Novel synthetic bis-indolic derivatives with antistaphylococcal activity, including against MRSA and VISA strains

Indole-based aryl sulfides target the cell wall of Staphylococcus aureus without detectable resistance

Sulfur-containing classes of the scaffold "Arylthioindoles" have been evaluated for antibacterial activity; they demonstrated excellent potency against methicillin-resistant Staphylococcus aureus (MRSA) as well as against vancomycin-resistant strains and a panel of clinical isolates of resistant strains. In this study, we have elucidated the mechanism of action of lead compounds, wherein they target the cell wall of S. aureus. Further, S. aureus failed to develop resistance against two lead compounds tested in a serial passage experiment in the presence of the compounds over a period of 40 days. Both the compounds demonstrated comparable in vivo efficacy with vancomycin in a neutropenic mice thigh infection model. The results of these antibacterial activities emphasize the excellent potential of thioethers for developing novel antibiotics and may fill in as a target for the adjustment of accessible molecules to develop new powerful antibacterial agents with fewer side effects.

4F-Indole Enhances the Susceptibility of Pseudomonas aeruginosa to Aminoglycoside Antibiotics

Infections caused by multidrug-resistant bacteria are becoming increasingly serious. The aminoglycoside antibiotics have been widely used to treat severe Gram-negative bacterial infections. Here, we reported that a class of small molecules, namely, halogenated indoles, can resensitize Pseudomonas aeruginosa PAO1 to aminoglycoside antibiotics such as gentamicin, kanamycin, tobramycin, amikacin, neomycin, ribosomalin sulfate, and cisomicin. We selected 4F-indole as a representative of halogenated indoles to investigate its mechanism and found that the two-component system (TCS) PmrA/PmrB inhibited the expression of multidrug efflux pump MexXY-OprM, allowing kanamycin to act intracellularly. Moreover, 4F-indole inhibited the biosynthesis of several virulence factors, such as pyocyanin, type III secretion system (T3SS), and type VI secretion system (T6SS) exported effectors, and reduced the swimming and twitching motility by suppressing the expression of flagella and type IV pili. This study suggests that the combination of 4F-indole and kanamycin can be more effective against P. aeruginosa PAO1 and affect its multiple physiological activities, providing a novel insight into the reactivation of aminoglycoside antibiotics. IMPORTANCE Infections caused by Pseudomonas aeruginosa have become a major public health crisis. Its resistance to existing antibiotics causes clinical infections that are hard to cure. In this study, we found that halogenated indoles in combination with aminoglycoside antibiotics could be more effective than antibiotics alone against P. aeruginosa PAO1 and preliminarily revealed the mechanism of the 4F-indole-induced regulatory effect. Moreover, the regulatory effect of 4F-indole on different physiological behaviors of P. aeruginosa PAO1 was analyzed by combined transcriptomics and metabolomics. We explain that 4F-indole has potential as a novel antibiotic adjuvant, thus slowing down the further development of bacterial resistance.

Bis-Indole Alkaloids Isolated from the Sponge Spongosorites calcicola Disrupt Cell Membranes of MRSA

Antimicrobial resistance (AMR) is a global health challenge with methicillin resistant Staphylococcus aureus (MRSA), a leading cause of nosocomial infection. In the search for novel antibiotics, marine sponges have become model organisms as they produce diverse bioactive compounds. We investigated and compared the antibacterial potential of 3 bis-indole alkaloids—bromodeoxytopsentin, bromotopsentin and spongotine A—isolated from the Northeastern Atlantic sponge Spongosorites calcicola. Antimicrobial activity was determined by MIC and time-kill assays. The mechanism of action of bis-indoles was assessed using bacterial cytological profiling via fluorescence microscopy. Finally, we investigated the ability of bis-indole alkaloids to decrease the cytotoxicity of pathogens upon co-incubation with HeLa cells through the measurement of mammalian cell lysis. The bis-indoles were bactericidal to clinically relevant Gram-positive pathogens including MRSA and to the Gram-negative gastroenteric pathogen Vibrio parahaemolyticus. Furthermore, the alkaloids were synergistic in combination with conventional antibiotics. Antimicrobial activity of the bis-indole alkaloids was due to rapid disruption and permeabilization of the bacterial cell membrane. Significantly, the bis-indoles reduced pathogen cytotoxicity toward mammalian cells, indicating their ability to prevent bacterial virulence. In conclusion, sponge bis-indole alkaloids are membrane-permeabilizing agents that represent good antibiotic candidates because of their potency against Gram-positive and Gram-negative bacterial pathogens.

