Polymyxin B-inspired non-hemolytic tyrocidine A analogues with significantly enhanced activity against gram-negative bacteria: How cationicity impacts cell specificity and antibacterial mechanism

Exploration of membrane-active cephalosporin derivatives as potent antibacterial agents against Staphylococcus aureus biofilms and persisters

Developing innovative antimicrobial agents is essential in the fight against drug-resistant bacteria, as well as biofilms and persistent bacteria. In this study, four series of amphiphilic cephalosporin derivatives were synthesized. Most of the compounds showed good activity against Gram-positive bacteria, among which membrane-active cephalosporin 15e showed high activity against Staphylococcus aureus. Furthermore, 15e can maintain antimicrobial activity in mammalian body fluids and does not develop detectable resistance. Antibacterial mechanism studies demonstrated that the compound 15e can destroy the bacterial cell membrane, causing leakage of intracellular nucleic acids and proteins. Moreover, it can also suppress bacterial metabolic activity and induce the accumulation of reactive oxygen species (ROS) in the bacteria. Of greater significance, compound 15e effectively prevented the formation of biofilms and eradicated established biofilms and persister cells. Notably, compound 15e exhibited potent in vivo antibacterial efficacy, which was better than cephalothin. These findings suggest that 15e has a potential to become a drug candidate for treating bacterial infections.

Nitazoxanide synergizes polymyxin B against Escherichia coli by depleting cellular energy

The rapid expansion of antibiotic-resistant bacterial diseases is a global burden on public health. It makes sense to repurpose and reposition already-approved medications for use as supplementary agents in synergistic combinations with existing antibiotics. Here, we demonstrate that the anthelmintic drug nitazoxanide (NTZ) synergistically enhances the effectiveness of the lipopeptide antibiotic polymyxin B in inhibiting gram-negative bacteria, including those resistant to polymyxin B. Mechanistic investigations revealed that nitazoxanide inhibited calcium influx and cell membrane depolarization, enhanced the affinity between polymyxin B and the extracellular membrane, and promoted intracellular ATP depletion and an increase in reactive oxygen species (ROS), thus enhancing the penetration and disruption of the Escherichia coli cell membrane by polymyxin B. The transcriptomic analysis revealed that the combination resulted in energy depletion by inhibiting both aerobic and anaerobic respiration patterns in bacterial cells. The increased bactericidal effect of polymyxin B on the E. coli ∆nuoC strain further indicates that NuoC could be a promising target for nitazoxanide. Furthermore, the combination of nitazoxanide and polymyxin B showed promising therapeutic effects in a mouse infection model infected with E. coli. Taken together, these results demonstrate the potential of nitazoxanide as a novel adjuvant to polymyxin B, to overcome antibiotic resistance and improve therapeutic outcomes in refractory infections.IMPORTANCEThe rapid spread of antibiotic-resistant bacteria poses a serious threat to public health. The search for potential compounds that can increase the antibacterial activity of existing antibiotics is a promising strategy for addressing this issue. Here, the synergistic activity of the FDA-approved agent nitazoxanide (NTZ) combined with polymyxin B was investigated in vitro using checkerboard assays and time-kill curves. The synergistic mechanisms of the combination of nitazoxanide and polymyxin B were explored by fluorescent dye, transmission electron microscopy (TEM), and transcriptomic analysis. The synergistic efficacy was evaluated in vivo by the Escherichia coli and mouse sepsis models. These results suggested that nitazoxanide, as a promising antibiotic adjuvant, can effectively enhance polymyxin B activity, providing a potential strategy for treating multidrug-resistant bacteria.

Design, synthesis and antibacterial evaluation of low toxicity amphiphilic-cephalosporin derivatives

Global public health is facing a serious problem as a result of the rise in antibiotic resistance and the decline in the discovery of new antibiotics. In this study, two series of amphiphilic-cephalosporins were designed and synthesized, several of which showed good antibacterial activity against both Gram-positive and Gram-negative bacteria. Structure-activity relationships indicated that the length of the hydrophobic alkyl chain significantly affects the antibacterial activity against Gram-negative bacteria. The best compound 2d showed high activity against drug-susceptible Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) with MICs of 0.5 and 2-4 μg/mL, respectively. Furthermore, 2d remained active in complex mammalian body fluids and had a longer post-antibiotic effect (PAE) than vancomycin. Mechanism studies indicated that compound 2d lacks membrane-damaging properties and can target penicillin-binding proteins to disrupt bacterial cell wall structure, inhibit the metabolic activity and induce the accumulation of reactive oxygen species (ROS) in bacteria. Compound 2d showed minimal drug resistance and was nontoxic to HUVEC and HBZY-1 cells with CC50 > 128 μg/mL. These findings suggest that 2d is a promising drug candidate for treating bacterial infections.

