Bacterial efflux inhibitors are widely distributed in land plants

Plant-Derived Antimicrobials and Their Crucial Role in Combating Antimicrobial Resistance

Antibiotic resistance emerged shortly after the discovery of the first antibiotic and has remained a critical public health issue ever since. Managing antibiotic resistance in clinical settings continues to be challenging, particularly with the rise of superbugs, or bacteria resistant to multiple antibiotics, known as multidrug-resistant (MDR) bacteria. This rapid development of resistance has compelled researchers to continuously seek new antimicrobial agents to curb resistance, despite a shrinking pipeline of new drugs. Recently, the focus of antimicrobial discovery has shifted to plants, fungi, lichens, endophytes, and various marine sources, such as seaweeds, corals, and other microorganisms, due to their promising properties. For this review, an extensive search was conducted across multiple scientific databases, including PubMed, Elsevier, ResearchGate, Scopus, and Google Scholar, encompassing publications from 1929 to 2024. This review provides a concise overview of the mechanisms employed by bacteria to develop antibiotic resistance, followed by an in-depth exploration of plant secondary metabolites as a potential solution to MDR pathogens. In recent years, the interest in plant-based medicines has surged, driven by their advantageous properties. However, additional research is essential to fully understand the mechanisms of action and verify the safety of antimicrobial phytochemicals. Future prospects for enhancing the use of plant secondary metabolites in combating antibiotic-resistant pathogens will also be discussed.

Hinokitiol Selectively Enhances the Antibacterial Activity of Tetracyclines against Staphylococcus aureus

The increasing prevalence of antibiotic resistance causes an urgent need for alternative agents to combat drug-resistant bacterial pathogens. Plant-derived compounds are promising candidates for the treatment of infections caused by antibiotic-resistant bacteria. Hinokitiol (β-thujaplicin), a natural tropolone derivative found in the heartwood of cupressaceous plants, has been widely used in oral and skin care products as an antimicrobial agent. The aim of this work was to study the synergy potential of hinokitiol with antibiotics against Staphylococcus aureus, which is an extremely successful opportunistic pathogen capable of causing nosocomial and community-acquired infections worldwide. The MIC was determined by the broth microdilution method, and the effect of combinations was evaluated through fractional inhibitory concentration indices (FICI). The mechanism behind this synergy was also investigated by using fluorescence spectroscopy and high-performance liquid chromatography (HPLC). The MICs of hinokitiol alone against most S. aureus strains were 32 μg/mL. Selectively synergistic activities (FICIs of ≤0.5) were observed for combinations of this phytochemical with tetracyclines against all tested strains of S. aureus. Importantly, hinokitiol at 1 μg/mL completely or partially reversed tetracycline resistance in staphylococcal isolates. The increased accumulation of tetracycline inside S. aureus in the presence of hinokitiol was observed. In addition, hinokitiol promoted the uptake of ethidium bromide (EB) in bacterial cells without membrane depolarization, suggesting that it may be an efflux pump inhibitor. IMPORTANCE The disease caused by S. aureus is a public health issue due to the continuing emergence of drug-resistant strains, particularly methicillin-resistant S. aureus (MRSA). Tetracyclines, one of the old classes of antimicrobials, have been used for the treatment of infections caused by S. aureus. However, the increased resistance to tetracyclines together with their toxicity have limited their use in the clinic. Here, we demonstrated that the combination of hinokitiol and tetracyclines displayed synergistic antibacterial activity against S. aureus, including tetracycline-resistant strains and MRSA, offering a potential alternative approach for the treatment of infections caused by this bacterium.

Role of Berberine as a Potential Efflux Pump Inhibitor against MdfA from Escherichia coli: In Vitro and In Silico Studies

Infections by Gram-negative pathogens are usually difficult to manage due to the drug export by efflux pumps. With the evolution and horizontal transfer of efflux pumps, there is an urgent need to discover safe and effective efflux pump inhibitors. Here, we found that the natural compound berberine (BBR), a traditional medicine for intestinal infection, is an inhibitor against the major facilitator superfamily (MFS) efflux pump MdfA in Escherichia coli. The impact of BBR on MdfA was evaluated in a recombinant E. coli reporter strain. We demonstrated that low levels of BBR significantly increased intracellular ciprofloxacin concentrations and restored antibiotic susceptibility of the reporter strain. At the same time, we conducted molecular dynamics simulations to investigate the mechanisms of BBR's effect on MdfA. Our data indicated that BBR can aggregate to the periplasmic and cytoplasmic sides of MdfA in both of its inward and outward conformations. Protein rigidities were affected to different degrees. More importantly, two major driving forces for the conformational transition, salt bridges and hydrophilic interactions with water, were changed by BBR's aggregation to MdfA, which affected its conformational transition. In summary, our data provide evidence for the extended application of BBR as an efflux pump inhibitor at a clinically meaningful level. We also reveal the mechanisms and provide insights into BBR's effect on the reciprocal motion of MdfA. IMPORTANCE In this work, we evaluated the role of berberine (BBR) as an inhibitor of the MFS efflux pump MdfA from E. coli. We demonstrated that low levels of BBR significantly increased intracellular ciprofloxacin concentrations and restored antibiotic susceptibility of the reporter strain. Molecular dynamics simulations revealed the effect of BBR on the conformational transition of MdfA. Our data suggested that driving forces for MdfA's conformational transition were affected by BBR and provided evidence for BBR's extended application as an effective inhibitor of MdfA.

