Antimicrobial sensitisers: Gatekeepers to avoid the development of multidrug-resistant bacteria

The resistance of multidrug-resistant bacteria to existing antibiotics forces the continued development of new antibiotics and antibacterial agents, but the high costs and long timeframe involved in the development of new agents renders the hope that existing antibiotics may again play a part. The "antibiotic adjuvant" is an indirect antibacterial strategy, but its vague concept has, in the past, limited the development speed of related drugs. In this review article, we put forward an accurate concept of a "non-self-antimicrobial sensitisers (NSAS)", to distinguish it from an "antibiotic adjuvant", and then discuss several scientific methods to restore bacterial sensitivity to antibiotics, and the sources and action mechanism of existing NSAS, in order to guide the development and further research of NSAS.

NhaA: A promising adjuvant target for colistin against resistant Escherichia coli

The emergence and widespread of multidrug-resistant Gram-negative bacteria have posed a severe threat to human health and environmental safety, escalating into a global medical crisis. Utilization of antibiotic adjuvants is a rapid approach to combat bacterial resistance effectively since the development of new antimicrobial agents is a formidable challenge. NhaA, driven by proton motive force, is a crucial secondary transporter on the cytoplasmic membrane of Escherichia coli. We found that 2-Aminoperimidine (2-AP), which is a specific inhibitor of NhaA, could enhance the activity of colistin against sensitive E. coli and reverse the resistance in mcr-1 positive E. coli. Mechanistic studies indicated that 2-AP induced dysfunction in cytoplasmic membrane through the suppression of NhaA, leading to metabolic inhibition and ultimately enhancing the sensitivity of E. coli to colistin. Moreover, 2-AP restored the efficacy of colistin against resistant E. coli in two animal infection models. Our findings reveal the potential of NhaA as a novel target for colistin adjuvants, providing new possibilities for the clinical application of colistin.

Gramine sensitizes Klebsiella pneumoniae to tigecycline killing

BACKGROUND

The presence of plasmid-mediated resistance-nodulation-division (RND) efflux pump gene cluster tmexCD1-toprJ1 and its related variants has been associated with heightened resistance to tigecycline, thus diminishing its effectiveness. In this study, we explored the potential of gramine, a naturally occurring indole alkaloid, as an innovative adjuvant to enhance the treatment of infections caused by K. pneumoniae carrying tmexCD-toprJ-like gene clusters.

METHODS

The synergistic potential of gramine in combination with antibiotics against both planktonic and drug-tolerant multidrug-resistant Enterobacterales was evaluated using the checkerboard microbroth dilution technique and time-killing curve analyses. Afterwards, the proton motive force (PMF) of cell membrane, the function of efflux pump and the activity of antioxidant system were determined by fluorescence assay and RT-PCR. The intracellular accumulation of tigecycline was evaluated by HPLC-MS/MS. The respiration rate, bacterial ATP level and the NAD+/NADH ratio were investigated to reveal the metabolism state. Finally, the safety of gramine was assessed through hemolytic activity and cytotoxicity assays. Two animal infection models were used to evaluate the in vivo synergistic effect.

RESULTS

Gramine significantly potentiated tigecycline and ciprofloxacin activity against tmexCD1-toprJ1 and its variants-positive pathogens. Importantly, the synergistic activity was also observed against bacteria in special physiological states such as biofilms and persister cells. The mechanism study showed that gramine possesses the capability to augment tigecycline accumulation within cells by disrupting the proton motive force (PMF) and inhibiting the efflux pump functionality. In addition, the bacterial respiration rate, intracellular ATP level and tricarboxylic acid cycle (TCA) were promoted under the treatment of gramine. Notably, gramine effectively restored tigecycline activity in multiple animal infection models infected by tmexCD1-toprJ1 positive K. pneumoniae (RGF105-1).

CONCLUSION

This study provides the first evidence of gramine's therapeutic potential as a novel tigecycline adjuvant for treating infections caused by K. pneumoniae carrying tmexCD-toprJ-like gene clusters.

