Novel synthetic polymyxins kill Gram-positive bacteria

Polymyxin B and ethylenediaminetetraacetic acid act synergistically against Pseudomonas aeruginosa and Staphylococcus aureus

Polymyxin B and ethylenediaminetetraacetic acid are antimicrobials possessing antibiofilm activity. They act by displacement and chelation, respectively, of divalent cations in bacterial membranes and may therefore act synergistically when applied in combination. If so, this combination of agents may be useful for the treatment of diseases like cystic fibrosis (CF), in which biofilms are present on the respiratory epithelium. We used checkerboard assays to investigate the synergy between these agents using reference strains Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 6538 in planktonic form. We then determined the efficacy of each agent against biofilms of both species grown on 96-pin lids and proceeded to combination testing against the P. aeruginosa reference strain and 10 clinical isolates from patients with CF. Synergism was observed for planktonic forms of both species and for biofilms of P. aeruginosa. The susceptibility of biofilms of P. aeruginosa clinical isolates to these agents was variable compared to the laboratory reference strain. This combination of agents may be useful in the management of biofilm-associated conditions, particularly those amenable to topical therapies. These results provide a basis upon which the antimicrobial and antibiofilm efficacy of preparations containing these agents may be enhanced.IMPORTANCEBacteria living in biofilms produce a protective matrix which makes them difficult to kill. Patients with severe respiratory disease often have biofilms. Polymyxin B is an antibiotic commonly used in topical medications, such as eye drops and nasal sprays. Ethylenediaminetetraacetic acid (EDTA) is used widely as a preservative in medication but also has antimicrobial properties. It has been hypothesized that Polymyxin B and EDTA could have a synergistic relationship: when used in combination their antimicrobial effect is enhanced. Here, we evaluated the levels at which Polymyxin B and EDTA work together to kill common pathogens Pseudomonas aeruginosa and Staphylococcus aureus. We found that Polymyxin B and EDTA were synergistic. This synergy may be useful in the management of planktonic infection with P. aeruginosa and S. aureus, or biofilm infection with P. aeruginosa. This synergy may be beneficial in the treatment of respiratory biofilms, in which P. aeruginosa biofilms are common.

Unexpected vulnerability of Enterococcus faecium to polymyxin B under anaerobic condition

Gram-positive Enterococcus faecium exhibited higher susceptibility (>4-fold) to polymyxin B (PMB), the canonical antimicrobial peptide against Gram-negative bacteria, under anaerobic condition than aerobic condition. Anaerobically grown E. faecium exhibited high vulnerability to PMB, leading to alteration of cell surface and morphology, as observed based on their high dansyl-PMB affinity (>2.9-fold), a proportion (>8.5-fold) of propidium iodide-stained cells, and observation of scanning electron microscopy results. Interestingly, our transcriptomic and chemical analyses revealed that enterocin B, produced anaerobically, imposes a burden on the cellular envelope when cells are exposed to PMB. This scenario was also supported by PMB susceptibility tests and killing curves, which showed that ΔentB knockout mutant cells were more resistant to PMB (32 µg/mL) compared to wild-type cells (4 µg/mL) under anaerobic condition. Fluorescent D-amino acid and BOCILLIN™-fluorescent profiling of transpeptidase activities in ΔentB mutant cells under anaerobic condition revealed similar levels of activity to those observed in WT cells under aerobic condition. The high level of secreted bacteriocins in WT under anaerobic condition likely led to significant membrane depolarization and loosening of the peptidoglycan layer, making the cells more permeable to PMB. Overall, our findings suggest that anaerobically produced bacteriocins, in conjunction with PMB, contribute to the killing of E. faecium by destabilizing its cell envelope.

Antibacterial and antibiofilm activity of 1-thiocarbamoyl-3,5-diaryl-4,5-dihydro-1H pyrazoles and thiazoles in multidrug-resistant pathogens

