Ramoplanin at bactericidal concentrations induces bacterial membrane depolarization in Staphylococcus aureus

Development of a natural product optimization strategy for inhibitors against MraY, a promising antibacterial target

MraY (phospho-N-acetylmuramoyl-pentapeptide-transferase) inhibitory natural products are attractive molecules as candidates for a new class of antibacterial agents to combat antimicrobial-resistant bacteria. Structural optimization of these natural products is required to improve their drug-like properties for therapeutic use. However, chemical modifications of these natural products are painstaking tasks due to complex synthetic processes, which is a bottleneck in advancing natural products to the clinic. Here, we develop a strategy for a comprehensive in situ evaluation of the build-up library, which enables us to streamline the preparation of the analogue library and directly assess its biological activities. We apply this approach to a series of MraY inhibitory natural products. Through construction and evaluation of the 686-compound library, we identify promising analogues that exhibit potent and broad-spectrum antibacterial activity against highly drug-resistant strains in vitro as well as in vivo in an acute thigh infection model. Structures of the MraY-analogue complexes reveal distinct interaction patterns, suggesting that these analogues represent MraY inhibitors with unique binding modes. We further demonstrate the generality of our strategy by applying it to tubulin-binding natural products to modulate their tubulin polymerization activities.

Fingolimod synergizes and reverses K. pneumoniae resistance to colistin

Klebsiella pneumoniae (K. pneumoniae) infection and the rapid spread of multi-drug resistant (MDR) bacteria pose a serious threat to global healthcare. Polymyxin E (colistin), a group of cationic antimicrobial polypeptides, is currently one of the last resort treatment options against carbapenem-resistant Gram-negative pathogens. The effectiveness of colistin has been compromised due to its intensive use. This study found that fingolimod (FLD), a natural product derivative, exhibited a significant synergistic bactericidal effect on K. pneumoniae when combined with colistin, both in vitro and in vivo. The checkerboard method was employed to assess the in vitro synergistic effect of FLD with colistin. FLD enhanced the susceptibility of bacteria to colistin and lowered effectively minimum inhibitory concentrations (MIC) when compared to colistin MIC, and the fractional inhibitory concentrations (FIC) value was less than 0.3. The time-kill curve demonstrated that the combination treatment of FLD and colistin had significant bactericidal efficacy. The in vitro concurrent administration of colistin and FLD resulted in heightening membrane permeability, compromising cell integrity, diminishing membrane fluidity, and perturbing membrane homeostasis. They also induced alterations in membrane potential, levels of reactive oxygen species, and adenosine triphosphate synthesis, ultimately culminating in bacterial death. Moreover, the combination of FLD with colistin significantly influenced fatty acid metabolism. In the mouse infection model, the survival rate of mice injected with K. pneumoniae was significantly improved to 67% and pathological damage was significantly relieved with combination treatment of FLD and colistin when compared with colistin treatment. This study highlights the potential of FLD in combining with colistin for treating infections caused by MDR isolates of K. pneumoniae.

Ramoplanin as a novel therapy for Neisseria gonorrhoeae infection: an in vitro and in vivo study in Galleria mellonella

Neisseria gonorrhoeae is a bacterial pathogen that causes gonorrhoea, a sexually transmitted infection. Increasing antimicrobial resistance in N. gonorrhoeae is providing motivation to develop new treatment options. In this study, we investigated the effectiveness of the antibiotic ramoplanin as a treatment for N. gonorrhoeae infection. We tested the effectiveness of ramoplanin in vitro against 14 World Health Organization (WHO) reference strains of N. gonorrhoeae and found that it was active against all 14 strains tested. Furthermore, in a Galleria mellonella infection model of N. gonorrhoeae WHO P, we demonstrated that ramoplanin was active in vivo without any evidence of toxicity. This suggests that ramoplanin might be a new promising antibiotic treatment for gonorrhoea.

Synthetic ramoplanin analogues are accessible by effective incorporation of arylglycines in solid-phase peptide synthesis

The threat of antimicrobial resistance to antibiotics requires a continual effort to develop alternative treatments. Arylglycines (or phenylglycines) are one of the signature amino acids found in many natural peptide antibiotics, but their propensity for epimerization in solid-phase peptide synthesis (SPPS) has prevented their use in long peptide sequences. We have now identified an optimized protocol that allows the synthesis of challenging non-ribosomal peptides including precursors of the glycopeptide antibiotics and an analogue of feglymycin (1 analogue, 20%). We have exploited this protocol to synthesize analogues of the peptide antibiotic ramoplanin using native chemical ligation/desulfurization (1 analogue, 6.5%) and head-to-tail macrocyclization in excellent yield (6 analogues, 3-9%), with these compounds extensively characterized by NMR (U-shaped structure) and antimicrobial activity assays (two clinical isolates). This method significantly reduces synthesis time (6-9 days) when compared with total syntheses (2-3 months) and enables drug discovery programs to include arylglycines in structure-activity relationship studies and drug development.

Efflux pump inhibitory potential of indole derivatives as an arsenal against norA over-expressing Staphylococcus aureus

NorA, an extensively studied efflux pump in Staphylococcus aureus, has been connected to fluoroquinolone, antiseptic, and disinfection resistance. Several studies have also emphasized how efflux pumps, including NorA, function as the first line of defense of S. aureus against antibiotics. In this study, we have screened some chemically synthesized indole derivatives for their activity as efflux pump inhibitors (EPIs). The derivative SMJ-5 was found to be a potent NorA efflux pump inhibitor among the screened indole derivatives, owing to increased ethidium bromide and norfloxacin accumulation in norA over-expressing S. aureus. The combination of SMJ-5 and ciprofloxacin demonstrated the eradication of S. aureus biofilm and prolonged the post-antibiotic effect more than ciprofloxacin alone. SMJ-5 was able to inhibit staphyloxanthin virulence. In in vitro time-kill trials and in vivo efficacy investigations, the combination enhanced the bactericidal activity of ciprofloxacin against S. aureus. Additionally, reverse transcription PCR results revealed that SMJ-5 also inhibits the NorA efflux pump indirectly at the transcriptional level. IMPORTANCE The NorA efflux pump is the most effective resistance mechanism in S. aureus. The clinical importance of NorA efflux pumps is demonstrated by the expression of pump genes in S. aureus strains in response to fluoroquinolones and biocides. Along with the repercussions of decreased fluoroquinolone sensitivity, increasing expression of efflux pump genes by their substrate necessitates the importance of efflux pump inhibitors. Reserpine and verapamil are clinically used to treat ailments and have proven NorA inhibitors, but, unfortunately, the concentration needed for these drugs to inhibit the pump is not safe in clinical settings. In the current study, we have screened some indole derivatives, and among them, SMJ-5 was reported to potentiate norfloxacin and ciprofloxacin at their sub-inhibitory concentration by inhibiting the norA gene transcriptionally. Here we highlight the promising points of this study, which could serve as a model to design a therapeutic EPI candidate against norA over-expressing S. aureus.

