Tuning the Effects of Bacterial Membrane Permeability through Photo-Isomerization of Antimicrobial Cationic Amphiphiles

Direct Growth Control of Antibiotic-Resistant Bacteria Using Visible-Light-Responsive Novel Photoswitchable Antibiotics

In addition to the discovery of new (modified) potent antibiotics to combat antibiotic resistance, there is a critical need to develop novel strategies that would restrict their off-target effects and unnecessary exposure to bacteria in our body and environment. We report a set of new photoswitchable arylazopyrazole-modified norfloxacin antibiotics that present a high degree of bidirectional photoisomerization, impressive fatigue resistance and reasonably high cis half-lives. The irradiated isomers of most compounds were found to exhibit nearly equal or higher antibacterial activity than norfloxacin against Gram-positive bacteria. Notably, against norfloxacin-resistant S. aureus bacteria, the visible-light-responsive p-SMe-substituted derivative showed remarkably high antimicrobial potency (MIC of 0.25 μg/mL) in the irradiated state, while the potency was reduced by 24-fold in case of its non-irradiated state. The activity was estimated to be retained for more than 7 hours. This is the first report to demonstrate direct photochemical control of the growth of antibiotic-resistant bacteria and to show the highest activity difference between irradiated and non-irradiated states of a photoswitchable antibiotic. Additionally, both isomers were found to be non-harmful to human cells. Molecular modellings were performed to identify the underlying reason behind the high-affinity binding of the irradiated isomer to topoisomerase IV enzyme.

Photoswitchable Antibiotic Hybrids: Spacer Length-Dependent Photochemical Control of Antibacterial Activity

Photopharmacology holds huge potential for the permanent (long-term) eradication of antibiotic resistance by the application of photoswitchable antibiotics. To construct such antibiotics, various methods have been employed to modify known antibiotics with photoswitches, such that the irradiated state shows activity comparable to or higher than that of the parent antibiotic and that a large activity difference between irradiated and nonirradiated states is achieved. However, most of those methods are ineffective when dealing with more than one drug with dissimilar structures. Here, we have demonstrated a new approach, in which two pharmacophores, one being a photoswitch, are covalently linked via a spacer of variable lengths, leading to a set of azopyrazole-norfloxacin antibiotic hybrids. All compounds showed a high degree of bidirectional photoisomerization, long thermal cis half-lives, and excellent photoresistance. Notably, the hybrid with an optimal four-carbon spacer length enabled the irradiated state to become 12-fold more potent than its nonirradiated state without losing much antimicrobial activity of norfloxacin. Only Gram-positive bacteria were found to be sensitive to this hybrid, and the full antibacterial potency of its irradiated state was found to be retained for nearly 24 h.

Azobenzene as Antimicrobial Molecules

Azo molecules, characterized by the presence of a -N=N- double bond, are widely used in various fields due to their sensitivity to external stimuli, ch as light. The emergence of bacterial resistance has pushed research towards designing new antimicrobial molecules that are more efficient than those currently in use. Many authors have attempted to exploit the antimicrobial activity of azobenzene and to utilize their photoisomerization for selective control of the bioactivities of antimicrobial molecules, which is necessary for antibacterial therapy. This review will provide a systematic and consequential approach to coupling azobenzene moiety with active antimicrobial molecules and drugs, including small and large organic molecules, such as peptides. A selection of significant cutting-edge articles collected in recent years has been discussed, based on the structural pattern and antimicrobial performance, focusing especially on the photoactivity of azobenzene and the design of smart materials as the most targeted and desirable application.

MadR mediates acyl CoA-dependent regulation of mycolic acid desaturation in mycobacteria

Our studies show that the mycolic acid desaturase regulator (MadR) acts as a molecular switch, controlling the desaturation and biosynthesis of mycolic acids, key lipids of the cell envelopes of mycobacteria. MadR works by a distinct mechanism wherein it binds various acyl-coenzyme As (aceyl-CoAs), but only saturated acyl-CoAs relieve DNA binding and repression. This suggests a unique mechanism that involves sensing of acyl-CoA pools as a checkpoint for coordinating mycolic acid remodeling and biosynthesis in response to cell surface perturbation. Our findings further our understanding of how mycobacteria control cell wall composition in response to stress across various environments ranging from soil to an intracellular niche in infected macrophages, with implications for understanding strategies for pathogenesis in the tubercle bacillus.

