Microbial efflux pump inhibition: tactics and strategies

3-O-Substituted Quercetin: an Antibiotic-Potentiating Agent against Multidrug-Resistant Gram-Negative Enterobacteriaceae through Simultaneous Inhibition of Efflux Pump and Broad-Spectrum Carbapenemases

The discovery of safe and efficient inhibitors against efflux pumps as well as metallo-β-lactamases (MBL) is one of the main challenges in the development of multidrug-resistant (MDR) reversal agents which can be utilized in the treatment of carbapenem-resistant Gram-negative bacteria. In this study, we have identified that introduction of an ethylene-linked sterically demanding group at the 3-OH position of the previously reported MDR reversal agent di-F-Q endows the resulting compounds with hereto unknown multitarget inhibitory activity against both efflux pumps and broad-spectrum β-lactamases including difficult-to-inhibit MBLs. A molecular docking study of the multitarget inhibitors against efflux pump, as well as various classes of β-lactamases, revealed that the 3-O-alkyl substituents occupy the novel binding sites in efflux pumps as well as carbapenemases. Not surprisingly, the multitarget inhibitors rescued the antibiotic activity of a carbapenem antibiotic, meropenem (MEM), in NDM-1 (New Delhi Metallo-β-lactamase-1)-producing carbapenem-resistant Enterobacteriaceae (CRE), and they reduced MICs of MEM more than four-fold (synergistic effect) in 8-9 out of 14 clinical strains. The antibiotic-potentiating activity of the multitarget inhibitors was also demonstrated in CRE-infected mouse model. Taken together, these results suggest that combining inhibitory activity against two critical targets in MDR Gram-negative bacteria, efflux pumps, and β-lactamases, in one molecule is possible, and the multitarget inhibitors may provide new avenues for the discovery of safe and efficient MDR reversal agents.

Nano-enabled antimicrobial thin films: design and mechanism of action

Antimicrobial thin films are types of protective coatings that are applied to surfaces such as medical devices, food packaging materials, water-resistant coatings, and other systems. These films prevent and reduce the spread of microbial organisms, including bacteria, fungi, and viruses. Antimicrobial thin films can be prepared from a variety of nanostructured materials including metal nanoparticles, metal oxides, plant materials, enzymes, bacteriocins and polymers. Their antimicrobial mechanism varies mostly based on the types of active agents from which the film is made of. Antimicrobial thin films are becoming increasingly popular microbial treatment methods due to their advantages such as enhanced stability, reduced toxicity levels, extended effectiveness over time and broad spectrum antimicrobial action without side effects on human health or the environment. This popularity and enhanced performance is mainly due to the extended possibility of film designs. Thin films offer convenient formulation methods which makes them suitable for commercial practices aiming at high turnover rates along with residential applications requiring frequent application cycles. This review focuses on recent developments in the possible processing methods and design approaches for assembling the various types of antimicrobial materials into nanostructured thin film-based delivery systems, along with mechanisms of action against microbes.

Bibliometric insights into the most influential papers on antibiotic adjuvants: a comprehensive analysis

Background: The utilization of antibiotic adjuvants presents a promising strategy for addressing bacterial resistance. Recently, the development of antibiotic adjuvants has attracted considerable attention from researchers in academia and industry. This study aimed to identify the most influential publications on antibiotic adjuvants and elucidate the hotspots and research trends in this field. Method: Original articles and reviews related to antibiotic adjuvants were retrieved from the Web of Science Core Collection database. The top 100 highly cited publications were selected and the visual analyses of publication outputs, countries, institutions, authors, journals, and keywords were conducted using Excel, VOSviewer, or CtieSpace software tools. Results: The top 100 cited publications concerning antibiotic adjuvants spanned the years 1977-2020, with citation counts ranging from 174 to 2,735. These publications encompassed 49 original articles and 51 reviews. The journal "Antimicrobial Agents and Chemotherapy" accounted for the highest number of publications (12%). The top 100 cited publications emanated from 39 countries, with the United States leading in production. Institutions in Canada and the United States exhibited the most substantial contributions to these highly cited publications. A total of 526 authors participated in these studies, with Robert E.W. Hancock, Laura J. V. Piddock, Xian-Zhi Li, Hiroshi Nikaido, and Olga Lomovskaya emerging as the most frequently nominated authors. The most common keywords included "E. coli", "P. aeruginosa", "S. aureus", "in-vitro activity", "antimicrobial peptide", "efflux pump inhibitor" "efflux pump", "MexAB-OprM" and "mechanism". These keywords underscored the hotspots of bacterial resistance mechanisms and the development of novel antibiotic adjuvants. Conclusion: Through the bibliometric analysis, this study identified the top 100 highly cited publications on antibiotic adjuvants. Moreover, the findings offered a comprehensive understanding of the characteristics and frontiers in this field.

Biocides in the Hospital Environment: Application and Tolerance Development

Hospital-acquired infections are a rising problem with consequences for patients, hospitals, and health care workers. Biocides can be employed to prevent these infections, contributing to eliminate or reduce microorganisms' concentrations at the hospital environment. These antimicrobials belong to several groups, each with distinct characteristics that need to be taken into account in their selection for specific applications. Moreover, their activity is influenced by many factors, such as compound concentration and the presence of organic matter. This article aims to review some of the chemical biocides available for hospital infection control, as well as the main factors that influence their efficacy and promote susceptibility decreases, with the purpose to contribute for reducing misusage and consequently for preventing the development of resistance to these antimicrobials.

Fighting Antimicrobial Resistance: Insights on How the Staphylococcus aureus NorA Efflux Pump Recognizes 2-Phenylquinoline Inhibitors by Supervised Molecular Dynamics (SuMD) and Molecular Docking Simulations

The superbug Staphylococcus aureus (S. aureus) exhibits several resistance mechanisms, including efflux pumps, that strongly contribute to antimicrobial resistance. In particular, the NorA efflux pump activity is associated with S. aureus resistance to fluoroquinolone antibiotics (e.g., ciprofloxacin) by promoting their active extrusion from cells. Thus, since efflux pump inhibitors (EPIs) are able to increase antibiotic concentrations in bacteria as well as restore their susceptibility to these agents, they represent a promising strategy to counteract bacterial resistance. Additionally, the very recent release of two NorA efflux pump cryo-electron microscopy (cryo-EM) structures in complex with synthetic antigen-binding fragments (Fabs) represents a real breakthrough in the study of S. aureus antibiotic resistance. In this scenario, supervised molecular dynamics (SuMD) and molecular docking experiments were combined to investigate for the first time the molecular mechanisms driving the interaction between NorA and efflux pump inhibitors (EPIs), with the ultimate goal of elucidating how the NorA efflux pump recognizes its inhibitors. The findings provide insights into the dynamic NorA-EPI intermolecular interactions and lay the groundwork for future drug discovery efforts aimed at the identification of novel molecules to fight antimicrobial resistance.