Antimicrobial Peptides and Small Molecules Targeting the Cell Membrane of Staphylococcus aureus

Clinical management of Staphylococcus aureus infections presents a challenge due to the high incidence, considerable virulence, and emergence of drug resistance mechanisms. The treatment of drug-resistant strains, such as methicillin-resistant S. aureus (MRSA), is further complicated by the development of tolerance and persistence to antimicrobial agents in clinical use. To address these challenges, membrane disruptors, that are not generally considered during drug discovery for agents against S. aureus, should be explored. The cell membrane protects S. aureus from external stresses and antimicrobial agents, but membrane-targeting antimicrobial agents are probably less likely to promote bacterial resistance. Nontypical linear cationic antimicrobial peptides (AMPs), highly modified AMPs such as daptomycin (lipopeptide), bacitracin (cyclic peptide), and gramicidin S (cyclic peptide), are currently in clinical use. Recent studies have demonstrated that AMPs and small molecules can penetrate the cell membrane of S. aureus, inhibit phospholipid biosynthesis, or block the passage of solutes between the periplasm and the exterior of the cell. In addition to their primary mechanism of action (MOA) that targets the bacterial membrane, AMPs and small molecules may also impact bacteria through secondary mechanisms such as targeting the biofilm, and downregulating virulence genes of S. aureus. In this review, we discuss the current state of research into cell membrane-targeting AMPs and small molecules and their potential mechanisms of action against drug-resistant physiological forms of S. aureus, including persister cells and biofilms.

Drug Repositioning as a Therapeutic Strategy against Streptococcus pneumoniae: Cell Membrane as Potential Target

A collection of repurposing drugs (Prestwick Chemical Library) containing 1200 compounds was screened to investigate the drugs' antimicrobial effects against planktonic cultures of the respiratory pathogen Streptococcus pneumoniae. After four discrimination rounds, a set of seven compounds was finally selected, namely (i) clofilium tosylate; (ii) vanoxerine; (iii) mitoxantrone dihydrochloride; (iv) amiodarone hydrochloride; (v) tamoxifen citrate; (vi) terfenadine; and (vii) clomiphene citrate (Z, E). These molecules arrested pneumococcal growth in a liquid medium and induced a decrease in bacterial viability between 90.0% and 99.9% at 25 µM concentration, with minimal inhibitory concentrations (MICs) also in the micromolar range. Moreover, all compounds but mitoxantrone caused a remarkable increase in the permeability of the bacterial membrane and share a common, minimal chemical structure consisting of an aliphatic amine linked to a phenyl moiety via a short carbon/oxygen linker. These results open new possibilities to tackle pneumococcal disease through drug repositioning and provide clues for the design of novel membrane-targeted antimicrobials with a related chemical structure.

Development of lipidated polycarbonates with broad-spectrum antimicrobial activity

Antimicrobial resistance is a global challenge owing to the lack of discovering effective antibiotic agents. Antimicrobial polymers containing the cationic groups and hydrophobic groups which mimic natural host-defense peptides (HDPs) show great promise in combating bacteria. Herein, we report the synthesis of lipidated polycarbonates bearing primary amino groups and hydrophobic moieties (including both the terminal long alkyl chain and hydrophobic groups in the sequences) by ring-opening polymerization. The hydrophobic/hydrophilic group ratios were adjusted deliberately and the lengths of the alkyl chains at the end of the polymers were modified to achieve the optimized combination for the lead polymers, which exhibited potent and broad-spectrum bactericidal activity against a panel of Gram-positive and Gram-negative bacteria. The polymers only showed very limited hemolytic activity, demonstrating their excellent selectivity. Comprehensive analyses using biochemical and biophysical assays revealed the strong interaction between the polymers and bacteria membranes. Moreover, the polymers also showed strong biofilm inhibition activity and did not readily induce antibiotic resistance. Our results suggest that lipidated polycarbonates could be a new class of antimicrobial agents.

Loss of outer membrane protein A (OmpA) impairs the survival of Salmonella Typhimurium by inducing membrane damage in the presence of ceftazidime and meropenem

OBJECTIVES

Salmonella enterica serovar Typhimurium is one of the significant non-typhoidal Salmonella serovars that causes gastroenteritis. The rapid development of antimicrobial resistance necessitates studying new antimicrobials and their therapeutic targets in this pathogen. Our study aimed to investigate the role of four prominent outer membrane porins of S. Typhimurium, namely OmpA, OmpC, OmpD and OmpF, in developing resistance against ceftazidime and meropenem.

METHODS

The antibiotic-mediated inhibition of bacterial growth was determined by measuring the absorbance and the resazurin assay. DiBAC4 (Bis-(1,3-Dibutylbarbituric Acid)Trimethine Oxonol), 2,7-dichlorodihydrofluoroscein diacetate (DCFDA) and propidium iodide were used to determine the outer membrane depolarization, reactive oxygen species (ROS) generation and subsequent killing of Salmonella. The expression of oxidative stress-response and efflux pump genes was quantified by quantitative RT-qPCR. HPLC was done to determine the amount of antibiotics that entered the bacteria. The damage to the bacterial outer membrane was studied by confocal and atomic force microscopy. The in vivo efficacy of ceftazidime and meropenem were tested in the C57BL/6 mouse model.

RESULTS

Deleting ompA reduced the survival of Salmonella in the presence of ceftazidime and meropenem. Massive outer membrane depolarization and reduced expression of oxidative stress-response genes in S. Typhimurium ΔompA hampered its growth in the presence of antibiotics. The enhanced uptake of antibiotics and decreased expression of efflux pump genes in S. Typhimurium ΔompA resulted in damage to the bacterial outer membrane. The clearance of the S. Typhimurium ΔompA from C57BL/6 mice with ceftazidime treatment proved the role of OmpA in rendering protection against β-lactam antibiotics.

CONCLUSIONS

OmpA protects S. Typhimurium from two broad-spectrum β-lactam antibiotics, ceftazidime and meropenem, by maintaining the stability of the outer membrane.

