drained, and the brain and other organs can be collected. This approach is highly amenable for studies involving drug delivery to the CNS and subsequent assessment of pharmacokinetic and pharmacodynamic activity.

An Efficient Approach for the Design and Synthesis of Antimicrobial Peptide-Peptide Nucleic Acid Conjugates

Peptide-Peptide Nucleic Acid (PNA) conjugates targeting essential bacterial genes have shown significant potential in developing novel antisense antimicrobials. The majority of efforts in this area are focused on identifying different PNA targets and the selection of peptides to deliver the peptide-PNA conjugates to Gram-negative bacteria. Notably, the selection of a linkage strategy to form peptide-PNA conjugate plays an important role in the effective delivery of PNAs. Recently, a unique Cysteine- 2-Cyanoisonicotinamide (Cys-CINA) click chemistry has been employed for the synthesis of cyclic peptides. Considering the high selectivity of this chemistry, we investigated the efficiency of Cys-CINA conjugation to synthesize novel antimicrobial peptide-PNA conjugates. The PNA targeting acyl carrier protein gene (acpP), when conjugated to the membrane-active antimicrobial peptides (polymyxin), showed improvement in antimicrobial activity against multidrug-resistant Gram-negative Acinetobacter baumannii. Thus, indicating that the Cys-CINA conjugation is an effective strategy to link the antisense oligonucleotides with antimicrobial peptides. Therefore, the Cys-CINA conjugation opens an exciting prospect for antimicrobial drug development.

Molecular mechanism underlying the TLR4 antagonistic and antiseptic activities of papiliocin, an insect innate immune response molecule

SignificanceSimilar to mammalian TLR4/MD-2, the Toll9/MD-2-like protein complex in the silkworm, Bombyx mori, acts as an innate pattern-recognition receptor that recognizes lipopolysaccharide (LPS) and induces LPS-stimulated expression of antimicrobial peptides such as cecropins. Here, we report that papiliocin, a cecropin-like insect antimicrobial peptide from the swallowtail butterfly, competitively inhibits the LPS-TLR4/MD-2 interaction by directly binding to human TLR4/MD-2. Structural elements in papiliocin, which are important in inhibiting TLR4 signaling via direct binding, are highly conserved among insect cecropins, indicating that its TLR4-antagonistic activity may be related to insect Toll9-mediated immune response against microbial infection. This study highlights the potential of papiliocin as a potent TLR4 antagonist and safe peptide antibiotic for treating gram-negative sepsis.

Structural Characterization and Immunoenhancing Effects of a Polysaccharide from the Soft Coral Lobophytum sarcophytoides

Previous studies on the soft coral Lobophytum sarcophytoides (Lobophytum sp.) are mainly about small molecules, and there has been no systematic research on polysaccharides. In the study, a novel polysaccharide (LCPs-1-A) with immunoenhancing functions was successfully extracted and purified from the soft coral Lobophytum sp. After preliminary analysis, our data indicated that LCPs-1-A was composed of glucose and had a molecular weight of 4.90 × 10⁶ Da. Moreover, our findings showed that LCPs-1-A could promote the proliferation and phagocytosis of RAW264.7 cells, stimulate the production of NO and ROS, and increase the mRNA expression of IL-1β, IL-6, and TNF-α, which indicated that LCPs-1-A had a good immunoenhancing activity. Through further studies, we found that LCPs-1-A might play an immunoenhancing role through the TLR4/NF-κB signaling pathway. Therefore, our results demonstrated that LCPs-1-A might be a natural immunostimulant for use in medical and food industries.

Eumelanin-Inspired Antimicrobial with Biocidal Activity against Methicillin-Resistant Staphylococcus aureus

The reliance on antibiotics and antimicrobials to treat bacterial infectious diseases is threatened by the emergence of antibiotic resistance and multi-drug-resistant organisms, thus having the potential to greatly impact human health. Thus, the discovery and development of antimicrobials capable of acting on antibiotic-resistant bacteria is a major area of significance in scientific research. Herein, we present the development of a eumelanin-inspired antimicrobial capable of killing methicillin-resistant Staphylococcus aureus (MRSA). By ligating quaternary ammonium-functionalized "arms" to a eumelanin-inspired indole with intrinsic antimicrobial activity, an antimicrobial agent with enhanced activity was prepared. This resulting antimicrobial, EIPE-1, had a minimum inhibitory concentration of 16 μg/mL (17.1 μM) against a clinical isolate of MRSA obtained from an adult cystic fibrosis patient. The biocidal activity occurred within 30 min of exposure and resulted in changes to the bacterial cell surface as visualized with a scanning electron microscope. Taken together, these studies demonstrate that EIPE-1 is effective at killing MRSA.