To find novel antibiotic drugs, six 1-thiocarbamoyl-3,5-diaryl-4,5-dihydro-1H derivatives named 1b, 1d (pyrazoles), 2a, 2b, 2c, and 2d (thiazoles) were evaluated in silico and in vitro. The in silico analyses were based on ADME pharmacokinetic parameters (absorption, distribution, metabolism, and excretion). The in vitro antibacterial activity was evaluated in Gram-positive and Gram-negative species (Staphylococcus aureus ATCC® 25904, Staphylococcus epidermidis ATCC® 35984, Klebsiella pneumoniae ATCC® 700603, and Acinetobacter baumannii ATCC® 19606), by determination of minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), kinetics curve, and antibiofilm assays. As results, the azoles have activity against the Gram-negative species K. pneumoniae ATCC® 700603 and A. baumannii ATCC® 19606. No antibacterial activity was observed for the Gram-positive bacteria evaluated. Thus, the azoles were evaluated against clinical isolates of K. pneumoniae carbapenemase (KPC) and A. baumannii multidrug-resistant (Ab-MDR). All azoles have antibacterial activity against Ab-MDR isolates (Gram-negative) with MIC values between 512 μg/mL and 1,024 μg/mL. Against KPC isolates the azoles 1b, 1d, and 2d present antibacterial activity (MIC = 1,024 μg/mL). In the kinetics curve assay, the 1b and 1d pyrazoles reduced significantly viable cells of Ab-MDR isolates and additionally inhibited 86.6 to 95.8% of the biofilm formation. The in silico results indicate high possibility to permeate the blood-brain barrier (2b) and was predict human gastrointestinal absorption (all evaluated azoles). Considering that the research and development of new antibiotics is a priority for drug-resistant pathogens, our study revealed the antibacterial and antibiofilm activity of novel azoles against K. pneumoniae and A. baumannii pathogens.

Simeprevir restores the anti-Staphylococcus activity of polymyxins

Methicillin-resistant Staphylococcus aureus (MRSA) infection poses a severe threat to global public health due to its high mortality. Currently, polymyxins are mainly used for the treatment of Gram-negative bacterial-related infection, while exhibiting limited antibacterial activities against Staphylococcus aureus (S. aureus). However, the combination of antibiotics with antibiotic adjuvants is a feasible strategy for the hard-treated infection and toxicity reducing. We will investigate the antibacterial activity of simeprevir (SIM), which treated for genotype 1 and 4 chronic hepatitis C, combined with polymyxins against MRSA through high-throughput screening technology. In our study, the synergistic antibacterial effect of SIM and polymyxins against S. aureus in vitro was found by checkerboard assay and time-growth curve. The cytotoxicity of SIM combined with polymyxin B sulfate [PB(S)] or polymyxin E (PE) in vitro was evaluated using CCK-8, human RBC hemolysis and scratch assays. In addition, we investigated the eradication of biofilm formation of S. aureus by biofilm inhibition assay and the killing of persister cells. Moreover, we evaluated the therapeutic effect and in vivo toxicity of the combination against MRSA in murine subcutaneous abscess model. Furthermore, it was preliminarily found that SIM significantly enhanced the destruction of MRSA membrane by SYTOX Green and DISC3(5) probes. In summary, these results reveal that the therapy of SIM combined with polymyxins (especially PE) is promising for the treatment of MRSA infection.

Progress towards the clinical use of antimicrobial peptides: challenges and opportunities

INTRODUCTION

To overcome the challenge of multidrug resistance, natural and synthetic peptides are candidates to become the basis of innovative therapeutics, featuring diverse mechanisms of action. Traditionally, the time elapsed from medical discoveries to their application is long. The urgency derived from the emergence of antibiotic resistance recommends an acceleration of research to put the new weapons in the hands of clinicians.

AREAS COVERED

This narrative review introduces ideas and suggestions of new strategies that may be used as a basis upon which to recommend reduced development times and to facilitate the arrival of new molecules in the fight against microbes.

EXPERT OPINION

Although studies on new innovative antimicrobial treatments are being conducted, sooner rather than later, more clinical trials, preclinical and translational research are needed to promote the development of innovative antimicrobial treatments for multidrug resistant infections. The situation is worrying, no less than that generated by pandemics such as the ones we have just experienced and conflicts such as world wars. Although from the point of view of human perception, resistance to antibiotics may not seem as serious as these other situations, it is possibly the hidden pandemic that most jeopardizes the future of medicine.