Efflux Pump Mediated Antimicrobial Resistance by Staphylococci in Health-Related Environments: Challenges and the Quest for Inhibition

The increasing emergence of antimicrobial resistance in staphylococcal bacteria is a major health threat worldwide due to significant morbidity and mortality resulting from their associated hospital- or community-acquired infections. Dramatic decrease in the discovery of new antibiotics from the pharmaceutical industry coupled with increased use of sanitisers and disinfectants due to the ongoing COVID-19 pandemic can further aggravate the problem of antimicrobial resistance. Staphylococci utilise multiple mechanisms to circumvent the effects of antimicrobials. One of these resistance mechanisms is the export of antimicrobial agents through the activity of membrane-embedded multidrug efflux pump proteins. The use of efflux pump inhibitors in combination with currently approved antimicrobials is a promising strategy to potentiate their clinical efficacy against resistant strains of staphylococci, and simultaneously reduce the selection of resistant mutants. This review presents an overview of the current knowledge of staphylococcal efflux pumps, discusses their clinical impact, and summarises compounds found in the last decade from plant and synthetic origin that have the potential to be used as adjuvants to antibiotic therapy against multidrug resistant staphylococci. Critically, future high-resolution structures of staphylococcal efflux pumps could aid in design and development of safer, more target-specific and highly potent efflux pump inhibitors to progress into clinical use.

An Update on Staphylococcus aureus NorA Efflux Pump Inhibitors

BACKGROUND

Methicillin-resistant and vancomycin-resistant Staphylococcus aureus are pathogens causing severe infectious diseases that pose real public health threats problems worldwide. In S. aureus, the most efficient multidrug-resistant system is the NorA efflux pump. For this reason, it is critical to identify efflux pump inhibitors.

OBJECTIVE

In this paper, we present an update of the new natural and synthetic compounds that act as modulators of antibiotic resistance through the inhibition of the S. aureus NorA efflux pump.

RESULTS

Several classes of compounds capable of restoring the antibiotic activity have been identified against resistant-S. aureus strains, acting as NorA efflux pump inhibitors. The most promising classes of compounds were quinolines, indoles, pyridines, phenols, and sulfur-containing heterocycles. However, the substantial degree structural diversity of these compounds makes it difficult to establish good structure- activity correlations that allow the design of compounds with more promising activities and properties.

CONCLUSION

Despite substantial efforts put forth in the search for new antibiotic adjuvants that act as efflux pump inhibitors, and despite several promising results, there are currently no efflux pump inhibitors authorized for human or veterinary use, or in clinical trials. Unfortunately, it appears that infection control strategies have remained the same since the discovery of penicillin, and that most efforts remain focused on discovering new classes of antibiotics, rather than trying to prolong the life of available antibiotics, and simultaneously fighting mechanisms of bacterial resistance.

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.