Mycobacterium tuberculosis has a lipid-rich cell envelope that is remodeled throughout infection to enable adaptation within the host. Few transcriptional regulators have been characterized that coordinate synthesis of mycolic acids, the major cell wall lipids of mycobacteria. Here, we show that the mycolic acid desaturase regulator (MadR), a transcriptional repressor of the mycolate desaturase genes desA1 and desA2, controls mycolic acid desaturation and biosynthesis in response to cell envelope stress. A madR-null mutant of M. smegmatis exhibited traits of an impaired cell wall with an altered outer mycomembrane, accumulation of a desaturated α-mycolate, susceptibility to antimycobacterials, and cell surface disruption. Transcriptomic profiling showed that enriched lipid metabolism genes that were significantly down-regulated upon madR deletion included acyl-coenzyme A (aceyl-CoA) dehydrogenases, implicating it in the indirect control of β-oxidation pathways. Electromobility shift assays and binding affinities suggest a unique acyl-CoA pool–sensing mechanism, whereby MadR is able to bind a range of acyl-CoAs, including those with unsaturated as well as saturated acyl chains. MadR repression of desA1/desA2 is relieved upon binding of saturated acyl-CoAs of chain length C₁₆ to C₂₄, while no impact is observed upon binding of shorter chain and unsaturated acyl-CoAs. We propose this mechanism of regulation as distinct to other mycolic acid and fatty acid synthesis regulators and place MadR as the key regulatory checkpoint that coordinates mycolic acid remodeling during infection in response to host-derived cell surface perturbation.

Serum Prevents Interactions between Antimicrobial Amphiphilic Aminoglycosides and Plasma Membranes

Antimicrobial cationic amphiphiles have broad-spectrum activity, and microbes do not readily develop resistance to these agents, highlighting their clinical and industrial potential. Cationic amphiphiles perturb the integrity of membranes leading to cell death, and the lack of discrimination between microbial and mammalian plasma membranes is thought to be one of the main barriers of using these agents for the treatment of systemic infections. Here, we describe the synthesis and study of 20 antimicrobial cationic amphiphiles that are derivatives of the aminoglycoside nebramine with different numbers of alkyl chain ethers that differ in length and degree of unsaturation. We determined antifungal activities and evaluated hemoglobin release from red blood cells as a measure of membrane selectivity and analyzed how serum influences these activities. Microscopic images revealed morphological transformations of red blood cells from the normal double-disc shape to empty ghost cells upon treatment with the cationic amphiphiles. Antifungal activity, hemolysis, and morphological changes in red blood cells decreased as the percentage of serum in the culture medium was increased. In images of red blood cells treated with fluorescently labeled amphiphilic nebramine probes, the accumulation of the cationic amphiphiles in the membranes decreased as serum concentration increased. This suggests that, in addition to its known effect of preventing the deformability of red blood cells, serum prevents interactions between cationic amphiphiles and the plasma membrane. The results of this study indicate that biological activities of cationic amphiphiles are abrogated in serum. Thus, these agents are suitable for external and industrial uses but probably not for effective treatment of systemic infections.

Self-assembling morphology-tunable single-component supramolecular antibiotics for enhanced antibacterial manipulation

This single-component supramolecular antibiotic can undergo reversible self-assembling morphology transitions under sequential ultrasonic and redox stimuli. The self-assemblies with different morphologies display effective antibacterial regulation.