Bioactive Lipodepsipeptides Produced by Bacteria and Fungi

Natural products are a vital source for agriculture, medicine, cosmetics and other fields. Lipodepsipeptides (LPDs) are a wide group of natural products distributed among living organisms such as bacteria, fungi, yeasts, virus, insects, plants and marine organisms. They are a group of compounds consisting of a lipid connected to a peptide, which are able to self-assemble into several different structures. They have shown different biological activities such as phytotoxic, antibiotic, antiviral, antiparasitic, antifungal, antibacterial, immunosuppressive, herbicidal, cytotoxic and hemolytic activities. Their biological activities seem to be due to their interactions with the plasma membrane (MP) because they are able to mimic the architecture of the native membranes interacting with their hydrophobic segment. LPDs also have surfactant properties. The review has been focused on the lipodepsipeptides isolated from fungal and bacterial sources, on their biological activity, on the structure–activity relationships of some selected LPD subgroups and on their potential application in agriculture and medicine. The chemical and biological characterization of lipodepsipeptides isolated in the last three decades and findings that resulted from SCI-FINDER research are reported. A critical evaluation of the most recent reviews dealing with the same argument has also been described.

Staphylococcus aureus is able to generate resistance to novel lipoglycopeptide antibiotic gausemycin A

Gausemycin A is the first member of the novel lipoglycopeptides family produced by Streptomyces roseoflavus INA-Ac-5812. Gausemycin A has a pronounced bactericidal activity against methicillin-resistant Staphylococcus aureus. However, the ability of S. aureus to be resistant to gausemycin A has not been investigated yet. Using serial passaging, we have obtained the resistant variant S. aureus 5812R, which is 80 times more resistant compared to the parent strain. Susceptibility testing of S. aureus 5812R revealed the acquisition of cross-resistance to daptomycin, cefazolin, tetracycline, and gentamicin, while the resistance to vancomycin, nisin, and ramoplanin was absent. Whole genome sequencing revealed single nucleotide polymorphism (SNP) and deletions in S. aureus 5812R, among which are genes encoding efflux pump (sepA), the two-component Kdp system (kdpE), and the component of isoprenoid biosynthesis pathway (hepT). Phenotypically, S. aureus 5812R resembles a small-colony variant, as it is slow-growing, forms small colonies, and is deficient in pigments. Profiling of fatty acids (FA) composition constituting the cytoplasmic membrane of S. aureus 5812R revealed the prevalence of anteiso-branched FA, while straight FA was slightly less present. The evidence also showed that the gausemycin A-resistant strain has increased expression of the cls2 gene of the cardiolipin synthase. The performed checkerboard assay pointed out that the combination of gausemycin A and ciprofloxacin showed a synergistic effect against S. aureus 5812R.

Model architectures for bacterial membranes

The complex composition of bacterial membranes has a significant impact on the understanding of pathogen function and their development towards antibiotic resistance. In addition to the inherent complexity and biosafety risks of studying biological pathogen membranes, the continual rise of antibiotic resistance and its significant economical and clinical consequences has motivated the development of numerous in vitro model membrane systems with tuneable compositions, geometries, and sizes. Approaches discussed in this review include liposomes, solid-supported bilayers, and computational simulations which have been used to explore various processes including drug-membrane interactions, lipid-protein interactions, host–pathogen interactions, and structure-induced bacterial pathogenesis. The advantages, limitations, and applicable analytical tools of all architectures are summarised with a perspective for future research efforts in architectural improvement and elucidation of resistance development strategies and membrane-targeting antibiotic mechanisms.

A Microbe-Derived Efflux Pump Inhibitor of the Resistance-Nodulation-Cell Division Protein Restores Antibiotic Susceptibility in Escherichia coli and Pseudomonas aeruginosa

The use of efflux pump inhibitors (EPIs) as potentiators along with the traditional antibiotics assists in the warfare against antibiotic-resistant superbugs. Efflux pumps of the resistance-nodulation-cell division (RND) family play crucial roles in multidrug resistance in Escherichia coli and Pseudomonas aeruginosa. Despite several efforts, clinically useful inhibitors are not available at present. This study describes ethyl 4-bromopyrrole-2-carboxylate (RP1) isolation, an inhibitor of RND transporters from the library of 4000 microbial exudates. RP1 acts synergistically with antibiotics by reducing their minimum inhibitory concentration in strains overexpressing archetype RND transporters (AcrAB-TolC and MexAB-OprM). It also improves the accumulation of Hoechst 33342 and inhibits its efflux (a hallmark of EPI functionality). The antibiotic-RP1 combinations prolong the postantibiotic effects and reduce the mutation prevention concentration of antibiotics. Additionally, from Biolayer Interferometry spectra, it appears that RP1 is bound to AcrB. RP1 displays low mammalian cytotoxicity, no Ca²⁺ channel inhibitory effects, and reduces the intracellular invasion of E. coli and P. aeruginosa in macrophages. Furthermore, the RP1-levofloxacin combination is nontoxic, well-tolerated, and notably effective in a murine lung infection model. In sum, RP1 is a potent EPI and worthy of further consideration as a potentiator to improve the effectiveness of existing antibiotics.

Repurposing Approved Drugs as Fluoroquinolone Potentiators to Overcome Efflux Pump Resistance in Staphylococcus aureus

Staphylococcus aureus is a versatile human commensal bacteria and pathogen that causes various community and hospital-acquired infections. The S. aureus efflux pump NorA which belongs to the major facilitator superfamily, confers resistance to a range of substrates. Many efflux pump inhibitors (EPIs) have been discovered, but none is clinically approved due to their undesirable toxicities. In this study, we have screened clinically approved drugs for possible NorA EPI-like activity. We identified six drugs that showed the best efflux pump inhibition in vitro, with a fractional inhibitory concentration index of ≤0.5, indicating synergism with hydrophilic fluoroquinolones. The mechanistic validation of efflux inhibitory potential was demonstrated in ethidium bromide-based accumulation and efflux inhibition assays. We further confirmed the functionality of EPIs by norfloxacin accumulation assay depicting more realistic proof of the conjecture. None of the EPIs disturbed membrane function or depleted the ATP synthesis levels in bacteria. Both raloxifene and pyrvinium displayed an increase in bactericidal activity of ciprofloxacin in time-kill kinetics, prolonged its post-antibiotic effect, and reduced the frequency of spontaneous resistant mutant development. The combination of EPIs with ciprofloxacin caused significant eradication of preformed biofilms. Moreover, in the murine thigh infection model, a single dose of pyrvinium combined with ciprofloxacin reduced the bacterial burden significantly compared to untreated control and ciprofloxacin alone, indicating the efficacy of the combination. Conclusively, this study represents approved drugs that can be repurposed and combined with antibiotics as NorA EPIs, having anti-biofilm properties to treat severe S. aureus infections at clinically relevant concentrations.