Nitroreductase-Responsive Photosensitizers for Selective Imaging and Photo-Inactivation of Intracellular Bacteria

Intracellular Staphylococcus aureus (S. aureus), especially the methicillin resistant staphylococcus aureus (MRSA), are difficult to detect and eradicate due to the protection by the host cells. Antibacterial photodynamic therapy (aPDT) offers promise in treating intracellular bacteria, provided that selective damage to the bacteria ranther than host cells can be realized. According to the different nitroreductase (NTR) levels in mammalian cells and S. aureus, herein NTR-responsive photosensitizers (PSs) (T)CyI-NO2 were designed and synthesized. The emission and 1O2 generation of (T)CyI-NO2 are quenched by the 4-nitrobenzyl group, but can be specifically switched on by bacterial NTR. Therefore, selective imaging and photo-inactivation of intracellular S. aureus and MRSA were achieved. Our findings may pave the way for the development of more efficient and selective aPDT agents to combat intractable intracellular infections.

Synergistic Photodynamic/Antibiotic Therapy with Photosensitive MOF-Based Nanoparticles to Eradicate Bacterial Biofilms

Bacterial biofilms pose a serious threat to human health, as they prevent the penetration of antimicrobial agents. Developing nanocarriers that can simultaneously permeate biofilms and deliver antibacterial agents is an attractive means of treating bacterial biofilm infections. Herein, photosensitive metal-organic framework (MOF) nanoparticles were developed to promote the penetration of antibiotics into biofilms, thereby achieving the goal of eradicating bacterial biofilms through synergistic photodynamic and antibiotic therapy. First, a ligand containing benzoselenadiazole was synthesized and incorporated into MOF skeletons to construct benzoselenadiazole-doped MOFs (Se-MOFs). The growth of the Se-MOFs could be regulated to obtain nanoparticles (Se-NPs) in the presence of benzoic acid. The singlet oxygen (¹O₂) generation efficiencies of the Se-MOFs and Se-NPs were evaluated. The results show that the Se-NPs exhibited a higher ¹O₂ generation efficacy than the Se-MOF under visible-light irradiation because the small size of the Se-NPs was conducive to the diffusion of ¹O₂. Afterward, an antibiotic drug, polymyxin B (PMB), was conjugated onto the surface of the Se-NPs via amidation to yield PMB-modified Se-NPs (PMB-Se-NPs). PMB-Se-NPs exhibit a synergistic antibacterial effect by specifically targeting the lipopolysaccharides present in the outer membranes of Gram-negative bacteria through surface-modified PMB. Benefiting from the synergistic therapeutic effects of antibiotic and photodynamic therapy, PMB-Se-NPs can efficiently eradicate bacterial biofilms at relatively low antibiotic doses and light intensities, providing a promising nanocomposite for combating biofilm infections.

Development of biaromatic core-linked antimicrobial peptide mimics: Substituent position significantly affects antibacterial activity and hemolytic toxicity

The development of bacterial resistance to the majority of clinically significant antimicrobials has made it more difficult to treat bacterial infections with conventional antibiotics. As part of ongoing research on antimicrobial peptide mimetics, a series of quaternary ammonium cationic compounds with various linkers were designed and synthesized, with some demonstrating high antibacterial activity against Gram-negative and Gram-positive bacteria. The structure-activity relationship study revealed that the spatial position of substituents had a significant impact on antibacterial activity and hemolytic toxicity. The best compound, 3e, has good antibacterial activity against Staphylococcus aureus [minimum inhibitory concentration (MIC = 1 μg/mL)] and the least hemolytic toxicity [hemolytic concentration (HC₅₀ = 905 μg/mL)], is stable in mammalian body fluids, and rarely induces bacterial resistance. The mechanism study revealed that the membrane action mode may be its potential bactericidal mechanism, and it can effectively cause the accumulation of intracellular reactive oxygen species (ROS) for killing bacteria. Importantly, 3e can effectively reduce the load of methicillin-resistant Staphylococcus aureus (MRSA) in mouse skin and has a higher in vivo bactericidal efficiency than vancomycin. These findings highlight the significance of divergent linkers in quaternary ammonium cations as antimicrobial peptide mimics and the potential of these cations to treat bacterial infections.