The Art of War with Pseudomonas aeruginosa: Targeting Mex Efflux Pumps Directly to Strategically Enhance Antipseudomonal Drug Efficacy

Pseudomonas aeruginosa (P. aeruginosa) poses a grave clinical challenge due to its multidrug resistance (MDR) phenotype, leading to severe and life-threatening infections. This bacterium exhibits both intrinsic resistance to various antipseudomonal agents and acquired resistance against nearly all available antibiotics, contributing to its MDR phenotype. Multiple mechanisms, including enzyme production, loss of outer membrane proteins, target mutations, and multidrug efflux systems, contribute to its antimicrobial resistance. The clinical importance of addressing MDR in P. aeruginosa is paramount, and one pivotal determinant is the resistance-nodulation-division (RND) family of drug/proton antiporters, notably the Mex efflux pumps. These pumps function as crucial defenders, reinforcing the emergence of extensively drug-resistant (XDR) and pandrug-resistant (PDR) strains, which underscores the urgency of the situation. Overcoming this challenge necessitates the exploration and development of potent efflux pump inhibitors (EPIs) to restore the efficacy of existing antipseudomonal drugs. By effectively countering or bypassing efflux activities, EPIs hold tremendous potential for restoring the antibacterial activity against P. aeruginosa and other Gram-negative pathogens. This review focuses on concurrent MDR, highlighting the clinical significance of efflux pumps, particularly the Mex efflux pumps, in driving MDR. It explores promising EPIs and delves into the structural characteristics of the MexB subunit and its substrate binding sites.

Deciphering the antimicrobial, antibiofilm and membrane stabilizing synergism of Mikania scandens (L.) Willd. leaves and stems substantiation through in vitro and in silico studies

Considering the traditional application of Mikania scandens (L.) Willd. against wounds and itching. Leaves (MSL) and stems (MSS) were sequentially extracted using solvents petroleum-ether, carbon-tetrachloride, chloroform, ethyl-acetate and ethanol. Disk-diffusion assay revealed the ethyl acetate MSL and MSS extracts were the prominent against ten bacteria, five carbapenem-resistant bacteria and one fungal strains. Subsequent quantitative antimicrobial analysis specified MSL extractives more potent over MSS with lower 1500 and 3500µg/ml MIC and MBC value in both gram-negative and positive bacteria. These sturdiest ethyl-acetate MSL extractives antimicrobial efficiency also fostered fungicidal activity having lower 100µg/ml MFC. Whereat, almost homologous 160-180 min timing noted liken to standard ciprofloxacin susceptibility in both strains, 75% biofilm inhibition at 2×MIC concentration along with 92±0.2% membrane stabilizing activities over synthetic counterparts prospected in preceding standard extractives. Computational molecular docking of MSL compounds supported this findings therefore forego this valuable synergistic insight as antimicrobial agents to efficiently eradicate human infections.

Comparative Genomic Analysis of Multi-Drug Resistant Pseudomonas aeruginosa Sequence Type 235 Isolated from Sudan

Pseudomonas aeruginosa (P. aeruginosa) is known to be associated with resistance to practically all known antibiotics. This is a cross-sectional, descriptive, laboratory-based analytical study in which 200 P. aeruginosa clinical isolates were involved. The DNA of the most resistant isolate was extracted and its whole genome was sequenced, assembled, annotated, and announced, strain typing was ascribed, and it was subjected to comparative genomic analysis with two susceptible strains. The rate of resistance was 77.89%, 25.13%, 21.61%, 18.09%, 5.53%, and 4.52% for piperacillin, gentamicin, ciprofloxacin, ceftazidime, meropenem, and polymyxin B, respectively. Eighteen percent (36) of the tested isolates exhibited a MDR phenotype. The most MDR strain belonged to epidemic sequence type 235. Comparative genomic analysis of the MDR strain (GenBank: MVDK00000000) with two susceptible strains revealed that the core genes were shared by the three genomes but there were accessory genes that were strain-specific, and this MDR genome had a low CG% (64.6%) content. A prophage sequence and one plasmid were detected in the MDR genome, but amazingly, it contained no resistant genes for drugs with antipseudomonal activity and there was no resistant island. In addition, 67 resistant genes were detected, 19 of them were found only in the MDR genome and 48 genes were efflux pumps, and a novel deleterious point mutation (D87G) was detected in the gyrA gene. The novel deleterious mutation in the gyrA gene (D87G) is a known position behind quinolone resistance. Our findings emphasize the importance of adoption of infection control strategies to prevent dissemination of MDR isolates.

Efflux, Signaling and Warfare in a Polymicrobial World

The discovery void of antimicrobial development has occurred at a time when the world has seen a rapid emergence and spread of antimicrobial resistance, the 'perfect storm' as it has often been described. While the discovery and development of new antibiotics has continued in the research sphere, the pipeline to clinic has largely been fed by derivatives of existing classes of antibiotics, each prone to pre-existing resistance mechanisms. A novel approach to infection management has come from the ecological perspective whereby microbial networks and evolved communities already possess small molecular capabilities for pathogen control. The spatiotemporal nature of microbial interactions is such that mutualism and parasitism are often two ends of the same stick. Small molecule efflux inhibitors can directly target antibiotic efflux, a primary resistance mechanism adopted by many species of bacteria and fungi. However, a much broader anti-infective capability resides within the action of these inhibitors, borne from the role of efflux in key physiological and virulence processes, including biofilm formation, toxin efflux, and stress management. Understanding how these behaviors manifest within complex polymicrobial communities is key to unlocking the full potential of the advanced repertoires of efflux inhibitors.

Antibiotic Adjuvants: A Versatile Approach to Combat Antibiotic Resistance

The problem of antibiotic resistance is on the rise, with multidrug-resistant strains emerging even to the last resort antibiotics. The drug discovery process is often stalled by stringent cut-offs required for effective drug design. In such a scenario, it is prudent to delve into the varying mechanisms of resistance to existing antibiotics and target them to improve antibiotic efficacy. Nonantibiotic compounds called antibiotic adjuvants which target bacterial resistance can be used in combination with obsolete drugs for an improved therapeutic regime. The field of "antibiotic adjuvants" has gained significant traction in recent years where mechanisms other than β-lactamase inhibition have been explored. This review discusses the multitude of acquired and inherent resistance mechanisms employed by bacteria to resist antibiotic action. The major focus of this review is how to target these resistance mechanisms by the use of antibiotic adjuvants. Different types of direct acting and indirect resistance breakers are discussed including enzyme inhibitors, efflux pump inhibitors, inhibitors of teichoic acid synthesis, and other cellular processes. The multifaceted class of membrane-targeting compounds with poly pharmacological effects and the potential of host immune-modulating compounds have also been reviewed. We conclude with providing insights about the existing challenges preventing clinical translation of different classes of adjuvants, especially membrane-perturbing compounds, and a framework about the possible directions which can be pursued to fill this gap. Antibiotic-adjuvant combinatorial therapy indeed has immense potential to be used as an upcoming orthogonal strategy to conventional antibiotic discovery.

Plant Secondary Metabolites on Efflux-Mediated Antibiotic Resistant Stenotrophomonas Maltophilia: Potential of Herbal-Derived Efflux Pump Inhibitors

During the process of adapting to metal contamination, plants produce secondary metabolites that have the potential to modulate multidrug-resistant (MDR) phenotypes; this is achieved by inhibiting the activity of efflux pumps to reduce the minimum inhibitory concentrations (MICs) of antimicrobial substrates. Our study evaluated the effect of secondary metabolites of belowground parts of Pteris vittata L. and Fallopia japonica, two metal-tolerant plants from northern Vietnam, on six antibiotic-resistant Stenotrophomonas maltophilia strains possessing efflux pump resistance mechanisms that were isolated from soil and clinical samples. The chemical composition of aqueous and dichloromethane (DCM) fractions extracted from P. vittata and F. japonica was determined using UHPLC-DAD-ESI/QTOF analysis. The antibacterial and efflux pump inhibitory activities of the four fractions were evaluated for the six strains (K279a, 0366, BurA1, BurE1, PierC1, and 502) using a microdilution assay at fraction concentrations of 62.5, 125, and 250 μg/mL. The DCM fraction of F. japonica exhibited remarkable antibacterial activity against strain 0366, with a MIC of 31.25 μg/mL. Furthermore, this fraction also significantly decreased gentamicin MIC: four-fold and eight-fold reductions for BurA1 and BurE1 strains, respectively (when tested at 250 μg/mL), and two-fold and eight-fold reductions for K279a and BurE1 strains, respectively (when tested at 125 μg/mL). Pure emodin, the main component identified in the DCM fraction of F. japonica, and sennidine A&B only reduced by half the MIC of gentamicin (when tested at 30 μg/mL). Our results suggest that the DCM fraction components of F. japonica underground parts may be potential candidates for new bacterial efflux pump inhibitors (EPIs).