IMPORTANCEStaphylococcus aureus is a frequent pathogen bacterium and the predominant cause of worsened nosocomial infections. Efflux pumps contribute to drug efflux and are reportedly associated with biofilm formation, thereby promoting difficult-to-treat biofilm-associated S. aureus infections. One strategy to combat these bacteria is to reduce active efflux and increase pathogen sensitivity to existing antibiotics. Repurposing approved drugs may solve the classical toxicity issues with previous efflux pump inhibitors and help reach sufficient plasma concentrations. We describe the in silico-based screening of FDA-approved drugs that identified six different molecules able to inhibit NorA pump (Major Facilitator Superfamily). Our study highlights that these compounds bind to and block the activity of the NorA pump and increase the sensitivity of S. aureus and methicillin-resistant S. aureus to fluoroquinolones. These drugs combined with fluoroquinolones significantly reduced the preformed biofilms and displayed significant efficacy in the murine thigh infection model when compared to untreated control and ciprofloxacin alone.

Antifungal Azoles as Tetracycline Resistance Modifiers in Staphylococcus aureus

Staphylococcus aureus has developed resistance to antimicrobials since their first use. The S. aureus major facilitator superfamily (MFS) efflux pump Tet(K) contributes to resistance to tetracyclines. The efflux pump diminishes antibiotic accumulation, and biofilm hampers the diffusion of antibiotics. None of the currently known compounds have been approved as efflux pump inhibitors (EPIs) for clinical use. In the current study, we screened clinically approved drugs for possible Tet(K) efflux pump inhibition. By performing in silico docking followed by in vitro checkerboard assays, we identified five azoles (the fungal ergosterol synthesis inhibitors) showing putative EPI-like potential with a fractional inhibitory concentration index of ≤0.5, indicating synergism. The functionality of the azoles was confirmed using ethidium bromide (EtBr) accumulation and efflux inhibition assays. In time-kill kinetics, the combination treatment with butoconazole engendered a marked increase in the bactericidal capacity of tetracycline. When assessing the off-target effects of the azoles, we observed no disruption of bacterial membrane permeability and polarization. Finally, the combination of azoles with tetracycline led to a significant eradication of preformed mature biofilms. This study demonstrates that azoles can be repurposed as putative Tet(K) EPIs and to reduce biofilm formation at clinically relevant concentrations. IMPORTANCE Staphylococcus aureus uses efflux pumps to transport antibiotics out of the cell and thus increases the dosage at which it endures antibiotics. Also, efflux pumps play a role in biofilm formation by the excretion of extracellular matrix molecules. One way to combat these pathogens may be to reduce the activity of efflux pumps and thereby increase pathogen sensitivity to existing antibiotics. We describe the in silico-based screen of clinically approved drugs that identified antifungal azoles inhibiting Tet(K), a pump that belongs to the major facilitator superfamily, and showed that these compounds bind to and block the activity of the Tet(K) pump. Azoles enhanced the susceptibility of tetracycline against S. aureus and its methicillin-resistant strains. The combination of azoles with tetracycline led to a significant reduction in preformed biofilms. Repurposing approved drugs may help solve the classical toxicity issues related to efflux pump inhibitors.

Emerging peptide antibiotics with therapeutic potential

This review covers some of the recent progress in the field of peptide antibiotics with a focus on compounds with novel or established mode of action and with demonstrated efficacy in animal infection models. Novel drug discovery approaches, linear and macrocyclic peptide antibiotics, lipopeptides like the polymyxins as well as peptides addressing targets located in the plasma membrane or in the outer membrane of bacterial cells are discussed.

Bacterial-Derived Plant Protection Metabolite 2,4-Diacetylphloroglucinol: Effects on Bacterial Cells at Inhibitory and Subinhibitory Concentrations

2,4-Diacetylphloroglucinol (2,4-DAPG) is a well-known bacterial secondary metabolite, however, its mechanism of inhibitory and subinhibitory action on bacterial cells is still poorly understood. The mechanism of 2,4-DAPG action on model bacterial strains was investigated using fluorescent spectroscopy and the action of the antibiotic was found to involve a rapid increase in membrane permeability that was accompanied by a reduction in its viability in nutrient-poor medium. At the same time, antibacterial action in nutrient-rich medium developed for several hours. Atomic force microscopy demonstrated time-dependent disturbances in the outer membrane of Escherichia coli when exposed to 2,4-DAPG, while Staphylococcusaureus cells have been visualized with signs of intracellular leakage. In addition, 2,4-DAPG inhibited the metabolic activity of S. aureus and E. coli bacterial cells in mature biofilms. Observed differences in the antibiofilm activity were dependent upon antibiotic concentration. The intracellular targets of the action of 2,4-DAPG were assessed using bacterial biosensors with inducible bioluminescence corresponding to DNA and protein damage. It was unable to register any positive response from either sensor. As a result, the bactericidal action of 2,4-DAPG is believed to be associated with the destruction of the bacterial barrier structures. The subinhibitory effect of 2,4-diacetylphloroglucinol was tested on quorum-sensing mediated processes in Pectobacterium carotovorum. Subinhibitory concentrations of 2,4-DAPG were found to lower the biosynthesis of acyl-homoserine lactones in P. carotovorum in a dose-dependent manner. Further investigation elucidated that 2,4-DAPG inhibits the metabolic activity of bacteria without affecting their viability.

Mode-of-Action of Antimicrobial Peptides: Membrane Disruption vs. Intracellular Mechanisms

Antimicrobial peptides are an attractive alternative to traditional antibiotics, due to their physicochemical properties, activity toward a broad spectrum of bacteria, and mode-of-actions distinct from those used by current antibiotics. In general, antimicrobial peptides kill bacteria by either disrupting their membrane, or by entering inside bacterial cells to interact with intracellular components. Characterization of their mode-of-action is essential to improve their activity, avoid resistance in bacterial pathogens, and accelerate their use as therapeutics. Here we review experimental biophysical tools that can be employed with model membranes and bacterial cells to characterize the mode-of-action of antimicrobial peptides.