Beyond Soil-Dwelling Actinobacteria: Fantastic Antibiotics and Where to Find Them

Bacterial secondary metabolites represent an invaluable source of bioactive molecules for the pharmaceutical and agrochemical industries. Although screening campaigns for the discovery of new compounds have traditionally been strongly biased towards the study of soil-dwelling Actinobacteria, the current antibiotic resistance and discovery crisis has brought a considerable amount of attention to the study of previously neglected bacterial sources of secondary metabolites. The development and application of new screening, sequencing, genetic manipulation, cultivation and bioinformatic techniques have revealed several other groups of bacteria as producers of striking chemical novelty. Biosynthetic machineries evolved from independent taxonomic origins and under completely different ecological requirements and selective pressures are responsible for these structural innovations. In this review, we summarize the most important discoveries related to secondary metabolites from alternative bacterial sources, trying to provide the reader with a broad perspective on how technical novelties have facilitated the access to the bacterial metabolic dark matter.

High Frequency of Class I and II Integrons and the Presence of aadA2 and dfrA12 Gene Cassettes in the Clinical Isolates of Acinetobacter baumannii from Shiraz, Southwest of Iran

Background: Acinetobacter baumannii is a global concern that causes healthcare-associated infections due to multidrug resistance against commercially available antimicrobial agents. Objectives: The present study was conducted to determine the antimicrobial susceptibility of A. baumannii isolates from clinical specimens in Shiraz, Iran. In addition, the possible relationship of susceptibility patterns with the presence of integrons and related gene cassettes is investigated. Methods: A. baumannii isolates were collected, and their susceptibility to various antibiotics was tested using the Kirby-Bauer disk diffusion method. Moreover, molecular analyses were performed to detect the presence of the OXA-51-like gene, as well as class I, II, and III integrons, and associated gene cassettes. Results: The majority of isolates were resistant to imipenem (99.4%), piperacillin (98.2%), gentamycin (98.2%), meropenem (97.7%), ceftazidime (95.4%), amikacin (95.4%), and trimethoprim-sulfamethoxazole (90.8%). All strains showed multidrug resistance to the tested antibiotics. The distribution analysis of integrons genes revealed that 90.2, 72.4, and 12.1% of the isolates carried intI1, intI2, and intI3 genes, respectively. Moreover, two types of prevalent gene cassettes, including aad and dfr, were detected in class 1 integron-carrying strains. Conclusions: The current study showed the high prevalence of A. baumannii isolates harboring integrons in our investigated medical center, which may indicate the distribution of multidrug resistance events. The different gene cassette arrays in the present study highlight the remarkable role of geographical issues in disseminating multidrug-resistant isolates. This could be attributed to distinct therapeutic interventions in different areas. The results demonstrate the necessity of continuous surveillance to prevent the distribution of multidrug resistance among A. baumannii strains in Iran.

Detection of mcr-1-mediated resistance to polymyxins in Enterobacterales using colistin disk chelator application

Objective. To evaluate the possibility of using the colistin disk chelator application (CDCA) method as simple and available screening tool for detection of mcr-1-mediated resistance to polymyxins in Enterobacterales. Materials and Methods. A total of 47 colistin-resistant Enterobacterales isolates obtained in 2014–2020 within multicenter MARATHON study were included in the experiment. Colistin susceptibility testing was performed using Mueller–Hinton broth microdilution method according to ISO 20776-1:2006. Interpretation of the results was performed according to EUCAST v.12.0 clinical breakpoints. MCR-genes were detected by multiplex real-time PCR. Phenotypic screening for mcr-expression was performed on Mueller–Hinton agar by application of dipicolinic acid in concentration of 1,000 mcg/disk in 10 µL volume per disk and 0.5 M solution of EDTA in 5 µL volume per disk. Chelating effect was registered by differences in zone of growth inhibition around colistin disks with and without chelator. Measurements were performed with the help of caliper in millimeters. Statistical data processing was carried out in accordance with guidelines for statistical analysis in medical researches using MS-Excel tool. Results. In 25 of 47 included in the experiment enterobacteria isolates mcr-genes were detected by molecular method. MCR-detection by CDCA method identified the average difference value of the zones of growth inhibition for colistin and its combination with EDTA and DPA as 4.1 mm and 3.7 mm respectively for mcr-positive isolates and 1.7 mm and 1.2 mm respectively for mcr-negative isolates. Statistical analysis estimated that a difference of ≥ 3 mm in zone of growth inhibition for combination of colistin with one of the chelating agents when compared to colistin only allows us to conclude that a studied isolated carries mcr-1-mediated resistance to polymyxins. In addition, sensitivity of the test was 96% and specificity was 91% if DPA is used, while EDTA showed only 88% sensitivity and 77% specificity. Conclusions. Proposed method appears as available technique for phenotypic screening of the Enterobacterales order for mcr-1-mediated resistance to polymyxins for practical laboratories in present conditions. The use of DPA is preferred because of better specificity and sensitivity rates.