Novel Small-molecule Antibacterials against Gram-positive Pathogens of Staphylococcus and Enterococcus Species

Defeat of the antibiotic resistance of pathogenic bacteria is one great challenge today and for the future. In the last century many classes of effective antibacterials have been developed, so that upcoming resistances could be met with novel drugs of various compound classes. Meanwhile, there is a certain lack of research of the pharmaceutical companies, and thus there are missing developments of novel antibiotics. Gram-positive bacteria are the most important cause of clinical infections. The number of novel antibacterials in clinical trials is strongly restricted. There is an urgent need to find novel antibacterials. We used synthetic chemistry to build completely novel hybrid molecules of substituted indoles and benzothiophene. In a simple one-pot reaction, two novel types of thienocarbazoles were yielded. Both indole substituted compound classes have been evaluated as completely novel antibacterials against the Staphylococcus and Enterococcus species. The evaluated partly promising activities depend on the indole substituent type. First lead compounds have been evaluated within in vivo studies. They confirmed the in vitro results for the new classes of small-molecule antibacterials.

Development of an imidazole salt catalytic system for the preparation of bis(indolyl)methanes and bis(naphthyl)methane

Imidazolium salts are shown to catalyze the rapid room temperature reaction of indoles or naphthol with aldehydes to provide bis(indolyl)methanes or bis(naphthol)methane in excellent yields and the reaction proceeds optimally in dichloromethane with no base additives. The reaction exhibits a broad substrate tolerance and occurs through nucleophilic activation of the indoles and naphthols through a cation-π interaction.

Efflux pump inhibitors of clinically relevant multidrug resistant bacteria

Bacterial infections are an increasingly serious issue worldwide. The inability of existing therapies to treat multidrug-resistant pathogens has been recognized as an important challenge of the 21st century. Efflux pumps are important in both intrinsic and acquired bacterial resistance and identification of small molecule efflux pump inhibitors (EPIs), capable of restoring the effectiveness of available antibiotics, is an active research field. In the last two decades, much effort has been made to identify novel EPIs. However, none of them has so far been approved for therapeutic use. In this article, we explore different structural families of currently known EPIs for multidrug resistance efflux systems in the most extensively studied pathogens (NorA in Staphylococcus aureus, AcrAB-TolC in Escherichia coli, and MexAB-OprM in Pseudomonas aeruginosa). Both synthetic and natural compounds are described, with structure-activity relationship studies and optimization processes presented systematically for each family individually. In vitro activities against selected test strains are presented in a unifying manner for all the EPIs described, together with the most important toxicity, pharmacokinetic and in vivo efficacy data. A critical evaluation of lead-likeness characteristics and the potential for clinical development of the most promising inhibitors of the three efflux systems is described. This overview of EPIs is a good starting point for the identification of novel effective antibacterial drugs.

Essential Oil, Extracts, and Sesquiterpenes Obtained From the Heartwood of Pilgerodendron uviferum Act as Potential Inhibitors of the Staphylococcus aureus NorA Multidrug Efflux Pump

Staphylococcus aureus is a serious human pathogen that is highly adaptive to environmental conditions and rapidly develops antibiotic resistance. The use of efflux pumps to reduce antibiotic concentrations at the intracellular level is one of the main mechanisms by which bacteria develop antibiotic resistance. The management of efflux pumps, specifically NorA, which is expressed by S. aureus strains, is a valuable strategy for restoring susceptibility in strains resistant to antibacterial agents. In recent years, many studies have focused on searching for natural substances that can reverse efflux pump-mediated resistance in S. aureus. Extracts and compounds obtained from plants can be efficient efflux pump inhibitors (EPIs) and represent a potentially patient-friendly strategy for controlling S. aureus. In the present study, we evaluated the ability of essential oils, petroleum ether extracts, dichloromethane extract (DCME) and six compounds isolated from the heartwood of Pilgerodendron uviferum (Cupressaceae) and two synthetic derivatives to inhibit efflux in NorA pumps in the following three S. aureus strains: K2378, which overexpressed the norA gene (norA++), K1902 (norA-deleted, ΔnorA) and the parental strain, NCTC 8325-4. Efflux activity was evaluated using a fluorometric method that measured the accumulation of the universal efflux pump substrate ethidium bromide (EtBr). Only DCME and the compounds 15-copaenol and epi-cubenol inhibited EtBr efflux by K2378. Even the lowest concentration of 15-copaenol exhibited a stronger inhibitory effect than carbonyl cyanide m-chlorophenyl hydrazone on EtBr efflux by K2378. 15-copaenal only showed inhibition of EtBr efflux in K2378 cells at 125 μg/mL, but not superior to the control inhibitor and 15-copaenyl acetate exerted no intrinsic EPI activity against K2378. Fractional inhibitory concentration index (FICI) values obtained in the checkerboard assays, indicated that all combinations between DCME, epi-cubenol and 15-copaenol, and tested antibiotics showed a synergistic effect in wild type, norA ++ and ΔnorA strains. Moreover, those were not toxic for the HeLa cell line at concentrations in which the synergistic effect and inhibitory activity of efflux pumps was determined. Other extracts and compounds obtained from P. uviferum did not display EtBr efflux-inhibiting activity against the evaluated S. aureus strains.