Causes of polymyxin treatment failure and new derivatives to fill the gap

Polymyxins are a class of antibiotics that were discovered in 1947 from programs searching for compounds effective in the treatment of Gram-negative infections. Produced by the Gram-positive bacterium Paenibacillus polymyxa and composed of a cyclic peptide chain with a peptide-fatty acyl tail, polymyxins exert bactericidal effects through membrane disruption. Currently, polymyxin B and colistin (polymyxin E) have been developed for clinical use, where they are reserved as "last-line" therapies for multidrug-resistant (MDR) infections. Unfortunately, the incidences of strains resistant to polymyxins have been increasing globally, and polymyxin heteroresistance has been gaining appreciation as an important clinical challenge. These phenomena, along with bacterial tolerance to this antibiotic class, constitute important contributors to polymyxin treatment failure. Here, we review polymyxins and their mechanism of action, summarize the current understanding of how polymyxin treatment fails, and discuss how the next generation of polymyxins holds promise to invigorate this antibiotic class.

Topical Antibiofilm Agents With Potential Utility in the Treatment of Chronic Rhinosinusitis: A Narrative Review

The role of bacterial biofilms in chronic and recalcitrant diseases is widely appreciated, and the treatment of biofilm infection is an increasingly important area of research. Chronic rhinosinusitis (CRS) is a complex disease associated with sinonasal dysbiosis and the presence of bacterial biofilms. While most biofilm-related diseases are associated with highly persistent but relatively less severe inflammation, the presence of biofilms in CRS is associated with greater severity of inflammation and recalcitrance despite appropriate treatment. Oral antibiotics are commonly used to treat CRS but they are often ineffective, due to poor penetration of the sinonasal mucosa and the inherently antibiotic resistant nature of bacteria in biofilms. Topical non-antibiotic antibiofilm agents may prove more effective, but few such agents are available for sinonasal application. We review compounds with antibiofilm activity that may be useful for treating biofilm-associated CRS, including halogen-based compounds, quaternary ammonium compounds and derivatives, biguanides, antimicrobial peptides, chelating agents and natural products. These include preparations that are currently available and those still in development. For each compound, antibiofilm efficacy, mechanism of action, and toxicity as it relates to sinonasal application are summarised. We highlight the antibiofilm agents that we believe hold the greatest promise for the treatment of biofilm-associated CRS in order to inform future research on the management of this difficult condition.

A novel decoy strategy for polymyxin resistance in Acinetobacter baumannii

Modification of the outer membrane charge by a polymyxin B (PMB)-induced PmrAB two-component system appears to be a dominant phenomenon in PMB-resistant Acinetobacter baumannii. PMB-resistant variants and many clinical isolates also appeared to produce outer membrane vesicles (OMVs). Genomic, transcriptomic, and proteomic analyses revealed that upregulation of the pmr operon and decreased membrane-linkage proteins (OmpA, OmpW, and BamE) are linked to overproduction of OMVs, which also promoted enhanced biofilm formation. The addition of OMVs from PMB-resistant variants into the cultures of PMB-susceptible A. baumannii and the clinical isolates protected these susceptible bacteria from PMB. Taxonomic profiling of in vitro human gut microbiomes under anaerobic conditions demonstrated that OMVs completely protected the microbial community against PMB treatment. A Galleria mellonella-infection model with PMB treatment showed that OMVs increased the mortality rate of larvae by protecting A. baumannii from PMB. Taken together, OMVs released from A. baumannii functioned as decoys against PMB.

Wrapped in a thick, protective outer membrane, Acinetobacter baumannii bacteria can sometimes cause serious infections when they find their way into human lungs and urinary tracts. Antibiotics are increasingly ineffective against this threat, which forces physicians to resort to polymyxin B, an old, positively-charged drug that ‘sticks’ to the negatively-charged proteins and fatty components at the surface of A. baumannii.

Scientists have noticed that when bacteria are exposed to lethal drugs, they often react by releasing vesicles, small ‘sacs’ made of pieces of the outer membranes which can contain DNA or enzymes. How this strategy protects the cells against antibiotics such as polymyxin B remains poorly understood.

To investigate this question, Park et al. examined different strains of A. baumannii, showing that bacteria resistant to polymyxin B had lower levels of outer membrane proteins but would release more vesicles. Adding vesicles from resistant strains to non-resistant A. baumannii cultures helped cells to survive the drugs. In fact, this protective effect extended to other species, shielding whole communities of bacteria against polymyxin B. In vivo, the vesicles protected bacteria in moth larvae infected with A. baumannii, leading to a higher death rate in the animals. Experiments showed that the negatively-charged vesicles worked as decoys, trapping the positively-charged polymyxin B away from its target.