The use of thymol, carvacrol and sorbic acid in microencapsules to control Salmonella Heidelberg, S. Minnesota and S. Typhimurium in broilers

Introduction

The control of Salmonella spp. in poultry involves different biosecurity actions and lately has been complicated by the emergence of multidrug resistant serovars. The application of organic acids and essential oils has been used with different approaches due to the antibacterial properties as food preservatives. The use of these molecules in animal feed to control enteric pathogens is a major interest within the poultry industry.

Methods

The use of a blend containing nature-identical compounds of sorbic acid (25%), thymol (9.5%) and carvacrol (2.5%) microencapsulated in a lipid matrix, was investigated in the present work, for the control of three Salmonella serovars (S. ser. Typhimurium, S. ser. Heidelberg and S. ser. Minnesota). Commercial broilers were challenged at 3 or at 33 days of age. Groups SH-1, SM-1 and ST-1, received treatment in the feed, at 2 kg/ton from 1-21 days of age and at 1 kg/ton from 35-42 days of age (last week), while groups SH-2, SM-2 and ST-2, were treated only during the last week receiving 2 kg/ton. Each treated group had an untreated control group, that was challenged at the same moment with the respective serovar (groups PCH, PCM and PCT). The challenge strains were enumerated in liver and cecal contents, weekly after challenge, at 7, 14, 21, 28, 35 and 42 days-of-age.

Results and discussion

Significant reduction was noticed at 7 and 14 days of age in all groups that received treatment during the initial phase (p < 0.05). Moreover, the body weight was significantly higher at the last experimental day (p < 0.05) in chickens that received treatment at the initial and at the final growth stages.

Deciphering the Role of β-Lactamase Inhibitors, Membrane Permeabilizers and Efflux Pump Inhibitors as Emerging Targets in Antibiotic Resistance

Antimicrobial drugs have been noticed to have reduce activity effective due to upsurge witnessed in resistance of microbes. To deal with viewpoint of such a circumstance, we must seek ways to prevent it or atleast mitigate its effects in order to provide its activity against the microbes. Hence, novel antimicrobials are the one of the most promising solution for ending antimicrobial resistance. Furthermore, due to the less development of newer antimicrobials in recent years, the only way to combat microbial resistance are various synergistic approaches of exploring antimicrobial drug combinations. This combination's efficacy is due to a synergistic chemical that re-sensitizes the resistant microbial strain. It has been observed that classes of β-lactamases inhibitors, efflux pump inhibitors and membrane permeabilizers are of particular relevance, since they can break resistance to the most commonly used antimicrobials. This review explains the readers that how these synergistic combinations can help to reduce or eliminate the microbial resistance supported with clinical evidence.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01045-6.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Augmenting Azoles with Drug Synergy to Expand the Antifungal Toolbox

Fungal infections impact the lives of at least 12 million people every year, killing over 1.5 million. Wide-spread use of fungicides and prophylactic antifungal therapy have driven resistance in many serious fungal pathogens, and there is an urgent need to expand the current antifungal arsenal. Recent research has focused on improving azoles, our most successful class of antifungals, by looking for synergistic interactions with secondary compounds. Synergists can co-operate with azoles by targeting steps in related pathways, or they may act on mechanisms related to resistance such as active efflux or on totally disparate pathways or processes. A variety of sources of potential synergists have been explored, including pre-existing antimicrobials, pharmaceuticals approved for other uses, bioactive natural compounds and phytochemicals, and novel synthetic compounds. Synergy can successfully widen the antifungal spectrum, decrease inhibitory dosages, reduce toxicity, and prevent the development of resistance. This review highlights the diversity of mechanisms that have been exploited for the purposes of azole synergy and demonstrates that synergy remains a promising approach for meeting the urgent need for novel antifungal strategies.

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.

Pseudomonas aeruginosa efflux pump superfamily (review of literature)

The significant increase in the number of antibiotic-resistant microorganisms observed in recent years is a public health problem worldwide. One of the molecular mechanisms for the formation of antimicrobial resistance in bacteria is the presence of efflux pumps. The review presents an analysis of experimental studies related to the study of efflux pumps in clinical strains of Pseudomonas aeruginosa, one of the representatives of hospital pathogens of the ESKAPE group. This review is intended for specialists developing new types of drugs against antibiotic-resistant strains, as well as researchers studying the mechanisms of bacterial resistance to antibiotics, heavy metals, biocides and other antimicrobial factors.

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.

Towards Advances in Medicinal Plant Antimicrobial Activity: A Review Study on Challenges and Future Perspectives

The increasing incidence of drug- resistant pathogens raises an urgent need to identify and isolate new bioactive compounds from medicinal plants using standardized modern analytical procedures. Medicinal plant-derived compounds could provide novel straightforward approaches against pathogenic bacteria. This review explores the antimicrobial activity of plant-derived components, their possible mechanisms of action, as well as their chemical potential. The focus is put on the current challenges and future perspectives surrounding medicinal plants antimicrobial activity. There are some inherent challenges regarding medicinal plant extracts and their antimicrobial efficacy. Appropriate and optimized extraction methodology plant species dependent leads to upgraded and selective extracted compounds. Antimicrobial susceptibility tests for the determination of the antimicrobial activity of plant extracts may show variations in obtained results. Moreover, there are several difficulties and problems that need to be overcome for the development of new antimicrobials from plant extracts, while efforts have been made to enhance the antimicrobial activity of chemical compounds. Research on the mechanisms of action, interplay with other substances, and the pharmacokinetic and/or pharmacodynamic profile of the medicinal plant extracts should be given high priority to characterize them as potential antimicrobial agents.

Effect of Carvacrol and Thymol on NorA efflux pump inhibition in multidrug-resistant (MDR) Staphylococcus aureus strains

Undue exposure to antimicrobials has led to the acquisition and development of sophisticated bacterial resistance mechanisms, such as efflux pumps, which are able to expel or reduce the intracellular concentration of various antibiotics, making them ineffective. Therefore, inhibiting this mechanism is a promising way to minimize the phenomenon of resistance in bacteria. In this sense, the present study sought to evaluate the activity of the Carvacrol (CAR) and Thymol (THY) terpenes as possible Efflux Pump Inhibitors (EPIs), by determining the Minimum Inhibitory Concentration (MIC) and the association of these compounds in subinhibitory concentrations with the antibiotic Norfloxacin and with Ethidium Bromide (EtBr) against strains SA-1199 (wild-type) and SA-1199B (overexpresses NorA) of Staphylococcus aureus. In order to verify the interaction of the terpenes with the NorA efflux protein, an in silico molecular modeling study was carried out. The assays used to obtain the MIC of CAR and THY were performed by broth microdilution, while the Efflux Pump inhibitory test was performed by the MIC modification method of the antibiotic Norfloxacin and EtBr. docking was performed using the Molegro Virtual Docker (MVD) program. The results of the study revealed that CAR and THY have moderate bacterial activity and are capable of reducing the MIC of Norfloxacin antibiotic and EtBr in strains of S. aureus carrying the NorA efflux pump. The docking results showed that these terpenes act as possible competitive NorA inhibitors and can be investigated as adjuvants in combined therapies aimed at reducing antibiotic resistance.