Discovery of adjuvants with antibacterial potentiation activity against carbapenemase-producing Enterobacterales based on in silico virtual screening

Bacterial multi-drug resistance has become a concern worldwide, especially after the emergence of carbapenemases. Adjuvants with antibacterial potentiation activity can resensitise drug-resistant strains to carbapenems. However, only a few adjuvants with antibacterial potentiation activity are currently available in clinical practice. Here, we first docked the library containing more than 30,000 small molecules to carbapenemases including Klebsiella pneumoniae carbapenemase 2 (KPC-2) and New Delhi metallo-β-lactamase-5 (NDM-5), through in silico virtual screening to obtain lead compounds against carbapenemase-producing Enterobacterales. Meanwhile, the in vitro antibacterial potentiation assays revealed that ibandronate, azacytidine, ribostamycin sulfate and cidofovir exhibited synergistic or additive activity in the presence of meropenem, with good biocompatibility based on red blood cell hemolysis and cell viability tests. Furthermore, the combination of meropenem and azacytidine showed high efficacy in a mouse sepsis model infected with an NDM-5-producing clinical strain, with a 100% survival rate, decreased bacterial burden and alleviated pathological deterioration. These results suggest that the virtual screening is a promising strategy to identify new antibiotic adjuvants targeting carbapenemase-producing Enterobacterales.

The enhancement effect of small molecule Lyb24 reveals AzoR as a novel target of polymyxin B

Given the important role of polymyxin B (PB) in the treatment of drug-resistant Gram-negative bacterial infections, the emergence of PB resistance poses a serious threat to public health. Adjuvant development is a supplementary strategy that can compensate for the lack of novel antibiotics by protecting PB. In this study, we found a small molecule named Lyb24 that showed weak antibacterial activity (minimum inhibitory concentration ≥ 10 μg/ml) but potentiated and revitalized the efficacy of PB against Gram-negative pathogens, including mcr-1- and mgrB-deletion-mediated PB-resistant strains. Our results showed that Lyb24 inhibits the translational levels of genes associated with the modification of lipid A. In addition, Lyb24 increases the permeability, disrupts the integrity and induces the depolarization of the membrane. We further found that both Lyb24 and PB could directly bind to AzoR and inhibit its activity. Structural analysis showed that Lyb24 binds to the isoalloxazine ring of flavin mononucleotide (FMN) through pi-pi stacking and loop η4 of AzoR. A pneumonia model was used to confirm that the activity against clinical PB-resistant Klebsiella pneumoniae was enhanced due to Lyb24 on PB. In conclusion, we provide a potential therapeutic regimen by combining Lyb24 and PB to treat Gram-negative-resistant bacterial infections. Our findings not only explain the synergistic effect of Lyb24, but also expand our knowledge on the mechanism of action of PB.

Metal-ruthenium complex based on dipyridylamine group as membrane-active antibacterial agent effectively decrease the development of drug-resistance on Staphylococcus aureus

Staphylococcus aureus (S. aureus), one of the Gram-positive bacteria, is easily to develop drug-resistance. Drug-resistant S. aureus infection leads to high morbidity and mortality. The complexes, namely [Ru(dpa)2(PSPIP)](PF6)2 (Ru1), [Ru(dpa)2(TSPIP)](PF6)2 (Ru2), and [Ru(dpa)2(TBPIP)](PF6)2 (Ru3), were synthesized using 2, 2'-dipyridylamine as an auxiliary ligand and three main ligands PSPIP, TSPIP, TBPIP. In vitro studies demonstrated that the Ru1-3 exhibited excellent antibacterial activity against S. aureus while showing low hemolytic toxicity to rabbit red blood cells. Notably, Ru3 was found to disrupt the bacterial cell membrane and alter its permeability through fluorescence staining and scanning electron microscopy (SEM) analysis. Furthermore, Ru3 displayed low toxicity in G. mellonella Larvae. Ru3 exhibited good activity against S. aureus in G. mellonella Larvae infection model and mouse skin infection model.To some extent, Ru3 inhibited biofilm formation on S. aureus as well as hemolytic toxin production, thereby attenuating the development of drug resistance without cross-resistance with other antibiotics. In addition, complex Ru3 exhibited a synergistic effect when combined with antibiotics amikacin, kanamycin, tobramycin and chloramphenicol, making it a valuable antibiotics adjuvant.

Synthesis of peptidomimetics as antibiotic adjuvants for combination with aztreonam to combat MDR Pseudomonas aeruginosa

Pseudomonas aeruginosa is one of the multipledrug-resistant (MDR) Gram-negative pathogens with few drugs available for treatment. Antibiotic adjuvant approach provides an alternative and complementary strategy. In this study, the stereo-structure-activity relationship of monobactams against MDR Gram-negative organisms was extended. Meanwhile, a series of novel peptidemimetic derivatives as antibiotic adjuvants was synthesized and evaluated for their synergistic effects with aztreonam (AZT) against P. aeruginosa, using dipeptide PAβN as the lead. Among the analogues, compound 22j showed a significant synergistic effect against MDR P. aeruginosa in vitro and in vivo, presumably through the mechanism of affecting the permeability of outer membrane. Thus, we identified 22j as a novel peptidemimetic lead compound to potentiate the activity of AZT against MDR P. aeruginosa, which is worthy of further development as antibiotic adjuvant candidates.