MazEF-rifampicin interaction suggests a mechanism for rifampicin induced inhibition of persisters

Background

Persistence is a natural phenomenon whereby a subset of a population of isogenic bacteria either grow slow or become dormant conferring them with the ability to withstand various stresses including antibiotics. In a clinical setting bacterial persistence often leads to the recalcitrance of various infections increasing the treatment time and cost. Additionally, some studies also indicate that persistence can also pave way for the emergence of resistant strains. In a laboratory setting this persistent phenotype is enriched in nutritionally deprived environments. Consequently, in a batch culture the late stationary phase is enriched with persistent bacteria. The mechanism of persister cell formation and its regulation is not well understood. Toxin-antitoxin (TA) systems have been implicated to be responsible for bacterial persistence and rifampicin is used to treat highly persistent bacterial strains. The current study tries to explore a possible interaction between rifampicin and the MazEF TA system that furthers the former’s success rate in treating persistent bacteria.

Results

In the current study we found that the population of bacteria in the death phase of a batch culture consists of metabolically inactive live cells resembling persisters, which showed higher membrane depolarization as compared to the log phase bacteria. We also observed an increase in the expression of the MazEF TA modules in this phase. Since rifampicin is used to kill the persisters, we assessed the interaction of rifampicin with MazEF complex. We showed that rifampicin moderately interacts with MazEF complex with 1:1 stoichiometry.

Conclusion

Our study suggests that the interaction of rifampicin with MazEF complex might play an important role in inhibition of persisters.

Supplementary information

The online version contains supplementary material available at 10.1186/s12860-020-00316-8.

Discovery of Six Ramoplanin Family Gene Clusters and the Lipoglycodepsipeptide Chersinamycin*

Ramoplanins and enduracidins are peptidoglycan lipid intermediate II-binding lipodepsipeptides with broad-spectrum activity against methicillin- and vancomycin-resistant Gram-positive pathogens. Targeted genome mining using probes from conserved sequences within the ramoplanin/enduracidin biosynthetic gene clusters (BGCs) was used to identify six microorganisms with BGCs predicted to produce unique lipodepsipeptide congeners of ramoplanin and enduracidin. Fermentation of Micromonospora chersina yielded a novel lipoglycodepsipeptide, called chersinamycin, which exhibited good antibiotic activity against Gram-positive bacteria (1-2 μg/mL) similar to the ramoplanins and enduracidins. The covalent structure of chersinamycin was determined by NMR spectroscopy and tandem mass spectrometry in conjunction with chemical degradation studies. These six new BGCs and isolation of a new antimicrobial peptide provide much-needed tools to investigate the fundamental aspects of lipodepsipeptide biosynthesis and to facilitate efforts to produce novel antibiotics capable of combating antibiotic-resistant infections.

An Update on Antimicrobial Peptides (AMPs) and Their Delivery Strategies for Wound Infections

Bacterial infections occur when wound healing fails to reach the final stage of healing, which is usually hindered by the presence of different pathogens. Different topical antimicrobial agents are used to inhibit bacterial growth due to antibiotic failure in reaching the infected site, which is accompanied very often by increased drug resistance and other side effects. In this review, we focus on antimicrobial peptides (AMPs), especially those with a high potential of efficacy against multidrug-resistant and biofilm-forming bacteria and fungi present in wound infections. Currently, different AMPs undergo preclinical and clinical phase to combat infection-related diseases. AMP dendrimers (AMPDs) have been mentioned as potent microbial agents. Various AMP delivery strategies that are used to combat infection and modulate the healing rate-such as polymers, scaffolds, films and wound dressings, and organic and inorganic nanoparticles-have been discussed as well. New technologies such as Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-associated protein (CRISPR-Cas) are taken into consideration as potential future tools for AMP delivery in skin therapy.

A Rapid Fluorescence-Based Microplate Assay to Investigate the Interaction of Membrane Active Antimicrobial Peptides with Whole Gram-Positive Bacteria

Background: Membrane-active antimicrobial peptides (AMPs) are interesting candidates for the development of novel antimicrobials. Although their effects were extensively investigated in model membrane systems, interactions of AMPs with living microbial membranes are less known due to their complexity. The aim of the present study was to develop a rapid fluorescence-based microplate assay to analyze the membrane effects of AMPs in whole Staphylococcus aureus and Staphylococcus epidermidis. Methods: Bacteria were exposed to bactericidal and sub-inhibitory concentrations of two membrane-active AMPs in the presence of the potential-sensitive dye 3,3′-dipropylthiadicarbocyanine iodide (diSC₃(5)) and the DNA staining dye propidium iodide (PI), to simultaneously monitor and possibly distinguish membrane depolarization and membrane permeabilization. Results: The ion channel-forming gramicidin D induced a rapid increase of diSC₃(5), but not PI fluorescence, with slower kinetics at descending peptide concentrations, confirming killing due to membrane depolarization. The pore-forming melittin, at sub-MIC and bactericidal concentrations, caused, respectively, an increase of PI fluorescence in one or both dyes simultaneously, suggesting membrane permeabilization as a key event. Conclusions: This assay allowed the distinction between specific membrane effects, and it could be applied in the mode of action studies as well as in the screening of novel membrane-active AMPs.

Tellimagrandin II, A Type of Plant Polyphenol Extracted from Trapa bispinosa Inhibits Antibiotic Resistance of Drug-Resistant Staphylococcus aureus

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) has become a critical global concern. Identifying new candidates of anti-S. aureus agents is urgently required because the therapeutic strategies for infected patients are limited currently. Therefore, the present study investigated whether Tellimagrandin II (TGII), a pure compound extracted from the shells of Trapa bispinosa, exhibits antibacterial effects against MRSA. We first showed that TGII exerted potent inhibitory activity against MRSA with a minimum inhibitory concentration of 128 μg/mL. The obtained fractional inhibitory concentration suggested that TGII could alone exert antistaphylococcal activity, and TGII combined with low doses of antibiotics displayed synergistic effects against MRSA. Moreover, we found that TGII exerted bactericidal activity by reducing the expression of mecA followed by the negative regulation of the penicillin-binding protein 2a (PBP2a) of MRSA. Transmission electron microscopy (TEM) images further confirmed that TGII destroyed the integrity of the cell wall of MRSA and caused the loss of cytoplasm content. In conclusion, we evidenced the antibacterial effects of TGII against MRSA, which enables the effective dose of current antibiotics to be reduced and the predicament of drug-resistant S. aureus isolates to be overcome.