Overview of polymyxin resistance in Enterobacteriaceae

Polymyxin antibiotics are disfavored owing to their potential clinical toxicity, especially nephrotoxicity. However, the dry antibiotic development pipeline, together with the increasing global prevalence of infections caused by multidrug-resistant (MDR) gram-negative bacteria, have renewed clinical interest in these polypeptide antibiotics. This review highlights the current information regarding the mechanisms of resistance to polymyxins and their molecular epidemiology. Knowledge of the resistance mechanisms and epidemiology of these pathogens is critical for the development of novel antibacterial agents and rapid treatment choices.

Comparative metabolomics revealed key pathways associated with the synergistic killing of multidrug-resistant Klebsiella pneumoniae by a bacteriophage-polymyxin combination

Resistance to the last-line polymyxins is emerging in multidrug-resistant Klebsiella pneumoniae and phage therapy is a promising alternative. However, phage monotherapy often rapidly causes resistance and few studies have examined antibiotic-phage combinations against K. pneumoniae. Here, we investigated the combination of polymyxin B with a novel phage pK8 against an mcr-1-carrying polymyxin-resistant clinical isolate Kp II-503 (polymyxin B MIC, 8 mg/L). The phage genome was sequenced and bacterial metabolomes were analysed at 4 and 24 h following the treatment with polymyxin B (16 mg/L), phage pK8 (10² PFU/mL) and their combination. Minimal metabolic changes across 24 h were observed with polymyxin B alone; whereas a significant inhibition of the citrate cycle, pentose phosphate pathway, amino acid and nucleotide metabolism occurred with the phage-polymyxin combination at both 4 and 24 h, but with phage alone only at 4 h. The development of resistance to phage alone was associated with enhanced membrane lipid and decreased amino acid biosynthesis in Kp II-503. Notably, cAMP, cGMP and cCMP were significantly enriched (3.1–6.6 log₂fold) by phage alone and the combination only at 4 h. This is the first systems pharmacology study to investigate the enhanced bacterial killing by polymyxin-phage combination and provides important mechanistic information on phage killing, resistance and antibiotic-phage combination in K. pneumoniae.

The war against bacteria, from the past to present and beyond

INTRODUCTION

The human defense against microorganisms dates back to the ancient civilizations, with attempts to use substances from vegetal, animal, or inorganic origin to fight infections. Today, the emerging threat of multidrug-resistant bacteria highlights the consequences of antibiotics inappropriate use, and the urgent need for novel effective molecules.

METHODS AND MATERIALS

We extensively researched on more recent data within PubMed, Medline, Web of Science, Elsevier's EMBASE, Cochrane Review for the modern pharmacology in between 1987 - 2021. The historical evolution included a detailed analysis of past studies on the significance of medical applications in the ancient therapeutic field.

AREAS COVERED

We examined the history of antibiotics development and discovery, the most relevant biochemical aspects of their mode of action, and the biomolecular mechanisms conferring bacterial resistance to antibiotics.

EXPERT OPINION

The list of pathogens showing low sensitivity or full resistance to most currently available antibiotics is growing worldwide. Long after the 'golden age' of antibiotic discovery, the most novel molecules should be carefully reserved to treat serious bacterial infections of susceptible bacteria. A correct diagnostic and therapeutic procedure can slow down the spreading of nosocomial and community infections sustained by multidrug-resistant bacterial strains.

Assessing the predictive performance of population pharmacokinetic models for intravenous polymyxin B in critically ill patients

Polymyxin B (PMB) has reemerged as a last‐line therapy for infections caused by multidrug‐resistant gram‐negative pathogens, but dosing is challenging because of its narrow therapeutic window and pharmacokinetic (PK) variability. Population PK (POPPK) models based on suitably powered clinical studies with appropriate sampling strategies that take variability into consideration can inform PMB dosing to maximize efficacy and minimize toxicity and resistance. Here we reviewed published PMB POPPK models and evaluated them using an external validation data set (EVD) of patients who are critically ill and enrolled in an ongoing clinical study to assess their utility. Seven published POPPK models were employed using the reported model equations, parameter values, covariate relationships, interpatient variability, parameter covariance, and unexplained residual variability in NONMEM (Version 7.4.3). The predictive ability of the models was assessed using prediction‐based and simulation‐based diagnostics. Patient characteristics and treatment information were comparable across studies and with the EVD (n = 40), but the sampling strategy was a main source of PK variability across studies. All models visually and statistically underpredicted EVD plasma concentrations, but the two‐compartment models more accurately described the external data set. As current POPPK models were inadequately predictive of the EVD, creation of a new POPPK model based on an appropriately powered clinical study with an informed PK sampling strategy would be expected to improve characterization of PMB PK and identify covariates to explain interpatient variability. Such a model would support model‐informed precision dosing frameworks, which are urgently needed to improve PMB treatment efficacy, limit resistance, and reduce toxicity in patients who are critically ill.