1,3,4-oxadiazole conjugates of capsaicin as potent NorA efflux pump inhibitors of Staphylococcus aureus

NorA efflux pump pertaining to the major facilitator superfamily (MFS) is known to play a key role in antibiotic and biocide resistance in Staphylococcus aureus (S. aureus). It accounts for the extrusion of antibiotics like fluoroquinolones (e.g. ciprofloxacin). Several compounds including synthetic and natural products have been identified as potential NorA efflux pump inhibitors (EPIs) and found to restore the antibacterial activity of antibiotics. However, none of the reported EPIs have reached to clinical approval probably due to their high toxicity profiles. Considering the NorA efflux pump inhibitory potential of capsaicin, a series of capsaicin-based 1,3,4 oxadiazole conjugates were prepared and evaluated for ciprofloxacin activity potentiating effect. Among the new capsaicinoids tested, 17i displayed a minimum effective concentration (MEC) of 12.5 µg/mL against NorA overexpressing S. aureus strain (SA1199B), whereas capsaicin showed MEC of 50 µg/mL. The kill kinetics curve for the combination showed that ciprofloxacin at a sub-inhibitory concentration (0.25 × MIC) was equipotent in effect, to its MIC. 17i has significantly decreased the ethidium bromide efflux confirming NorA inhibition as the mode of action. Mutation prevention concentration of the ciprofloxacin was reduced in combination with 17i.In silico studies revealed the binding efficiency and binding affinity of 17i with NorA. This compound may serve as a template for the further drug discovery.

Amino acid-derived defense metabolites from plants: A potential source to facilitate novel antimicrobial development

For millennia, humanity has relied on plants for its medicines, and modern pharmacology continues to reexamine and mine plant metabolites for novel compounds and to guide improvements in biological activity, bioavailability, and chemical stability. The critical problem of antibiotic resistance and increasing exposure to viral and parasitic diseases has spurred renewed interest into drug treatments for infectious diseases. In this context, an urgent revival of natural product discovery is globally underway with special attention directed toward the numerous and chemically diverse plant defensive compounds such as phytoalexins and phytoanticipins that combat herbivores, microbial pathogens, or competing plants. Moreover, advancements in “omics,” chemistry, and heterologous expression systems have facilitated the purification and characterization of plant metabolites and the identification of possible therapeutic targets. In this review, we describe several important amino acid–derived classes of plant defensive compounds, including antimicrobial peptides (e.g., defensins, thionins, and knottins), alkaloids, nonproteogenic amino acids, and phenylpropanoids as potential drug leads, examining their mechanisms of action, therapeutic targets, and structure–function relationships. Given their potent antibacterial, antifungal, antiparasitic, and antiviral properties, which can be superior to existing drugs, phytoalexins and phytoanticipins are an excellent resource to facilitate the rational design and development of antimicrobial drugs.

Camphor and Eucalyptol-Anticandidal Spectrum, Antivirulence Effect, Efflux Pumps Interference and Cytotoxicity

Candidaalbicans represents one of the most common fungal pathogens. Due to its increasing incidence and the poor efficacy of available antifungals, finding novel antifungal molecules is of great importance. Camphor and eucalyptol are bioactive terpenoid plant constituents and their antifungal properties have been explored previously. In this study, we examined their ability to inhibit the growth of different Candida species in suspension and biofilm, to block hyphal transition along with their impact on genes encoding for efflux pumps (CDR1 and CDR2), ergosterol biosynthesis (ERG11), and cytotoxicity to primary liver cells. Camphor showed excellent antifungal activity with a minimal inhibitory concentration of 0.125-0.35 mg/mL while eucalyptol was active in the range of 2-23 mg/mL. The results showed camphor's potential to reduce fungal virulence traits, that is, biofilm establishment and hyphae formation. On the other hand, camphor and eucalyptol treatments upregulated CDR1;CDR2 was positively regulated after eucalyptol application while camphor downregulated it. Neither had an impact on ERG11 expression. The beneficial antifungal activities of camphor were achieved with an amount that was non-toxic to porcine liver cells, making it a promising antifungal compound for future development. The antifungal concentration of eucalyptol caused cytotoxic effects and increased expression of efflux pump genes, which suggests that it is an unsuitable antifungal candidate.

The 1,8-naphthyridines sulfonamides are NorA efflux pump inhibitors

OBJECTIVE

Efflux pumps are transmembrane proteins associated with bacterial resistance mechanisms. Bacteria use these proteins to actively transport antibiotics to the extracellular medium, preventing the pharmacological action of these drugs. This study aimed to evaluate in vitro the antibacterial activity of 1,8-naphthyridines sulfonamides, as well as their ability to inhibit efflux systems of Staphylococcus aureus strains expressing different levels of the NorA efflux pump.

METHODS

The broth microdilution test was performed to assess antibacterial activity. Efflux pump inhibition was evaluated in silico by molecular docking and in vitro by fluorometric tests, and the minimum inhibitory concentration (MIC) was determined. The MIC was determined in the association between 1,8-naphthyridine and norfloxacin or ethidium bromide.

RESULTS

The 1,8-naphthyridines did not show direct antibacterial activity. However, they effectively reduced the MIC of multidrug-resistant bacteria by associating with norfloxacin and ethidium bromide, in addition to increasing the fluorescence emission. In silico analysis addressing the binding between NorA and 1,8-naphthyridines suggests that hydrogen bonds and hydrophilic interactions represent the interactions with the most favourable binding energy, corroborating the experimental data.

CONCLUSION

Our data suggest that 1,8-naphthyridines sulfonamides inhibit bacterial resistance through molecular mechanisms associated with inhibition of the NorA efflux pump in S. aureus strains.

Photodynamic Inactivation of Bacteria with Porphyrin Derivatives: Effect of Charge, Lipophilicity, ROS Generation, and Cellular Uptake on Their Biological Activity In Vitro

Resistance of microorganisms to antibiotics has led to research on various therapeutic strategies with different mechanisms of action, including photodynamic inactivation (PDI). In this work, we evaluated a cationic, neutral, and anionic meso-tetraphenylporphyrin derivative’s ability to inactivate the Gram-negative and Gram-positive bacteria in a planktonic suspension under blue light irradiation. The spectroscopic, physicochemical, redox properties, as well as reactive oxygen species (ROS) generation capacity by a set of photosensitizers varying in lipophilicity were investigated. The theoretical calculations were performed to explain the distribution of the molecular charges in the evaluated compounds. Moreover, logP partition coefficients, cellular uptake, and phototoxicity of the photosensitizers towards bacteria were determined. The role of a specific microbial efflux pump inhibitor, verapamil hydrochloride, in PDI was also studied. The results showed that E. coli exhibited higher resistance to PDI than S. aureus (3–5 logs) with low light doses (1–10 J/cm²). In turn, the prolongation of irradiation (up to 100 J/cm²) remarkably improved the inactivation of pathogens (up to 7 logs) and revealed the importance of photosensitizer photostability. The PDI potentiation occurs after the addition of KI (more than 3 logs extra killing). Verapamil increased the uptake of photosensitizers (especially in E. coli) due to efflux pump inhibition. This effect suggests that PDI is mediated by ROS, the electrostatic charge interaction, and the efflux of photosensitizers (PSs) regulated by multidrug-resistance (MDR) systems. Thus, MDR inhibition combined with PDI gives opportunities to treat more resistant bacteria.