Therapeutic compounds targeting Lipid II for antibacterial purposes

Resistance against commonly used antibiotics has emerged in all bacterial pathogens. In fact, there is no antibiotic currently in clinical use against which resistance has not been reported. In particular, rapidly increasing urbanization in developing nations are sites of major concern. Additionally, the widespread practice by physicians to prescribe antibiotics in cases of viral infections puts selective pressure on antibiotics that still remain effective and it will only be a matter of time before resistance develops on a large scale. The biosynthesis pathway of the bacterial cell wall is well studied and a validated target for the development of antibacterial agents. Cell wall biosynthesis involves two major processes; 1) the biosynthesis of cell wall teichoic acids and 2) the biosynthesis of peptidoglycan. Key molecules in these pathways, including enzymes and precursor molecules are attractive targets for the development of novel antibacterial agents. In this review, we will focus on the major class of natural antibacterial compounds that target the peptidoglycan precursor molecule Lipid II; namely the glycopeptides, including the novel generation of lipoglycopeptides. We will discuss their mechanism-of-action and clinical applications. Further, we will briefly discuss additional peptides that target Lipid II such as the lantibiotic nisin and defensins. We will highlight recent developments and future perspectives.

Tridecaptin M, a New Variant Discovered in Mud Bacterium, Shows Activity against Colistin- and Extremely Drug-Resistant Enterobacteriaceae

The World Health Organization has categorized the Gram-negative superbugs, which are inherently impervious to many antibiotics, as critical priority pathogens due to the lack of effective treatments. The breach in our last-resort antibiotic (i.e., colistin) by extensively drug-resistant and pan-drug-resistant Enterobacteriaceae strains demands the immediate development of new therapies.

The World Health Organization has categorized the Gram-negative superbugs, which are inherently impervious to many antibiotics, as critical priority pathogens due to the lack of effective treatments. The breach in our last-resort antibiotic (i.e., colistin) by extensively drug-resistant and pan-drug-resistant Enterobacteriaceae strains demands the immediate development of new therapies. In the present study, we report the discovery of tridecaptin M, a new addition to the family, and its potential against colistin-resistant Enterobacteriaceae in vitro and in vivo. Also, we performed mode-of-action studies using various fluorescent probes and studied the hemolytic activity and mammalian cytotoxicity in two cell lines. Tridecaptin M displayed strong antibacterial activity (MICs of 2 to 8 μg ml⁻¹) against clinical strains of Klebsiella pneumoniae (which were resistant to colistin, carbapenems, third- and fourth-generation cephalosporins, fluoroquinolones, fosfomycin, and other antibiotics) and mcr-1-positive Escherichia coli strains. Unlike polymyxins, tridecaptin M did not permeabilize the outer membrane or cytoplasmic membrane. It blocked ATP synthesis in bacteria by dissipating the proton motive force. The compound exhibited negligible acquired resistance, low in vitro cytotoxicity and hemolytic activity, and no significant acute toxicity in mice. It also showed promising efficacy in a thigh infection model of colistin-resistant K. pneumoniae. Altogether, these results demonstrate the future prospects of this class of antibiotics to address the unmet medical need to circumvent colistin resistance in extensively drug-resistant Enterobacteriaceae infections. The work also emphasizes the importance of natural products in our shrunken drug discovery pipeline.

In-vitro studies on a natural lantibiotic, paenibacillin: A new-generation antibacterial drug candidate to overcome multi-drug resistance

The alarming burden of antibiotic resistance in nosocomial pathogens warrants the discovery and development of new and effective antimicrobial compounds. Small cationic antimicrobial peptides seem to be a promising therapeutic alternative to fight multi-drug resistance. This study investigated the in-vitro potential of a previously reported lantibiotic, paenibacillin, from the clinical perspective. An antimicrobial peptide, M152-P4, was isolated, purified and characterized from a mud isolate, and its susceptibility was determined in clinical isolates of Staphylococcus aureus and Enterococcus spp. Time-kill kinetics, resistance, probable mode of action, haemolytic activity and mammalian cytotoxicity were investigated. M152-P4 was identified as paenibacillin based on mass spectroscopy data, amino acid analysis and biosynthetic gene cluster analysis. It had potent antibacterial activity against the Gram-positive pathogens tested, with minimum inhibitory concentrations from 0.1 to 1.56 µM. It appeared very challenging for S. aureus to develop resistance to this compound. Also, paenibacillin penetrated the outer layer of bacteria, and depolarized the membrane completely by creating pores in the plasma membrane with better potential than nisin. Paenibacillin showed no haemolysis up to 60 µM, and the half maximal inhibitory concentration on mammalian cell lines was >100 µM. These results highlight the excellent antibacterial properties of paenibacillin in clinically relevant pathogens. It is stable in the presence of serum, and non-haemolytic and non-cytotoxic even above the therapeutic concentration. Further research efforts regarding toxicity and in-vivo efficacy are necessary to develop paenibacillin as a next-generation therapeutic drug to overcome multi-drug resistance in Gram-positive pathogens.

Daptomycin Resistant Staphylococcus aureus Clinical Strain With Novel Non-synonymous Mutations in the mprF and vraS Genes: A New Insight Into Daptomycin Resistance

Objectives: Daptomycin (DAP) resistance in Staphylococcus aureus is uncommon but there are increasing reports of the emergence of resistance during DAP therapy. Most clinical DAP-resistant S. aureus isolates investigated carried mutations in the mprF gene. The aim of this study was to identify mutations between a clinical pair of methicillin-susceptible S. aureus (MSSA) isolates (DAP-susceptible and DAP-resistant). Additionally, the activity of genes previously associated with DAP resistance was assessed.

Materials and Methods: Two MSSA isolates from patient with left-sided endocarditis were analyzed by whole genome sequencing (WGS) and reverse transcription-quantitative real-time PCR (RT-qPCR). The first isolate, DAP-susceptible, was obtained before initiation of treatment and the second isolate, DAP-resistant, was recovered after 4 weeks of DAP therapy.

Results: Comparison of complete genomes of DAP-susceptible and its DAP-resistant variant identified two non-synonymous and one synonymous mutations. The non-synonymous mutations consisted of a S829L substitution in mprF and a T331I substitution in vraS. The RT-qPCR experiments revealed an increased expression of vraS, dltA, mprF, and sceD genes in DAP-resistant variant. Strikingly, the expression of dltA and mprF genes was significantly downregulated by DAP.

Conclusion: The mprF and vraS genes were previously associated with DAP resistance, however, none of the mutations described in this study had been previously identified and linked to DAP resistance. Moreover, we provide a new insight into the DAP action on S. aureus, in which the expression of key genes in DAP resistance is decreased by the antibiotic.