Antifungal Activities of 4″,6″-Disubstituted Amphiphilic Kanamycins

Amphiphilic kanamycins derived from the classic antibiotic kanamycin have attracted interest due to their novel bioactivities beyond inhibition of bacteria. In this study, the recently described 4″,6″-diaryl amphiphilic kanamycins reported as inhibitors of connexin were examined for their antifungal activities. Nearly all 4″,6″-diaryl amphiphilic kanamycins tested had antifungal activities comparable to those of 4″,6″-dialkyl amphiphilic kanamycins, reported previously against several fungal strains. The minimal growth inhibitory concentrations (MICs) correlated with the degree of amphiphilicity (cLogD) of the di-substituted amphiphilic kanamycins. Using the fluorogenic dyes, SYTOX^TM Green and propidium iodide, the most active compounds at the corresponding MICs or at 2×MICs caused biphasic dye fluorescence increases over time with intact cells. Further lowering the concentrations to half MICs caused first-order dye fluorescence increases. Interestingly, 4×MIC or 8×MIC levels resulted in fluorescence suppression that did not correlate with the MIC and plasma membrane permeabilization. The results show that 4″,6″-diaryl amphiphilic kanamycins are antifungal and that amphiphilicity parameter cLogD is useful for the design of the most membrane-active versions. A cautionary limitation of fluorescence suppression was revealed when using fluorogenic dyes to measure cell-permeation mechanisms with these antifungals at high concentrations. Finally, 4″,6″-diaryl amphiphilic kanamycins elevate the production of cellular reactive oxygen species as other reported amphiphilic kanamycins.

Trojan Antibiotics: New Weapons for Fighting Against Drug Resistance

Bacterial resistance has caused a global healthcare emergency due to the buildup of antibiotics in the environment. Novel approaches that enable highly efficient bactericide and auto inactivation are highly desired. Past researches mainly focused on the on-off bactericidal ability of antibiotics, which often displays unsatisfactory bactericidal efficiency. Herein, we report a Trojan antibiotic that considers the affinity of antibiotics to bacteria. A disguised host-guest supramolecule based on cucurbituril (CB[7]) and a bola-type azobenzene compound with glycosylamine heads at both ends is synthesized. This supramolecule has a surface fully decorated with sugar-like components, which are highly analogous to wall components of bacteria. This Trojan antibiotic is benign to a wide spectrum of bacteria at a weak basic pH of approximately 9.0 under daylight conditions. However, this antibiotic becomes a potent bactericide toward both Gram-negative and Gram-positive bacteria at pH 4.0 under 365 nm UV irradiation. The dual use of pH and UV light greatly enhances the efficiency of the bactericidal effect so that the 50% minimum inhibitory concentration (MIC₅₀) of the Trojan antibiotic is at least 10 times smaller than that of conventional drugs, and the removal of the UV source and reversal of pH automatically stop the antibacterial behavior, which prevents the buildup of active antimicrobial materials in the environment. We expect that the presented Trojan supramolecular strategy may open up a new paradigm in the fight against bacterial resistance.

Switchable modulation of bacterial growth and biofilm formation based on supramolecular tripeptide amphiphiles

A minimalistic dual-responsive supramolecular tripeptide system was developed for switchable control of bacterial growth and biofilm formation.

Derivatives of Ribosome-Inhibiting Antibiotic Chloramphenicol Inhibit the Biosynthesis of Bacterial Cell Wall

Here, we describe the preparation and evaluation of α,β-unsaturated carbonyl derivatives of the bacterial translation inhibiting antibiotic chloramphenicol (CAM). Compared to the parent antibiotic, two compounds containing α,β-unsaturated ketones (1 and 4) displayed a broader spectrum of activity against a panel of Gram-positive pathogens with a minimum inhibitory concentration range of 2-32 μg/mL. Interestingly, unlike the parent CAM, these compounds do not inhibit bacterial translation. Microscopic evidence and metabolic labeling of a cell wall peptidoglycan suggested that compounds 1 and 4 caused extensive damage to the envelope of Staphylococcus aureus cells by inhibition of the early stage of cell wall peptidoglycan biosynthesis. Unlike the effect of membrane-disrupting antimicrobial cationic amphiphiles, these compounds did not rapidly permeabilize the bacterial membrane. Like the parent antibiotic CAM, compounds 1 and 4 had a bacteriostatic effect on S. aureus. Both compounds 1 and 4 were cytotoxic to immortalized nucleated mammalian cells; however, neither caused measurable membrane damage to mammalian red blood cells. These data suggest that the reported CAM-derived antimicrobial agents offer a new molecular scaffold for development of novel bacterial cell wall biosynthesis inhibiting antibiotics.