Bark extract of Cassia sieberiana DC. (Caesalpiniaceae) displayed good antibacterial activity against MDR gram-negative phenotypes in the presence of phenylalanine-arginine β-naphthylamide

BACKGROUND

Multidrug-resistant (MDR) bacteria remain a major cause of morbidity and mortality globally. The present study was designed to investigate the in vitro antibacterial activities of crude methanol extract and constituents isolated by Column Chromatography (CC) from Cassia sieberiana bark (CSB) against ten MDR Gram-negative bacteria, as well as the mechanisms of action of the most active sample.

METHODS

The antibacterial activity of the tested samples (extract, the fractions and their compounds isolated by CC and the structures obtained by exploiting ¹H and ¹³C Nuclear magnetic resonance (NMR) spectra) in the presence and absence of an efflux pumps inhibitor, phenylalanine-arginine β-naphthylamide (PAβN), was evaluated using the micro-dilution method. The effects of the most active sample were evaluated on the cell growth kinetic and on the bacterial H⁺-ATPase proton pumps.

RESULTS

Phytochemical composition of the crude extract showed a rather selective distribution of secondary metabolites (presence of polyphenols, tannins, steroids, triterpenes, flavonoids, alkaloids, saponins and absence of anthocyanins, anthraquinones). The tested samples displayed different antibacterial activities with minimal inhibitory concentrations (MICs) ranging from 64 to 512 μg/mL. Crude extract (CS) and fraction CSc showed the highest inhibitory spectra, both inhibiting all of the studied bacteria except Enterobacter aerogenes EA27 strain. Fraction CSc exerted bactericidal effects on most bacteria meanwhile, crude extract (CS) and sub-fraction CSc2 exerted bacteriostatic effects. Compounds 1 (spectaline) and 2 (iso-6-cassine) inhibited the growth of 70% (Escherichia coli ATCC8739 and AG102, Klebsiella pneumoniae ATCC11296, Enterobacter aerogenes ATCC13048 and EA27, Providencia stuartii ATCC29916, Pseudomonas aeruginosa PA01) and 60% (Escherichia coli ATCC8739, Klebsiella pneumoniae ATCC11296 and KP55, Providencia stuartii ATCC29916, Pseudomonas aeruginosa PA01 and PA124) of bacteria respectively with MICs ranging from 128 to 512 μg/mL. In the presence of PAβN, the activities of crude extract CS, fraction CAc and sub-fraction CSc2 strongly increased on most bacteria strains as their MICs significantly decreased. Sub-fraction CSc2 inhibited the H⁺-ATPase proton pumps and altered growth kinetic of Escherichia coli ATCC8739.

CONCLUSION

The overall results justify the traditional use of C. sieberiana for the treatment of bacterial infections.

Atomistic insight and modeled elucidation of conessine towards Pseudomonas aeruginosa efflux pump

Drug-resistant Pseudomonas aeruginosa efflux pump extrudes antibiotics from cells for survival. Efflux pump inhibitor (EPI) thus becomes an interesting alternative to handle the drug-resistant bacteria. Conessine, a natural steroidal alkaloid from Holarrhena antidysenterica, previously exhibited efflux pump inhibitory potential. Our molecular docking and molecular dynamics (MD) studies provided atomistic information as well as the interaction of conessine with bacterial MexB efflux pump in phospholipid bilayer membrane to further the previous experimental report. Herein, the binding site and proposed mode of action of conessine were identified compared to known/commercial EPIs such as PAβN or designed-synthetic P9D. Our results explained conessine binding mode of action as an effective agent against the MexB efflux pump. The MD simulation also suggested that conessine was able to affect glycine loop (G-loop) flexibility, and the reduced G-loop flexibility due to conessine could hinder an antibiotics extrusion. In addition, our study suggested the conessine core structure buried in a hydrophobic region in the efflux pump similar to other known EPIs. Our finding could cope as a key for the design and development of the conessine derivative as novel EPI against P. aeruginosa.Communicated by Ramaswamy H. Sarma.

Efflux pump inhibitors: new updates

The discovery of antibiotics ought to have ended the issue of bacterial infections, but this was not the case as it has led to the evolution of various mechanisms of bacterial resistance against various antibiotics. The efflux pump remains one of the mechanisms through which organisms develop resistance against antibiotics; this is because organisms can extrude most of the clinically relevant antibiotics from the interior cell environment to the exterior environment via the efflux pumps. Efflux pumps are thought to contribute significantly to biofilm formation as highlighted by various studies. Therefore, the inhibition of these efflux pumps can be a potential way of improving the activity of antibiotics, particularly now that the discovery of novel antibiotics is becoming tedious. Efflux pump inhibitors (EPIs) are molecules that can inhibit efflux pumps; they have been considered potential therapeutic agents for rejuvenating the activity of antibiotics that have already lost their activity against bacteria. However, studies are yet to determine the specific substrates for such pumps; the effect of altered efflux activity of these pumps on biofilm formation is still being investigated. A clear knowledge of the involvement of efflux pumps in biofilm development could aid in developing new agents that can interfere with their function and help to prevent biofilms formation; thereby, improving the outcome of treatment strategies. This review focuses on the novel update of EPIs and discusses the evidence of the roles of efflux pumps in biofilm formation; the potential approaches towards overcoming the increasing problem of biofilm-based infections are also discussed.

Promising strategies for future treatment of Klebsiella pneumoniae biofilms

Klebsiella pneumoniae is a Gram-negative pathogenic bacterium that has the ability to aggregate as biofilm, representing one of the main agents in hospital infections, showing high rates of resistance to antibiotics. The K. pneumoniae biofilm aggregates are composed mainly of extracellular polysaccharides, eDNA and proteins. Besides, biofilms can attach to medical devices, such as endotracheal tubes and catheters, but are most dangerous on body surfaces. Here, we discuss the recent findings about the resistance mechanisms of K. pneumoniae biofilms, including genes and protein involved in 'classic', multidrug-resistant and hypervirulent strains, and also virulence factors. In addition, we also explore new strategies for possible treatment of these biofilms, and recently discovered molecules which may lead to future treatments.

Effect of α-Bisabolol and Its β-Cyclodextrin Complex as TetK and NorA Efflux Pump Inhibitors in Staphylococcus aureus Strains

Efflux pumps are proteins present in the plasma membrane of bacteria, which transport antibiotics and other compounds into the extracellular medium, conferring resistance. The discovery of natural efflux pump inhibitors is a promising alternative. α-Bisabolol is a sesquiterpene isolated from several plants such as Matricaria chamomilla L. and has important properties such as antibacterial and anti-inflammatory activity. Currently, the formation of inclusion complexes with β-Cyclodextrin has been used for improving the physicochemical characteristics of the host molecule. This study evaluated the effect of α-Bisabolol, in isolation and in complexation with β-Cyclodextrin, as TetK and NorA efflux pump inhibitors in Staphylococcus aureus strains. The minimum inhibitory concentration (MIC) was determined. Subsequently, inhibitory activity over the pumps was observed by an MIC reduction for the antibiotics, by using subinhibitory concentrations (MIC/8) in combination with tetracycline and norfloxacin. The MIC of the compounds was ≥1024 μg/mL. α-Bisabolol potentiated the action of tetracycline and reduced the MIC of norfloxacin to a clinically relevant concentration. The complexed substance showed synergism however, the effect of the isolated α-Bisabolol was superior to the complex. These results indicate α-Bisabolol is a potential substance to be used as an efflux pump inhibitor.

