Antibiotics with novel mode of action as new weapons to fight antimicrobial resistance

Public health case for microbiome-sparing antibiotics and new opportunities for drug development

Although antibiotics remain a cornerstone of modern medicine, the issues of widespread antibiotic resistance and collateral damage to the microbiome from antibiotic use are driving a need for drug developers to consider more tailored, patient-directed products to avoid antibiotic-induced perturbations of the structure and function of the indigenous microbiota. This perspective summarizes a cascade of microbiome health effects that is initiated by antibiotic-mediated microbiome disruption at an individual level and ultimately leads to infection and transmission of multidrug-resistant pathogens across patient populations. The scientific evidence behind each of the key steps of this cascade is presented. The interruption of this cascade through the use of highly targeted, microbiome-sparing antibiotics aiming to improve health outcomes is discussed. Further, this perspective reflects on some key clinical trial design and reimbursement considerations to be addressed as part of the drug development path.

Anti-staphylococcal activity of a polyphenol-rich citrus extract: synergy with β-lactams and low proficiency to induce resistance

Introduction

Antibiotic resistance represents one of the most significant threats to public health in the 21st century. Polyphenols, natural molecules with antibacterial activity produced by plants, are being considered as alternative antimicrobial strategies to manage infections caused by drug-resistant bacteria. In this study, we investigated the antibacterial activity of a polyphenol mixture extracted from citrus fruits, against both antibiotic-susceptible and resistant strains of Staphylococcus aureus and Staphylococcus epidermidis.

Methods

Broth microdilution and time-kill curve experiments were used to test the extract anti-staphylococcal activity. Cytotoxicity was assessed by the hemolysis assay. The interaction between the mixture and antibiotics was investigated by the checkerboard assay. The effect of B alone and in combination with oxacillin on the membrane potential was investigated by the 3,3'-dipropylthiadicarbocyanine iodide assay. The ability of the extract to induce the development of resistance was verified by propagating S. aureus for 10 transfers in the presence of sub-inhibitory concentrations.

Results

The citrus extract was found to be active against all Staphylococcus strains at remarkably low concentrations (0.0031 and 0.0063%), displaying rapid bactericidal effects without being toxic on erythrocytes. In particular, B was found to rapidly cause membrane depolarization. When combined with methicillin, meropenem, and oxacillin, the mixture displayed synergistic activity exclusively against methicillin-resistant strains. We additionally show that the sequential exposure of S. aureus to sub-inhibitory concentrations did not induce the development of resistance against the extract.

Discussion

Overall, these findings support the potential use of the citrus extract as promising option to manage staphylococcal infections and suggest that it may counteract the mechanism behind methicillin-resistance.

The antimicrobial potential of adarotene derivatives against Staphylococcus aureus strains

Multidrug-resistant (MDR) pathogens are severely impacting our ability to successfully treat common infections. Here we report the synthesis of a panel of adarotene-related retinoids showing potent antimicrobial activity on Staphylococcus aureus strains (including multidrug-resistant ones). Fluorescence and molecular dynamic studies confirmed that the adarotene analogues were able to induce conformational changes and disfunctions to the cell membrane, perturbing the permeability of the phospholipid bilayer. Since the major obstacle for developing retinoids is their potential cytotoxicity, a selected candidate was further investigated to evaluate its activity on a panel of human cell lines. The compound was found to be well tolerated, with IC50 5-15-fold higher than the MIC on S. aureus strains. Furthermore, the adarotene analogue had a good pharmacokinetic profile, reaching a plasma concentration of about 6 μM after 0.5 h after administration (150 mg/kg), at least twice the MIC observed against various bacterial strains. Moreover, it was demonstrated that the compound potentiated the growth-inhibitory effect of the poorly bioavailable rifaximin, when used in combination. Overall, the collected data pave the way for the development of synthetic retinoids as potential therapeutics for hard-to-treat infectious diseases caused by antibiotic-resistant Gram-positive pathogens.

Plant-based compositions for the treatment of local and topical bacterial infections: a patent review

Aim: To examine recent patents related to plants with antimicrobial effects. Methods: Our investigation was carried out using the Espacenet database, searching patents related to International Patent Classification (IPC) codes A61P31/04, and A61K36/00 between 2008 and 2023. Results: The study began with 360 patents, of which 15 were found to be duplicates; 144 were not related to the research topic and six were not available. Then, 48 patents were related to other administration routes. The final selection was 47 patents related to local administration compositions. The potential for these herbal compositions to be marketed as over-the-counter medicines was evident. Conclusion: Developing new plant-origin anti-infective agents could provide promising solutions for combating resistant infections and offer accessible treatment options.