Antimicrobial Activity of Galangin and Its Effects on Murein Hydrolases of Vancomycin-Intermediate Staphylococcus aureus (VISA) Strain Mu50

Backgroud: Antibiotic treatment for infections caused by vancomycin-intermediate Staphylococcus aureus (VISA) strains is challenging, and only a few effective and curative methods have been developed to combat these strains. This study aimed to investigate the antimicrobial activity of galangin against S. aureus and its effects on the murein hydrolases of VISA strain Mu50. This is the first report on these effects of galangin, and it may help to improve the treatment for VISA infections by demonstrating the effective use of galangin.

METHODS

Firstly, the minimum inhibitory concentration (MIC) and growth curve were used to investigate the antimicrobial activity of galangin against S. aureus. Secondly, transmission electron microscopy (TEM) was used to observe morphological changes of VISA strain Mu50. Thirdly, Triton X-100-induced autolysis and cell wall hydrolysis assays were performed to determine the activities of the murein hydrolases of Mu50. Finally, fluorescence real-time quantitative PCR was used to investigate the expression of the murein hydrolase-related Mu50 genes.

RESULTS

The results indicated that the MIC of galangin was 32 μg/mL against ATCC25293, N315, and Mu50, and galangin could significantly suppress the bacterial growth (p < 0.05) with concentrations of 4, 8 and 16 μg/mL, compared with control group (0 μg/mL). To explore the possible reasons of bacteriostatic effects of galangin, we observed morphological changes using TEM which showed that the division of Mu50 daughter cells treated with galangin was obviously inhibited. Considering the vital role of murein hydrolases in cellular division, assays were performed, and galangin markedly decreased Triton X-100-induced autolysis and cell wall hydrolysis. Galangin also significantly inhibited the expression of the murein hydrolase genes (atl, lytM, and lytN) and their regulatory genes (cidR, cidA, and cidB).

CONCLUSIONS

Our findings indicated that galangin can effectively inhibit murein hydrolase activity as well as the growth of VISA strain Mu50.

Pharmaceutical Approaches to Target Antibiotic Resistance Mechanisms

There is urgent need for new therapeutic strategies to fight the global threat of antibiotic resistance. The focus of this Perspective is on chemical agents that target the most common mechanisms of antibiotic resistance such as enzymatic inactivation of antibiotics, changes in cell permeability, and induction/activation of efflux pumps. Here we assess the current landscape and challenges in the treatment of antibiotic resistance mechanisms at both bacterial cell and community levels. We also discuss the potential clinical application of chemical inhibitors of antibiotic resistance mechanisms as add-on treatments for serious drug-resistant infections. Enzymatic inhibitors, such as the derivatives of the β-lactamase inhibitor avibactam, are closer to the clinic than other molecules. For example, MK-7655, in combination with imipenem, is in clinical development for the treatment of infections caused by carbapenem-resistant Enterobacteriaceae and Pseudomonas aeruginosa, which are difficult to treat. In addition, other molecules targeting multidrug-resistance mechanisms, such as efflux pumps, are under development and hold promise for the treatment of multidrug resistant infections.