Synergistic Effects of Efflux Pump Modulators on the Azole Antifungal Susceptibility of Microsporum canis

The microbiologic and clinical resistance of dermatophytes is seldom reported, and the mechanisms associated with resistance are not well known. This study investigated the effect of efflux pump modulators (EPMs) (i.e., haloperidol HAL and promethazine PTZ) and their inhibiting activity on the minimum inhibitory concentrations of itraconazole (ITZ) and fluconazole (FLZ) against selected M. canis strains. M. canis strains with low (≤ 1 μg/ml itraconazole and < 64 μg/ml fluconazole) and high (> 1 μg/ml itraconazole and ≥ 64 μg/ml fluconazole) azole MIC values were tested using Checkerboard microdilution assay. The disk diffusion assay, the minimum fungicidal concentration and the time-kill assay were also performed in order to confirm the results of checkerboard microdilution assay. The MIC values of ITZ and FLZ of M. canis decreased in the presence of subinhibitory concentrations of HAL and PTZ, the latter being more effective with a greater increased susceptibility. Synergism was observed in all strains with high azole MICs (FICI < 0.5) and no synergism in the strains with low azole MICs. A fungicidal activity was observed after 48 h of incubation when ITZ and FLZ were tested in combination with HAL or PTZ. These results suggest that the drug efflux pumps are involved in the defense mechanisms to azole drugs in M. canis strains. The synergism might be related to an increased expression of efflux pump genes, eventually resulting in azole resistance phenomena. Complementary studies on M. canis resistance are advocated in order to investigate the molecular mechanisms of this phenomenon.

The Role of Drug Repurposing in the Development of Novel Antimicrobial Drugs: Non-Antibiotic Pharmacological Agents as Quorum Sensing-Inhibitors

Background: The emergence of multidrug-resistant organisms (MDROs) is a global public health issue, severely hindering clinicians in administering appropriate antimicrobial therapy. Drug repurposing is a drug development strategy, during which new pharmacological applications are identified for already approved drugs. From the viewpoint of the development of virulence inhibitors, inhibition of quorum sensing (QS) is a promising route because various important features in bacterial physiology and virulence are mediated by QS-dependent gene expression. Methods: Forty-five pharmacological agents, encompassing a wide variety of different chemical structures and mechanisms of action, were tested during our experiments. The antibacterial activity of the compounds was tested using the broth microdilution method. Screening and semi-quantitative assessment of QS-inhibition by the compounds was performed using QS-signal molecule-producing and indicator strains. Results: Fourteen pharmaceutical agents showed antibacterial activity in the tested concentration range, while eight drugs (namely 5-fluorouracil, metamizole-sodium, cisplatin, methotrexate, bleomycin, promethazine, chlorpromazine, and thioridazine) showed dose-dependent QS-inhibitory activity in the in vitro model systems applied during the experiments. Conclusions: Virulence inhibitors represent an attractive alternative strategy to combat bacterial pathogens more efficiently. Some of the tested compounds could be considered potential QS-inhibitory agents, warranting further experiments involving additional model systems to establish the extent of their efficacy.

Identification of AcrAB-TolC Efflux Pump Genes and Detection of Mutation in Efflux Repressor AcrR from Omeprazole Responsive Multidrug-Resistant Escherichia coli Isolates Causing Urinary Tract Infections

Antimicrobial resistance poses a threat in the treatment of infectious diseases in Bangladesh as well as in the world. Multidrug-resistant (MDR) Enterobacteriaceae, the most common cause of one such infectious disease, urinary tract infection (UTI), has contributed to the escalating problem of selecting empiric antibiotics against UTIs. The aim of this study was to investigate the presence of the efflux pump in MDR Escherichia coli isolates from UTI in the North-East region of Bangladesh, to isolate and characterize the AcrAB-TolC efflux pump genes of these locally isolated strains and to do mutation analysis of the efflux pump repressor AcrR gene to understand the AcrAB-TolC efflux pump mechanism. In the presence of omeprazole, an efflux pump inhibitor, every MDR E. coli isolate showed increased susceptibility to at least 1 of the 7 antibiotics investigated, indicating that efflux pump might be involved in their antibiotic resistance. Omeprazole decreased the minimum inhibitory concentration of every antibiotics being investigated by 2- to 8-fold. DNA and the deduced amino acid sequences of the polymerase chain reaction (PCR) products analyzed by bioinformatics tools revealed that the chromosomal AcrAB-TolC and AcrR genes were present in all MDR and antibiotic-susceptible E. coli isolates. However, the deduced amino acid sequences of the amplification refractory mutation system (ARMS) PCR product of the AcrR gene revealed that the substitution of arginine to cysteine at position 45 of AcrR was observed only in the MDR E. coli whose antibiotic susceptibility increased in the presence of omeprazole. Data reported herein support the notion that the increased antibiotic susceptibility of the MDR E. coli isolates in the presence of omeprazole might be due to efflux pump(s) inhibition and the AcrAB-TolC efflux pump might be a contributor to antibiotic resistance when the mutation of arginine to cysteine occurs at position 45 of AcrR.

Strategies to Overcome Antimicrobial Resistance (AMR) Making Use of Non-Essential Target Inhibitors: A Review

Antibiotics have always been considered as one of the most relevant discoveries of the twentieth century. Unfortunately, the dawn of the antibiotic era has sadly corresponded to the rise of the phenomenon of antimicrobial resistance (AMR), which is a natural process whereby microbes evolve in such a way to withstand the action of drugs. In this context, the identification of new potential antimicrobial targets and/or the identification of new chemical entities as antimicrobial drugs are in great demand. To date, among the many possible approaches used to deal with antibiotic resistance is the use of antibiotic adjuvants that hit bacterial non-essential targets. In this review, the author focuses on the discovery of antibiotic adjuvants and on new tools to study and reduce the prevalence of resistant bacterial infections.

Synergistic activity of filtrates of Lactobacillus rhamnosus and Saccharomyces boulardii and antibacterial preparations against Corynebacterium spp

We present the results of the first study of the combined influence of the biologically active substances Lactobacillus rhamnosus GG ATCC 53103 and Saccharomyces boulardii, obtained by the author’s method, and antibacterial agents on Corynebacterium spp. The first area of research was the study of increasing the sensitivity of toxigenic microorganisms to antimicrobial drugs due to the consecutive effects of the structural components and metabolites of L. rhamnosus GG and S. boulardii and antibacterial drugs on Corynebacterium spp. tox+. The greatest increase in the sensitivity of test-cultures of corynebacteria to penicillin (by 19.4 mm), imipenem (by 15.0 mm), vancomycin (by 12.0 mm), gentamicin (by 11.0 mm), ciprofloxacin (by 9.8 mm), erythromycin (by 9.6 mm), cefotaxime (by 9.5 mm) occurred due to the products of lactobacteria and a combination of metabolites of lactobacteria and saccharomycetes. The second area of research was the study of the synergic activity of substances L. rhamnosus GG and S. boulardii and traditional antibacterial drugs manifested by their simultaneous effect on Corynebacterium spp. Maximum potentiation of azithromycin (by 4.6 mm), erythromycin (by 4.5 mm), cefotaxime (by 2.2 mm), ceftriaxone (by 1.6 mm) and ampicillin (by 1.0 mm) relative to corynebacteria was also observed under the influence of lactobacteria metabolites and a combination of lactobacteria and saccharomycetes metabolites. Different degrees of manifestation of the combined action of biologically active substances L. rhamnosus GG and S. boulardii with antibiotics were determined, which depended on the selected combinations, the method of influence on the microorganism, the individual sensitivity of the test-cultures, the activity of the test filtrates and the initial concentration of the producers used to obtain the products of vital activity of lactobacteria and saccharomyces. The presented complexes of structural components and metabolites of L. rhamnosus GG and S. boulardii, obtained without the use of traditional nutrient media, by increasing the bioavailability of pathogenic pathogens can reduce the required concentration of the antibiotic, continuing their use, and suspend the likelihood of pathogens developing resistance to microorganisms. This makes them promising candidates both for the development of "accompaniment-preparations" for antibiotics for the additional therapy of infectious diseases of different etiology, and for the creation of a new direction of antimicrobial agents with multifunctional capabilities. Synergistic activity of filtrates L. rhamnosus GG and S. boulardii and antibacterial preparations against Corynebacterium spp.