Experimental Study on the Inhibition of Bacteria and Algae by Jussiaea stipulacea Ohwi Extract

Nowadays, plant allelopathy, as a new type of biological algal and/or bacterial inhibition technology, has attracted extensive attention. Eight substances were isolated and identified from Jussiaea stipulacea Ohwi, and five concentration gradients, as well as a control (0, 1.25, 5, 10, 20, and 50 mg/L) were set, with three parallels in each group, and then sampled and detected at 24, 48, 72, and 96 h. When the concentration was 50 mg/L, the inhibition rate of Anabaena was as high as 74.8%, 69.2%, and 70.7% for ursolic acid, kaempferol, and luteolin, respectively. Streptococcus iniae and Aeromonas hydrophila were cultured to a logarithmic phase, and their final concentrations reached 1000, 500, 250, 125, 62.50, 31.25, 15.63, and 7.81 μg/mL. Luteolin and gallic acid showed an inhibitory effect on S iniae and A hydrophila at 1000 μg/mL. We found that allelochemicals also had a certain bacteriostatic effect, among which luteolin has great development potential.

Flavonoids as Inhibitors of Bacterial Efflux Pumps

Flavonoids are widely occurring secondary plant constituents, and are abundant in vegetable and fruit diets as well as herbal medicines. Therapeutic treatment options for bacterial infections are limited due to the spread of antimicrobial resistances. Hence, in a number of studies during the last few years, different classes of plant secondary metabolites as resistance-modifying agents have been carried out. In this review, we present the role of flavonoids as inhibitors of bacterial efflux pumps. Active compounds could be identified in the subclasses of chalcones, flavan-3-ols, flavanones, flavones, flavonols, flavonolignans and isoflavones; by far the majority of compounds were aglycones, although some glycosides like kaempferol glycosides with p-coumaroyl acylation showed remarkable results. Staphylococcus aureus NorA pump was the focus of many studies, followed by mycobacteria, whereas Gram-negative bacteria are still under-investigated.

Elucidation of Phytochemical Content of Cupressus macrocarpa Leaves: In Vitro and In Vivo Antibacterial Effect against Methicillin-Resistant Staphylococcus aureus Clinical Isolates

Methicillin-resistant Staphylococcus aureus (MRSA) is an important human pathogen that causes various infections. The increasing resistance of MRSA to different antibiotics is widely spreading; therefore, plant extracts may be novel therapeutic alternatives. The phytochemical profiling of Cupressus macrocarpa Hartw. ex Gordon leaves in vitro, and in vivo, antimicrobial potential of its extracts against MRSA clinical isolates were explored. A phytochemical tentative identification of 49 compounds was performed in the leaves using LC-ESI-MS/MS; in addition, isolation, and structure elucidation of hesperidin and eriocitrin were achieved for the first time. The diethyl ether extract (DEEL) exhibited the best antibacterial effect with MIC values ranging from 2 to 8 µg/mL, which significantly reduced the growth and efflux activity in 48.78% and 29.26% of isolates, respectively. qRT-PCR showed a significant down expression of norA and norB genes, which significantly affected the bacterial cell morphology and had a non-significant effect on membrane depolarization (using flow cytometry). In a rat model, four groups were wounded and treated with normal saline or DEEL, or infected with MRSA, or infected and treated with DEEL. The regeneration of the epidermis, maturation of granulation tissue, and reduction of inflammatory cell infiltration were observed after treatment with DEEL. Thus, C. macrocarpa leaves may be a promising source for new antimicrobials against MRSA.

Interactions with Microbial Proteins Driving the Antibacterial Activity of Flavonoids

Flavonoids are among the most abundant natural bioactive compounds produced by plants. Many different activities have been reported for these secondary metabolites against numerous cells and systems. One of the most interesting is certainly the antimicrobial, which is stimulated through various molecular mechanisms. In fact, flavonoids are effective both in directly damaging the envelope of Gram-negative and Gram-positive bacteria but also by acting toward specific molecular targets essential for the survival of these microorganisms. The purpose of this paper is to present an overview of the most interesting results obtained in the research focused on the study of the interactions between flavonoids and bacterial proteins. Despite the great structural heterogeneity of these plant metabolites, it is interesting to observe that many flavonoids affect the same cellular pathways. Furthermore, it is evident that some of these compounds interact with more than one target, producing multiple effects. Taken together, the reported data demonstrate the great potential of flavonoids in developing innovative systems, which can help address the increasingly serious problem of antibiotic resistance.