Making a chink in their armor: Current and next-generation antimicrobial strategies against the bacterial cell envelope

Gram-negative bacteria are uniquely equipped to defeat antibiotics. Their outermost layer, the cell envelope, is a natural permeability barrier that contains an array of resistance proteins capable of neutralizing most existing antimicrobials. As a result, its presence creates a major obstacle for the treatment of resistant infections and for the development of new antibiotics. Despite this seemingly impenetrable armor, in-depth understanding of the cell envelope, including structural, functional and systems biology insights, has promoted efforts to target it that can ultimately lead to the generation of new antibacterial therapies. In this article, we broadly overview the biology of the cell envelope and highlight attempts and successes in generating inhibitors that impair its function or biogenesis. We argue that the very structure that has hampered antibiotic discovery for decades has untapped potential for the design of novel next-generation therapeutics against bacterial pathogens.

Gigantol restores the sensitivity of mcr carrying multidrug-resistant bacteria to colistin

BACKGROUND

The emergence and wide spread of plasmid-mediated colistin resistance gene (mcr-1) and its mutants have immensely limited the efficacy of colistin in treating multidrug-resistant (MDR) Gram-negative bacterial infections. The development of synergistic combinations of antibiotics with a natural product that coped with the resistance of MDR bacteria was an economic strategy to restore antibiotics activity. Herein, we investigated gigantol, a bibenzyl phytocompound, for restoring in vitro and in vivo, the sensitivity of mcr-positive bacteria to colistin.

METHODS

The synergistic activity of gigantol and colistin against multidrug-resistant Enterobacterales was studied via checkerboard assay and time-killing curve. Subsequently, the transcription and protein expression levels of mcr-1 gene were determined by RT-PCR and Western blots. The interaction of gigantol and MCR-1 was simulated via molecular docking and verified via site-directed mutagenesis of MCR-1. Hemolytic activity and cytotoxicity assay were used to evaluate the safety of gigantol. Finally, the in vivo synergistic effect was evaluated via two animal infection models.

RESULTS

Gigantol restored the activity of colistin against mcr-positive bacteria E.coli B2 (MIC from 4 μg/ml to 0.25 μg/ml), Salmonella 15E343 (MIC from 8 μg/ml to 1 μg/ml), K. pneumoniae 19-2-1 (MIC from 32 μg/ml to 2 μg/ml) carrying mcr-1, mcr-3, mcr-8, respectively. Mechanistic studies revealed that gigantol down-regulated the expression of genes involved in LPS-modification, reduced the MCR-1 products and inhibited the activity of MCR-1 by binding to amino acid residues Tyr287 and Pro481 in its D-glucose-binding pocket. Safety evaluation showed that the addition of gigantol relieves the hemolysis caused by colistin. Compared with monotherapy, the combination of gigantol and colistin significantly improved the survival rate of Gallgallella mellonella larvae and mice infected by E.coli B2. Moreover, there was a considerable decrease in the bacterial load present in the viscera of mice.

CONCLUSION

Our results confirmed that gigantol was a potential colistin adjuvant, and could be used to tackle multi-drug resistant Gram-negative pathogen infections combined with colistin.

Valorisation of the diterpene podocarpic acid - Antibiotic and antibiotic enhancing activities of polyamine conjugates

As part of our search for new antimicrobials and antibiotic adjuvants, a series of podocarpic acid-polyamine conjugates have been synthesized. The library of compounds made use of the phenolic and carboxylic acid moieties of the diterpene allowing attachment of polyamines (PA) of different lengths to afford a structurally-diverse set of analogues. Evaluation of the conjugates for intrinsic antimicrobial properties identified two derivatives of interest: a PA3-4-3 (spermine) amide-bonded variant 7a that was a non-cytotoxic, non-hemolytic potent growth inhibitor of Gram-positive Staphylococcus aureus (MRSA) and 9d, a PA3-8-3 carbamate derivative that was a non-toxic selective antifungal towards Cryptococcus neoformans. Of the compound set, only one example exhibited activity towards Gram-negative bacteria. However, in the presence of sub-therapeutic amounts of either doxycycline (4.5 µM) or erythromycin (2.7 μM) several analogues were observed to exhibit weak to modest antibiotic adjuvant properties against Pseudomonas aeruginosa and/or Escherichia coli. The observation of strong cytotoxicity and/or hemolytic properties for subsets of the library, in particular those analogues bearing methyl ester or n-pentylamide functionality, highlighted the fine balance of structural requirements and lipophilicity for antimicrobial activity as opposed to mammalian cell toxicity.