Involvement of multiple influx and efflux transporters in the accumulation of cationic fluorescent dyes by Escherichia coli

Background

It is widely believed that most xenobiotics cross biomembranes by diffusing through the phospholipid bilayer, and that the use of protein transporters is an occasional adjunct. According to an alternative view, phospholipid bilayer transport is negligible, and several different transporters may be involved in the uptake of an individual molecular type. We recognise here that the availability of gene knockout collections allows one to assess the contributions of all potential transporters, and flow cytometry based on fluorescence provides a convenient high-throughput assay for xenobiotic uptake in individual cells.

Results

We used high-throughput flow cytometry to assess the ability of individual gene knockout strains of E coli to take up two membrane-permeable, cationic fluorescent dyes, namely the carbocyanine diS-C3(5) and the DNA dye SYBR Green. Individual strains showed a large range of distributions of uptake. The range of modal steady-state uptakes for the carbocyanine between the different strains was 36-fold. Knockouts of the ATP synthase α- and β-subunits greatly inhibited uptake, implying that most uptake was ATP-driven rather than being driven by a membrane potential. Dozens of transporters changed the steady-state uptake of the dye by more than 50% with respect to that of the wild type, in either direction (increased or decreased); knockouts of known influx and efflux transporters behaved as expected, giving credence to the general strategy. Many of the knockouts with the most reduced uptake were transporter genes of unknown function (‘y-genes’). Similarly, several overexpression variants in the ‘ASKA’ collection had the anticipated, opposite effects. Similar results were obtained with SYBR Green (the range being approximately 69-fold). Although it too contains a benzothiazole motif there was negligible correlation between its uptake and that of the carbocyanine when compared across the various strains (although the membrane potential is presumably the same in each case).

Conclusions

Overall, we conclude that the uptake of these dyes may be catalysed by a great many transporters of putatively broad and presently unknown specificity, and that the very large range between the ‘lowest’ and the ‘highest’ levels of uptake, even in knockouts of just single genes, implies strongly that phospholipid bilayer transport is indeed negligible. This work also casts serious doubt upon the use of such dyes as quantitative stains for representing either bioenergetic parameters or the amount of cellular DNA in unfixed cells (in vivo). By contrast, it opens up their potential use as transporter assay substrates in high-throughput screening.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1561-0) contains supplementary material, which is available to authorized users.

Review on plant antimicrobials: a mechanistic viewpoint

Microbial resistance to classical antibiotics and its rapid progression have raised serious concern in the treatment of infectious diseases. Recently, many studies have been directed towards finding promising solutions to overcome these problems. Phytochemicals have exerted potential antibacterial activities against sensitive and resistant pathogens via different mechanisms of action. In this review, we have summarized the main antibiotic resistance mechanisms of bacteria and also discussed how phytochemicals belonging to different chemical classes could reverse the antibiotic resistance. Next to containing direct antimicrobial activities, some of them have exerted in vitro synergistic effects when being combined with conventional antibiotics. Considering these facts, it could be stated that phytochemicals represent a valuable source of bioactive compounds with potent antimicrobial activities.

Antibiotic Adjuvants: Make Antibiotics Great Again!

Multiple approaches have been developed to combat bacterial resistance. However, the combination of antibiotic resistance mechanisms by bacteria and the limited number of effective antibiotics available decreases the effective interventions for the treatment of current bacterial infections. This review covers the many ways that bacteria resist antibiotics including antibiotic target modification, the use of efflux pumps, and antibiotic inactivation. As a pertinent example, the use of beta lactamase inhibitors in combination with β-lactam containing antibiotics is discussed in detail. The solution to emerging antibiotic resistance may involve combination therapies of existing antibiotics and potentiating adjuvants, which re-empower the antibiotic agent to become efficacious against the resistant strain of interest. We report herein that a reasoned adjuvant design permits one to perform polypharmacy on bacteria by not only providing greater internal access to the codosed antibiotics but also by de-energizing the efflux pumps used by the bacteria to escape antibiotic action.

The Continuing Threat of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus has been an exceptionally successful pathogen, which is still relevant in modern age-medicine due to its adaptability and tenacity. This bacterium may be a causative agent in a plethora of infections, owing to its abundance (in the environment and in the normal flora) and the variety of virulence factors that it possesses. Methicillin-resistant S. aureus (MRSA) strains—first described in 1961—are characterized by an altered penicillin-binding protein (PBP2a/c) and resistance to all penicillins, cephalosporins, and carbapenems, which makes the β-lactam armamentarium clinically ineffective. The acquisition of additional resistance determinants further complicates their eradication; therefore, MRSA can be considered as the first representative of multidrug-resistant bacteria. Based on 230 references, the aim of this review is to recap the history, the emergence, and clinical features of various MRSA infections (hospital-, community-, and livestock-associated), and to summarize the current advances regarding MRSA screening, typing, and therapeutic options (including lipoglycopeptides, oxazolidinones, anti-MRSA cephalosporins, novel pleuromutilin-, tetracycline- and quinolone-derivatives, daptomycin, fusidic acid, in addition to drug candidates in the development phase), both for an audience of clinical microbiologists and infectious disease specialists.

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.

A permeability-increasing drug synergizes with bacterial efflux pump inhibitors and restores susceptibility to antibiotics in multi-drug resistant Pseudomonas aeruginosa strains

Resistance to antibiotics poses a major global threat according to the World Health Organization. Restoring the activity of existing drugs is an attractive alternative to address this challenge. One of the most efficient mechanisms of bacterial resistance involves the expression of efflux pump systems capable of expelling antibiotics from the cell. Although there are efflux pump inhibitors (EPIs) available, these molecules are toxic for humans. We hypothesized that permeability-increasing antimicrobial peptides (AMPs) could lower the amount of EPI necessary to sensitize bacteria to antibiotics that are efflux substrates. To test this hypothesis, we measured the ability of polymyxin B nonapeptide (PMBN), to synergize with antibiotics in the presence of EPIs. Assays were performed using planktonic and biofilm-forming cells of Pseudomonas aeruginosa strains overexpressing the MexAB-OprM efflux system. Synergy between PMBN and EPIs boosted azithromycin activity by a factor of 2,133 and sensitized P. aeruginosa to all tested antibiotics. This reduced several orders of magnitude the amount of inhibitor needed for antibiotic sensitization. The selected antibiotic-EPI-PMBN combination caused a 10 million-fold reduction in the viability of biofilm forming cells. We proved that AMPs can synergize with EPIs and that this phenomenon can be exploited to sensitize bacteria to antibiotics.