Detection and Investigation of Eagle Effect Resistance to Vancomycin in Clostridium difficile With an ATP-Bioluminescence Assay

Vancomycin was bactericidal against Clostridium difficile at eightfold the minimum inhibitory concentration (MIC) using a traditional minimum bactericidal concentration (MBC) assay. However, at higher concentrations up to 64 × MIC, vancomycin displayed a paradoxical “more-drug-kills-less” Eagle effect against C. difficile. To overcome challenges associated with performing the labor-intensive agar-based MBC method under anaerobic growth conditions, we investigated an alternative more convenient ATP-bioluminescence assay to assess the Eagle effect in C. difficile. The commercial BacTiter-Glo^TM assay is a homogenous method to determine bacterial viability based on quantification of bacterial ATP as a marker for metabolic activity. The ATP-bioluminescence assay was advantageous over the traditional MBC-type assay in detecting the Eagle effect because it reduced assay time and was simple to perform; measurement of viability could be performed in less than 10 min outside of the anaerobic chamber. Using this method, we found C. difficile survived clinically relevant, high concentrations of vancomycin (up to 2048 μg/mL). In contrast, C. difficile did not survive high concentrations of metronidazole or fidaxomicin. The Eagle effect was also detected for telavancin, but not for teicoplanin, dalbavancin, oritavancin, or ramoplanin. All four pathogenic strains of C. difficile tested consistently displayed Eagle effect resistance to vancomycin, but not metronidazole or fidaxomicin. These results suggest that Eagle effect resistance to vancomycin in C. difficile could be more prevalent than previously appreciated, with potential clinical implications. The ATP-Bioluminescence assay can thus be used as an alternative to the agar-based MBC assay to characterize the Eagle effect against a variety of antibiotics, at a wide-range of concentrations, with much greater throughput. This may facilitate improved understanding of Eagle effect resistance and promote further research to understand potential clinical relevance.

Repurposing auranofin as an intestinal decolonizing agent for vancomycin-resistant enterococci

Multidrug-resistant enterococcal pathogens, especially vancomycin-resistant enterococci (VRE), are among the pathogens that require new antibiotic innovation. The colonization of the gut represents a major pathway by which VRE can cause infection and spread to other patients. In the current study, auranofin (FDA-approved rheumatoid arthritis drug) is evaluated for its potential use as a decolonizing agent for VRE. Auranofin was found to exert potent antimicrobial activity against a wide range of enterococcal clinical isolates with a minimum inhibitory concentration of 1 μg/mL. No resistant mutants could be developed against auranofin over the course of 14 passages. Auranofin was also found to exert potent anti-biofilm activity against VRE. Auranofin was superior to linezolid, the drug of choice for VRE infection treatment, in the in vivo mouse model. Auranofin significantly reduced the VRE burden in feces, cecum, and ileum contents after 8 days of treatment. Accordingly, this study provides valuable evidence that auranofin has significant promise as a novel gastrointestinal decolonizing agent for VRE.

Structure, Function, and Biosynthetic Origin of Octapeptin Antibiotics Active against Extensively Drug-Resistant Gram-Negative Bacteria

Resistance to the last-resort antibiotic colistin is now widespread and new therapeutics are urgently required. We report the first in toto chemical synthesis and pre-clinical evaluation of octapeptins, a class of lipopeptides structurally related to colistin. The octapeptin biosynthetic cluster consisted of three non-ribosomal peptide synthetases (OctA, OctB, and OctC) that produced an amphiphilic antibiotic, octapeptin C4, which was shown to bind to and depolarize membranes. While active against multi-drug resistant (MDR) strains in vitro, octapeptin C4 displayed poor in vivo efficacy, most likely due to high plasma protein binding. Nuclear magnetic resonance solution structures, empirical structure-activity and structure-toxicity models were used to design synthetic octapeptins active against MDR and extensively drug-resistant (XDR) bacteria. The scaffold was then subtly altered to reduce plasma protein binding, while maintaining activity against MDR and XDR bacteria. In vivo efficacy was demonstrated in a murine bacteremia model with a colistin-resistant P. aeruginosa clinical isolate.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

Antimicrobial Peptides Targeting Gram-Positive Bacteria

Antimicrobial peptides (AMPs) have remarkably different structures as well as biological activity profiles, whereupon most of these peptides are supposed to kill bacteria via membrane damage. In order to understand their molecular mechanism and target cell specificity for Gram-positive bacteria, it is essential to consider the architecture of their cell envelopes. Before AMPs can interact with the cytoplasmic membrane of Gram-positive bacteria, they have to traverse the cell wall composed of wall- and lipoteichoic acids and peptidoglycan. While interaction of AMPs with peptidoglycan might rather facilitate penetration, interaction with anionic teichoic acids may act as either a trap for AMPs or a ladder for a route to the cytoplasmic membrane. Interaction with the cytoplasmic membrane frequently leads to lipid segregation affecting membrane domain organization, which affects membrane permeability, inhibits cell division processes or leads to delocalization of essential peripheral membrane proteins. Further, precursors of cell wall components, especially the highly conserved lipid II, are directly targeted by AMPs. Thereby, the peptides do not inhibit peptidoglycan synthesis via binding to proteins like common antibiotics, but form a complex with the precursor molecule, which in addition can promote pore formation and membrane disruption. Thus, the multifaceted mode of actions will make AMPs superior to antibiotics that act only on one specific target.

Novel cationic peptide TP359 down-regulates the expression of outer membrane biogenesis genes in Pseudomonas aeruginosa: a potential TP359 anti-microbial mechanism

Background

Antimicrobial peptides (AMPs) are a class of antimicrobial agents with broad-spectrum activities. Several reports indicate that cationic AMPs bind to the negatively charged bacterial membrane causing membrane depolarization and damage. However, membrane depolarization and damage may be insufficient to elicit cell death, thereby suggesting that other mechanism(s) of action could be involved in this phenomenon. In this study, we investigated the antimicrobial activity of a novel antimicrobial peptide, TP359, against two strains of Pseudomonas aeruginosa, as well as its possible mechanisms of action.