Novel functionalized selenium nanowires as antibiotic adjuvants in multiple ways to overcome drug resistance of multidrug-resistant bacteria

Methicillin-resistant Staphylococcus (MRS) is a multi-drug resistant bacteria that pose a serious threat to human health. Antibacterial nanomaterials are becoming a promising antibiotic substitute or antibiotic adjuvants. In this work, selenium nanowires were modified with nano‑silver (Ag NPs) with antibacterial activity and [Ru(bpy)₂dppz]²⁺ with fluorescent labeling of DNA (SRA), and the antibacterial activity, antibacterial mechanism and biological toxicity of SRA synergistic antibiotics were studied. In vitro, antibacterial results show that SRA (12 μg/mL) improves the antibacterial activity of various antibiotics against resistant bacteria and significantly slows the development of bacterial resistance to antibiotics. Studies on antibacterial mechanisms have shown that SRA synergistic antibiotics destroy drug-resistant bacteria through a combination of physical (physical damage) and chemical pathways (destruction of biofilm, membrane depolarization, cell membrane destruction, adenosine triphosphate consumption and reactive oxygen species production). Transcriptomics analysis found that SRA affects bacterial activity by affecting bacterial biosynthesis, ATP synthesis and biofilm formation. Furthermore, SRA synergistic antibiotics can accelerate wound healing of bacterial infection by reducing the inflammatory response. The toxicity evaluation results show that SRA has extremely low cellular and in vivo toxicity. SRA has the potential of clinical application as multiple antibiotic adjuvants to deal with resistant bacterial infections.

Synergistic antibacterial activity of tetrandrine combined with colistin against MCR-mediated colistin-resistant Salmonella

It has been recognized that colistin resistance is a growing problem that seriously impairs the clinical efficacy of colistin against bacterial infections. One strategy that has been proven to have therapeutic effect is to overcome the widespread emergence of antibiotic-resistant pathogens by combining existing antibiotics with promising non-antibiotic agents. In this work, antibiotic susceptibility testing, checkerboard assays and time-kill curves were used to investigate the antibacterial activity of the individual drugs and the potential synergistic activity of the combination. The molecular mechanisms of tetrandrine in combination with colistin were analyzed using fluorometric assay and Real-time PCR. To predict possible interactions between tetrandrine and MCR-1, molecular docking assay was taken. Finally, we evaluated the in vivo efficacy of tetrandrine in combination with colistin against MCR-positive Salmonella. Overall, the combination of tetrandrine and colistin showed significant synergistic activity. In-depth mechanistic analysis showed that the combination of tetrandrine with colistin enhances the membrane-damaging ability of colistin, undermines the functions of proton motive force (PMF) and efflux pumps in MCR-positive bacteria. The results of molecular docking and RT-PCR analyses showed that tetrandrine not only affects the expression of mcr-1 but is also an effective MCR-1 inhibitor. Compared with colistin monotherapy, the combination of tetrandrine with colistin significantly reduced the bacterial load in vivo. Our findings demonstrated that tetrandrine serves as a potential colistin adjuvant against MCR-positive Salmonella.

Phenotypic screening of compound libraries as a platform for the identification of antibiotic adjuvants: Identification of colistin adjuvants from a natural product library

The identification of antibiotic adjuvants, small molecules that potentiate the activity of conventional antibiotics, provides an orthogonal approach to the development of new antibiotics in the fight against drug resistant bacterial infections. Methods to identify novel adjuvants could potentially aid efforts to overcome the increasing prevalence of resistance and stave off the onset of a "post-antibiotic era." Phenotypic whole cell screens allow for the identification of hits with the necessary properties to access their biomolecular target, and may also facilitate the discovery of novel adjuvant targets. A phenotypic screening platform is outlined, in which a natural product library was explored for activity with antibiotics from several mechanistically distinct classes against clinically important bacterial species. General approaches to delineating the mechanism of action of hit compounds identified from phenotypic screens are described, followed by specific approaches to uncovering the mechanism of action of the colistin adjuvants identified from the natural product screen.