Streptothricin-F Inhibition of FtsZ Function: A Promising Approach for Controlling Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) is a significant pathogenic bacterium affecting the kiwifruit industry. This study investigated the target sites of streptothricin-F (ST-F), produced by Streptomyces lavendulae gCLA4. The inhibition of ST-F on Psa was examined by the microscopic structural differences of Psa before and after treatment with ST-F, as well as the interaction between ST-F and cell division-related proteins. The results revealed filamentation of Psa after ST-F treatment, and fluorescence microscopy showed that ST-F inhibited the formation of the Z-ring composed of FtsZ protein. In vitro experiments and molecular docking demonstrated that ST-F can bind to FtsZ with a binding energy of 0.4 μM and inhibit FtsZ's GTP-dependent polymerization reaction. In addition, ST-F does not exert inhibitory effects on cell division in Psa strains overexpressing ftsZ. In conclusion, FtsZ is one of the target sites for ST-F inhibition of Psa, highlighting its potential as a therapeutic target for controlling Psa-induced kiwifruit bacterial canker.

Friend or Foe: Protein Inhibitors of DNA Gyrase

DNA gyrase is essential for the successful replication of circular chromosomes, such as those found in most bacterial species, by relieving topological stressors associated with unwinding the double-stranded genetic material. This critical central role makes gyrase a valued target for antibacterial approaches, as exemplified by the highly successful fluoroquinolone class of antibiotics. It is reasonable that the activity of gyrase could be intrinsically regulated within cells, thereby helping to coordinate DNA replication with doubling times. Numerous proteins have been identified to exert inhibitory effects on DNA gyrase, although at lower doses, it can appear readily reversible and therefore may have regulatory value. Some of these, such as the small protein toxins found in plasmid-borne addiction modules, can promote cell death by inducing damage to DNA, resulting in an analogous outcome as quinolone antibiotics. Others, however, appear to transiently impact gyrase in a readily reversible and non-damaging mechanism, such as the plasmid-derived Qnr family of DNA-mimetic proteins. The current review examines the origins and known activities of protein inhibitors of gyrase and highlights opportunities to further exert control over bacterial growth by targeting this validated antibacterial target with novel molecular mechanisms. Furthermore, we are gaining new insights into fundamental regulatory strategies of gyrase that may prove important for understanding diverse growth strategies among different bacteria.

Schematic-portfolio of potent anti-microbial scaffolds targeting DNA gyrase: Unlocking ways to overcome resistance

Drug development process demands validation of specific drug target impeding the Multi Drug Resistance (MDR). DNA gyrase, as a bacterial target has been in trend for developing newer antibacterial candidates due to its absence in higher eukaryotes. The fluoroquinolones are the leading molecules in the drug discovery pipeline for gyrase inhibition due to its diversity. The fluoroquinolones like levofloxacin and moxifloxacin have been listed in class A drugs for treating MDR. Gatifloxacin and ciprofloxacin also proved its efficacy against MDR TB and MDR enteric fever in adults, whereas nemonoxacin can induce anti-MDR activity of other antibiotics already suggested by studies. Though fluoroquinolones already proved its effectiveness against gyrase, other molecules viz., benzothiazinone, phenyl pyrrolamide, substituted oxadiazoles, triazolopyrimidine, arylbenzothiazole, coumarinyl amino alcohols and ciprofloxacin uracil, can inhibit the target more precisely. The structure-activity-relationships of the different scaffolds along with their synthetic strategies have been deciphered in the current review. Also, the naturally occurring compounds along with their extraction procedure have also been highlighted as potent DNA gyrase inhibitors. In addition to fluoroquinolone, the natural compounds novobiocin and simocyclinone could also inhibit the gyrase, impressively which has been designed with the gyrase structure for better understanding. Herein, ongoing clinical development of some novel drugs possessing triazaacenaphthylenes, spiropyrimidinetriones, and oxazolidinone-quinolone hybrids have been highlighted which could further assist the future generation antibiotic development corroborating gyrase as a potential target against MDR pathogens.

Monoterpene Thiols: Synthesis and Modifications for Obtaining Biologically Active Substances

Monoterpene thiols are one of the classes of natural flavors that impart the smell of citrus fruits, grape must and wine, black currants, and guava and are used as flavoring agents in the food and perfume industries. Synthetic monoterpene thiols have found an application in asymmetric synthesis as chiral auxiliaries, derivatizing agents, and ligands for metal complex catalysis and organocatalysts. Since monoterpenes and monoterpenoids are a renewable source, there are emerging trends to use monoterpene thiols as monomers for producing new types of green polymers. Monoterpene thioderivatives are also known to possess antioxidant, anticoagulant, antifungal, and antibacterial activity. The current review covers methods for the synthesis of acyclic, mono-, and bicyclic monoterpene thiols, as well as some investigations related to their usage for the preparation of the compounds with antimicrobial properties.