Surface configuration of microarc oxidized Ti with regionally loaded chitosan hydrogel containing ciprofloxacin for improving biological performance

The bacterial colonization and poor osseointegration of Ti implants significantly compromise their applications in load-bearing bone repair and replacement. To endorse the Ti with both excellent bioactivity and antibacterial ability, we developed a microarc oxidation coating that was modified uniformly by hydroxyapatite (HA) nanodots arrays and loaded regionally with chitosan hydrogel containing ciprofloxacin. The bonding between the HA nanodots covered coating and the chitosan hydrogel is further enhanced via silanization and chemical grafting of glutaraldehyde. Benefiting from the regionally loaded structure of the chitosan hydrogel, the chitosan hydrogel unloaded area can promote the cell adhesion and proliferation with excellent bioactivity, though relatively low OD value of cck8 has been observed at the beginning of the cell culturing. Whereas, the OD value of cck8 rises with the prolongation of the cell culturing time due to the degradation of the regionally loaded chitosan hydrogel. With the help of the laden ciprofloxacin in chitosan hydrogels, the sample effectively sterilizes the bacterial with a bacteriostatic ring. Therefore, regional loading of chitosan hydrogel containing ciprofloxacin on the modified microarc oxidation coating is a good approach to endorse Ti with both excellent bioactivity and antibacterial ability.

Lysine-Tethered Stable Bicyclic Cationic Antimicrobial Peptide Combats Bacterial Infection in Vivo

Antimicrobial peptides (AMPs) have attracted great attention as next generation antibiotics for the treatment of multidrug-resistant (MDR) bacterial infections. Poor proteolytic stability has however undermined clinical applications of AMPs. A novel peptide cyclization approach is described to enhance the in vivo antibacterial activity of AMPs. Bicyclic antimicrobial peptides were synthesized by cross-linking the ε-amino groups of three lysine residues with a 1,3,5-trimethylene benzene spacer. In a proof of principal study, four bicyclic peptides were synthesized from the cationic AMP OH-CM6. One bicyclic peptide retained strong antimicrobial activity and low toxicity but exhibited a prolonged half-life in serum. Antibacterial activity was consequently improved in vivo without renal or hepato-toxicity. The novel peptide cyclization approach represents an important tool for enhancing AMP proteolytic stability for improved treatment of bacterial infection.

Polarity-Sensitive Fluorescent Probe for Reflecting the Packing Degree of Bacterial Membrane Lipids

The maintenance of an intact membrane structure is of great importance for bacteria to execute various biological functions. However, chemical probes for monitoring the dynamic changes of bacterial membranes are barely reported. Herein, we, for the first time, report a novel polarity-sensitive probe for reflecting the packing degree of bacterial membrane lipids. Specifically, we synthesize a membrane-targeting fluorescent probe (TICT-lipid) that possesses both twist intramolecular charge transfer and aggregation-induced emission properties. TICT-lipid exhibits sensitive responses to the minute difference in the packing degree of membrane lipids, facilitating rapid differentiation of Gram-negative and Gram-positive bacteria. Interestingly, in the presence of membrane-disrupting antibiotics, the localization of TICT-lipid shifts from the outer membrane to the cell membrane by outputting blue-shifted and enhanced emission, making the mechanism of action of antibiotics clearly visible. TICT-lipid is a polarity-sensitive fluorescent probe, holding great promise in the study of membrane-related bacterial processes and antibiotic screening.

Current synthetic chemistry towards cyclic antimicrobial peptides

Antimicrobial peptides (AMPs) have great potentials for developing novel antibiotics against multi-drug resistant (MDR) bacteria. However, the clinical application of AMPs is limited due to their poor protease stability and high hemolytic toxicity. Various strategies have been widely explored to improve the pharmacological properties of natural or artificial antimicrobial peptides, including D- or non-natural amino acid residue replacement, backbone modification, cyclization, PEGlytion, and lipidation. Among others, peptide cyclization, which has been widely applied to enhance the biostability and target selectivity of bioactive peptide, is a very appealing and promising strategy for developing novel antibiotics based on AMPs. Herein, we summarize the current strategies for synthesizing cyclic antimicrobial peptides and the resulting influence of peptide cyclization on the biological activities.