Taken together, the findings by Park et al. highlight a new strategy that allows certain strains of bacteria to protect themselves from antibiotics, while also benefitting the rest of the microbial community.

The antibacterial activity of peptide dendrimers and polymyxin B increases sharply above pH 7.4

pH-activity profiling reveals that antimicrobial peptide dendrimers (AMPDs) kill Klebsiella pneumoniae and Methicillin-resistant Staphylococcus aureus (MRSA) at pH = 8.0, against which they are inactive at pH = 7.4, due to stronger electrostatic binding to bacterial cells at higher pH. A similar effect occurs with polymyxin B and might be general for polycationic antimicrobials.

Mechanisms of bactericidal action and resistance of polymyxins for Gram-positive bacteria

Polymyxins are cationic antimicrobial peptides used as the last-line therapy to treat multidrug-resistant Gram-negative bacterial infections. The bactericidal activity of polymyxins against Gram-negative bacteria relies on the electrostatic interaction between the positively charged polymyxins and the negatively charged lipid A of lipopolysaccharide (LPS). Given that Gram-positive bacteria lack an LPS-containing outer membrane, it is generally acknowledged that polymyxins are less active against Gram-positive bacteria. However, Gram-positive bacteria produce negatively charged teichoic acids, which may act as the target of polymyxins. More and more studies suggest that polymyxins have potential as a treatment for Gram-positive bacterial infection. This mini-review discusses recent advances in the mechanism of the antibacterial activity and resistance of polymyxins in Gram-positive bacteria.Key Points• Teichoic acids play a key role in the action of polymyxins on Gram-positive bacteria.• Polymyxin kills Gram-positive bacteria by disrupting cell surface and oxidative damage.• Modification of teichoic acids and phospholipids contributes to polymyxin resistance in Gram-positive bacteria.• Polymyxins have potential as a treatment for Gram-positive bacterial infection.

When Combined with Colistin, an Otherwise Ineffective Rifampicin-Linezolid Combination Becomes Active in Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii

The synergistic action of colistin, with two antibiotics active in Gram-positive bacteria but unable to kill gram negatives (linezolid and rifampicin), was investigated, since triple combinations are emerging as a tool to overtake multidrug resistance. Checkerboard determinations demonstrated that, when combined with colistin, the combination of linezolid and rifampicin turns active in multidrug-resistant Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii. Thus, the presence of sublethal concentrations of colistin resulted in a strongly synergistic interaction between these two drugs. Moreover, the minimum inhibitory concentrations of linezolid–rifampicin combinations in the presence of colistin were lower than the maximal concentrations of these antimicrobials ain blood. These findings suggest the use of this triple combination as an effective treatment of multidrug-resistant (MDR) bacterial infections.

Efficacy of combinations of colistin with other antimicrobials involves membrane fluidity and efflux machinery

Objective

Despite its use was abandoned several decades ago, the polycationic peptide colistin has become the last hope to treat severe infections caused by multidrug-resistant Gram-negative bacteria. Thus, the development of colistin resistance may seriously compromise the efficacy of treatment. Moreover, colistin has high toxicity being dose dependent. A potentially effective strategy to avoid resistance may be to combine colistin with other antimicrobials. This may help in the rescue of old antimicrobials and in reducing toxic undesired effects.

Methods

Antimicrobial susceptibility determination, efflux machinery function measurements in different conditions and measurement of inhibition of the extrusion by colistin were performed. Moreover, modifications of anisotropy of the membranes by using fluorescent dyes was accomplished.

Results

Sub-inhibitory concentrations of colistin have a synergistic effect with several antimicrobials that act intracellularly (targeting protein synthesis and DNA replication). This effect was demonstrated through the uptake increases of acridine orange. in Pseudomonas aeruginosa, Escherichia coli and Acinetobacter baumanii but also in an intrinsically colistin-resistant species as Serratia marcescens. Measurements of the anisotropy of bacterial membranes, as a measure of membrane fluidity, showed significant changes indicative of colistin activity.

Conclusion

The alterations in the cellular efflux machinery that resulted in higher intracellular concentrations of acridine orange, and likely of other antimicrobials combined with data of membrane fluidity and measured synergism in vitro allow us to envisage the use of these combinations to fight infections caused by multidrug–resistant bacteria.