Repurposing primaquine as a polyamine conjugate to become an antibiotic adjuvant

In our search for new antibiotic adjuvants as a novel strategy to deal with the emergence of multi-drug resistant (MDR) bacteria, a series of succinylprimaquine-polyamine (SPQ-PA) conjugates and derivatives of a cationic amphiphilic nature have been prepared. Evaluation of these primaquine conjugates for intrinsic antimicrobial properties and the ability to restore the antibiotic activity of doxycycline identified two derivatives, SPQ-PA3-8-3 and SPQ-PA3-10-3 that exhibited intrinsic activity against the Gram-positive bacteria Staphylococcus aureus and the yeast Cryptococcus neoformans. None of the analogues were active against the Gram-negative bacterium Pseudomonas aeruginosa. However, in the presence of a sub-therapeutic amount of doxycycline (4.5 µM), both SPQ-PA3-4-3 and SPQ-PA3-10-3 compounds displayed potent antibiotic adjuvant properties against P. aeruginosa, with MIC's of 6.25 µM. A series of derivatives were prepared to investigate the structure-activity relationship that explored the influence of both a simplified aryl lipophilic substituent and variation of the length of the polyamine scaffold on observed intrinsic antimicrobial properties and the ability to potentiate the action of doxycycline against P. aeruginosa.

Feasibility of an inhaled antibiotic/adjuvant dry powder combination using an experimental design approach

The global increase of multidrug resistant bacteria and the lack of new classes of antibiotic especially those targeting Gram-negative pathogens are leaving the clinicians disarmed to treat numerous bacterial infections. Recently, the design of adjuvants able to enhance antibiotics activities appears to be one of the most promising investigated solutions to circumvent this problem. In this context, we have recently identified a new polyamino-isoprenyl derivative NV716 able to potentiate, at a very low concentration the activity of doxycycline against resistant P. aeruginosa bacterial strains by increasing its intracellular concentration. In this study we will report an experimental protocol to optimize a dry powder for inhalation ensuring the simultaneous delivery of an antibiotic (doxycycline) and an adjuvant (the polyaminoisoprenyl derivative NV716 since aerosol therapy could allow a rapid drug administration and target the respiratory system by avoiding the first pass effect and minimizing undesirable systemic effects. Thus, an experimental design was carried out permitting to identify the influence of several factors on the aerosolization efficiency of our combination and allowing us to find the right composition and manufacture leading to the best optimization of the simultaneous delivery of the two compounds in the form of an inhalable powder. More precisely, the powders of the two active ingredients were prepared by freeze drying and their aerosolization was improved by the addition of carrier particles of lactose inhalation grade. Under these conditions, the best formulation was defined by combining the optimal factors leading to the best aerodynamic properties' values (the lowest MMAD (Mass Median Aerodynamic Diameter) and the highest FPF (Fraction of Fine Particles)) without even using sophisticated engineering techniques. Finally, our results suggest that these molecules could be successfully delivered at the requested concentration in the lungs and then able to decrease drug consumption as well as increase treatment efficacy.

Relationship between Adaptive Changing of Lysophosphatidylethanolamine Content in the Bacterial Envelope and Ampicillin Sensitivity of Yersinia pseudotuberculosis

The low permeability of porin channels is the possible reason for Gram-negative bacterial resistance to antibiotics. The adaptive accumulation of lysophosphatidylethanolamine (LPE) in Yersinia pseudotuberculosis induces conformational changes of OmpF porin that may hinder the transport of antibiotics through this channel. The present study was aimed to test whether the changes in LPE content affect the resistance of bacteria to ampicillin. The addition of glucose to the culture medium was shown to simultaneously increase the level of LPE and minimum inhibitory concentration (MIC) for ampicillin of Y. pseudotuberculosis cells 6- and 2-fold, respectively. However, the coadministration of glucose and polyphenol extract from buckwheat husks reduced the content of LPE 2-fold and restored MIC to the control value. Thus, PBEH can be used as antibiotic adjuvant to improve an antibiotic's ability to cross the outer membrane. The present work demonstrated: (i) the role of adaptive changes in the lipid composition of Y. pseudotuberculosis in the development of antibiotic resistance, and (ii) the promising use of PBEH in combination therapy to increase the susceptibility of Gram-negative bacteria to the conventional β-lactam antibiotics, probably attenuating in vivo a previously demonstrated effect of LPE on the conformation and function of the OmpF channel.

Second Generation Modifiers of Colistin Resistance Show Enhanced Activity and Lower Inherent Toxicity

We recently reported a 2-aminoimidazole-based antibiotic adjuvant that reverses colistin resistance in two species of Gram-negative bacteria. Mechanistic studies in Acinetobacter baumannii demonstrated that this compound downregulated the PmrAB two-component system and abolished a lipid A modification that is required for colistin resistance. We now report the synthesis and evaluation of two separate libraries of substituted 2-aminoimidazole analogues based on this parent compound. From these libraries, a new small molecule was identified that lowers the minimum inhibitory concentration of colistin by up to 32-fold greater than the parent compound while also displaying less inherent bacterial effect, thereby minimizing the likelihood of resistance evolution.