Results

TP359 proved to be bactericidal against P. aeruginosa as confirmed by the reduced bacteria counts, membrane damage and cytoplasmic membrane depolarization. In addition, it was non-toxic to mouse J774 macrophages and human lung A549 epithelial cells. Electron microscopy analysis showed TP359 bactericidal effects by structural changes of the bacteria from viable rod-shaped cells to those with cell membrane damages, proceeding into the efflux of cytoplasmic contents and emergence of ghost cells. Gene expression analysis on the effects of TP359 on outer membrane biogenesis genes underscored marked down-regulation, particularly of oprF, which encodes a major structural and outer membrane porin (OprF) in both strains studied, indicating that the peptide may cause deregulation of outer membrane genes and reduced structural stability which could lead to cell death.

Conclusion

Our data shows that TP359 has potent antimicrobial activity against P aeruginosa. The correlation between membrane damage, depolarization and reduced expression of outer membrane biogenesis genes, particularly oprF may suggest the bactericidal mechanism of action of the TP359 peptide.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0808-2) contains supplementary material, which is available to authorized users.

New Found Hope for Antibiotic Discovery: Lipid II Inhibitors

Research into antibacterial agents has recently gathered pace in light of the disturbing crisis of antimicrobial resistance. The development of modern tools offers the opportunity of reviving the fallen era of antibacterial discovery through uncovering novel lead compounds that target vital bacterial cell components, such as lipid II. This paper provides a summary of the role of lipid II as well as an overview and insight into the structural features of macrocyclic peptides that inhibit this bacterial cell wall component. The recent discovery of teixobactin, a new class of lipid II inhibitor has generated substantial research interests. As such, the significant progress that has been achieved towards its development as a promising antibacterial agent is discussed.

Insight into membrane selectivity of linear and branched polyethylenimines and their potential as biocides for advanced wound dressings

We report here structure-property relationship between linear and branched polyethylene imines by examining their antimicrobial activities against wide range of pathogens. Both the polymers target the cytoplasmic membrane of bacteria and yeasts, eliciting rapid microbicidal properties. Using multiscale molecular dynamic simulations, we showed that, in both fully or partially protonated forms LPEI discriminates between mammalian and bacterial model membranes whereas BPEI lacks selectivity for both the model membranes. Simulation results suggest that LPEI forms weak complex with the zwitterionic lipids whereas the side chain amino groups of BPEI sequester the zwitterionic lipids by forming tight complex. Consistent with these observations, label-free cell impedance measurements, cell viability assays and high content analysis indicate that BPEI is cytotoxic to human epithelial and fibroblasts cells. Crosslinking of BPEI onto electrospun gelatin mats attenuate the cytotoxicity for fibroblasts while retaining the antimicrobial activity against Gram-positive and yeasts strains. PEI crosslinked gelatin mats elicit bactericidal activity by contact-mediated killing and durable to leaching for 7days. The potent antimicrobial activity combined with enhanced selectivity of the crosslinked ES gelatin mats would expand the arsenel of biocides in the management of superficial skin infections. The contact-mediated microbicidal properties may avert antimicrobial resistance and expand the diversity of applications to prevent microbial contamination.

STATEMENT OF SIGNIFICANCE

Current commercially available advanced wound dressings are either impregnated with metallic silver or silver salts which have side effects or may not avert antimicrobial resistance. In this article, we have used multidisciplinary approach comprising of computational, chemical and biological methods to understand the antimicrobial properties and biocompatibility of linear (LPEI) and branched (BPEI) polyethylenimines. We then applied this knowledge to develop dual purpose wound dressings containing these polymers, which encourages healing while maintain antimicrobial activity. In addition, the approach can be expanded to rationalize the antimicrobial vs. cytotoxicity of other cationic polymers and the method of crosslinking would enhance their potentials as biocides for advanced materials.

Production of the Ramoplanin Activity Analogue by Double Gene Inactivation

Glycopeptides such as vancomycin and telavancin are essential for treating infections caused by Gram-positive bacteria. But the dwindling availability of new antibiotics and the emergence of resistant bacteria are making effective antibiotic treatment increasingly difficult. Ramoplanin, an inhibitor of bacterial cell wall biosynthesis, is a highly effective antibiotic against a wide range of Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus, vancomycin-intermediate resistant Clostridium difficile and vancomycin-resistant Enterococcus sp. Here, two tailoring enzyme genes in the biosynthesis of ramoplanin were deleted by double in-frame gene knockouts to produce new ramoplanin derivatives. The deschlororamoplanin A2 aglycone was purified and its structure was identified with LC-MS/MS. Deschlororamoplanin A2 aglycone and ramoplanin aglycone showed similar activity to ramoplanin A2. The results showed that α-1,2-dimannosyl disaccharide at Hpg¹¹ and chlorination at Chp¹⁷ in the ramoplanin structure are not essential for its antimicrobial activity. This work provides new precursor compounds for the semisynthetic modification of ramoplanin.

Short cationic lipopeptides as effective antibacterial agents: Design, physicochemical properties and biological evaluation

The spread of drug-resistant bacteria has imparted a sense of urgency in the search for new antibiotics. In an effort to develop a new generation of antibacterial agents, we have designed de novo charged lipopeptides inspired by natural antimicrobial peptides. These short lipopeptides are composed of cationic lysine and hydrophobic lipoamino acids that replicate the amphiphilic properties of natural antimicrobial peptides. The resultant lipopeptides were found to self-assemble into nanoparticles. Some were effective against a variety of Gram-positive bacteria, including strains resistant to methicillin, daptomycin and/or vancomycin. The lipopeptides were not toxic to human kidney and liver cell lines and were highly resistant to tryptic degradation. Transmission electron microscopy analysis of bacteria cells treated with lipopeptide showed membrane-damage and lysis with extrusion of cytosolic contents. With such properties in mind, these lipopeptides have the potential to be developed as new antibacterial agents against drug-resistant Gram-positive bacteria.

Identification of an iridium(III) complex with anti-bacterial and anti-cancer activity

Group 9 transition metal complexes have been widely explored as therapeutic agents due to their unique geometry, their propensity to undergo ligand exchanges with biomolecules and their diverse steric and electronic properties. These metal complexes can offer distinct modes of action in living organisms compared to carbon-based molecules. In this study, we investigated the antimicrobial and anti-proliferative abilities of a series of cyclometallated iridium(III) complexes. The iridium(III) complex 1 inhibited the growth of S. aureus with MIC and MBC values of 3.60 and 7.19 μM, respectively, indicating its potent bactericidal activity. Moreover, complex 1 also exhibited cytotoxicity against a number of cancer cell lines, with particular potency against ovarian, cervical and melanoma cells. This cyclometallated iridium(III) complex is the first example of a substitutionally-inert, Group 9 organometallic compound utilized as a direct and selective inhibitor of S. aureus.