The Concept of an Ideal Antibiotic: Implications for Drug Design

The emergence and spread of antibiotic-resistant pathogens is a major public health issue, which requires global action of an intersectoral nature. Multidrug-resistant (MDR) pathogens-especially "ESKAPE" bacteria-can withstand lethal doses of antibiotics with various chemical structures and mechanisms of action. Pharmaceutical companies are increasingly turning away from participating in the development of new antibiotics, due to the regulatory environment and the financial risks. There is an urgent need for innovation in antibiotic research, as classical discovery platforms (e.g., mining soil Streptomycetes) are no longer viable options. In addition to discovery platforms, a concept of an ideal antibiotic should be postulated, to act as a blueprint for future drugs, and to aid researchers, pharmaceutical companies, and relevant stakeholders in selecting lead compounds. Based on 150 references, the aim of this review is to summarize current advances regarding the challenges of antibiotic drug discovery and the specific attributes of an ideal antibacterial drug (a prodrug or generally reactive compound with no specific target, broad-spectrum antibacterial activity, adequate penetration through the Gram-negative cell wall, activity in biofilms and in hard-to-treat infections, accumulation in macrophages, availability for oral administration, and for use in sensitive patient groups).

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.

Identification and Characterization of Menadione and Benzethonium Chloride as Potential Treatments of Pierce's Disease of Grapevines

Xylella fastidiosa infects a wide range of plant hosts and causes Pierce's disease (PD) of grapevines. The type 1 multidrug resistance (MDR) efflux system is essential for pathogenicity and survival of bacterial pathogens in planta. X. fastidiosa, with a single MDR system, is significantly more vulnerable to inhibition by small-molecule treatments than most bacterial pathogens that typically carry redundant MDR systems. A high-throughput cell viability assay using a green fluorescent protein-marked strain of X. fastidiosa Temecula 1 was developed to screen two Prestwick combinatorial small-molecule libraries of drugs and phytochemicals (1,600 chemicals in total) approved by the Food and Drug Administration and European Medicines Agency for cell growth inhibition. The screens revealed 215 chemicals that inhibited bacterial growth by >50% at 50 µM concentrations. Seven chemicals proved to lyse X. fastidiosa cells at 25 µM, including four phytochemicals. Menadione (2-methyl-1,4-naphthoquinone, vitamin K) from the phytochemical library and benzethonium chloride (a topical disinfectant) from the chemical library both showed significant bactericidal activity against X. fastidiosa. Both menadione and benzethonium chloride foliar spray (15 and 5 mM, respectively) and soil drench (5 and 25 mM, respectively) treatments were equally effective in reducing PD symptoms by 54 to 59% and revealed that the effects of both chemical treatments became systemic. However, menadione was phytotoxic when applied as a foliar spray at effective concentrations, causing significant loss of photosynthetic capacity.

A Protein Complex from Human Milk Enhances the Activity of Antibiotics and Drugs against Mycobacterium tuberculosis

Mycobacterium tuberculosis, the causative agent of human tuberculosis (TB), has surpassed HIV/AIDS as the leading cause of death from a single infectious agent. The increasing occurrence of drug-resistant strains has become a major challenge for health care systems and, in some cases, has rendered TB untreatable. However, the development of new TB drugs has been plagued with high failure rates and costs. Alternative strategies to increase the efficacy of current TB treatment regimens include host-directed therapies or agents that make M. tuberculosis more susceptible to existing TB drugs. In this study, we show that HAMLET, an α-lactalbumin-oleic acid complex derived from human milk, has bactericidal activity against M. tuberculosis HAMLET consists of a micellar oleic acid core surrounded by a shell of partially denatured α-lactalbumin molecules and unloads oleic acid into cells upon contact with lipid membranes. At sublethal concentrations, HAMLET potentiated a remarkably broad array of TB drugs and antibiotics against M. tuberculosis For example, the minimal inhibitory concentrations of rifampin, bedaquiline, delamanid, and clarithromycin were decreased by 8- to 16-fold. HAMLET also killed M. tuberculosis and enhanced the efficacy of TB drugs inside macrophages, a natural habitat of M. tuberculosis Previous studies showed that HAMLET is stable after oral delivery in mice and nontoxic in humans and that it is possible to package hydrophobic compounds in the oleic acid core of HAMLET to increase their solubility and metabolic stability. The potential of HAMLET and other liprotides as drug delivery and sensitization agents in TB chemotherapy is discussed here.

The Quest for Novel Antimicrobial Compounds: Emerging Trends in Research, Development, and Technologies

Pathogenic antibiotic resistant bacteria pose one of the most important health challenges of the 21st century. The overuse and abuse of antibiotics coupled with the natural evolutionary processes of bacteria has led to this crisis. Only incremental advances in antibiotic development have occurred over the last 30 years. Novel classes of molecules, such as engineered antibodies, antibiotic enhancers, siderophore conjugates, engineered phages, photo-switchable antibiotics, and genome editing facilitated by the CRISPR/Cas system, are providing new avenues to facilitate the development of antimicrobial therapies. The informatics revolution is transforming research and development efforts to discover novel antibiotics. The explosion of nanotechnology and micro-engineering is driving the invention of antimicrobial materials, enabling the cultivation of "uncultivable" microbes and creating specific and rapid diagnostic technologies. Finally, a revival in the ecological aspects of microbial disease management, the growth of prebiotics, and integrated management based on the "One Health" model, provide additional avenues to manage this health crisis. These, and future scientific and technological developments, must be coupled and aligned with sound policy and public awareness to address the risks posed by rising antibiotic resistance.

In vitro effects of promethazine on cell morphology and structure and mitochondrial activity of azole-resistant Candida tropicalis

The aim of this study was to evaluate the effect of promethazine on the antifungal minimum inhibitory concentrations against planktonic cells and mature biofilms of Candida tropicalis, as well as investigate its potential mechanisms of cell damage against this yeast species. Three C. tropicalis isolates (two azole-resistant and one azole-susceptible) were evaluated for their planktonic and biofilm susceptibility to promethazine alone and in combination with itraconazole, fluconazole, voriconazole, amphotericin B, and caspofungin. The antifungal activity of promethazine against C. tropicalis was investigated by performing time-kill curve assays and assessing rhodamine 6G efflux, cell size/granularity, membrane integrity, and mitochondrial transmembrane potential, through flow cytometry. Promethazine showed antifungal activity against planktonic cells and biofilms at concentrations of 64 and 128 μg/ml, respectively. The addition of two subinhibitory concentrations of promethazine reduced the antifungal MICs for all tested azole drugs against planktonic growth, reversing the resistance phenotype to all azoles. Promethazine decreased the efflux of rhodamine 6G in an azole-resistant strain. Moreover, promethazine decreased cell size/granularity and caused membrane damage, and mitochondrial membrane depolarization. In conclusion, promethazine presented synergy with azole antifungals against resistant C. tropicalis and exhibited in vitro cytotoxicity against C. tropicalis, altering cell size/granularity, membrane integrity, and mitochondrial function, demonstrating potential mechanisms of cell damage against this yeast species.

Steroidal alkaloids and conessine from the medicinal plant Holarrhena antidysenterica restore antibiotic efficacy in a Galleria mellonella model of multidrug-resistant Pseudomonas aeruginosa infection

Background

This study aimed to evaluate the efficacy of combinations of steroidal alkaloids and conessine from the Thai medicinal plant Holarrhena antidysenterica with antibiotics against Pseudomonas aeruginosa strains possessing different efflux-pump-mediated multidrug-resistant (MDR) phenotypes in a Galleria mellonella infection model.

Methods

P. aeruginosa strains with defined mutations that result in the overexpression of the MexAB-OprM, MexCD-OprJ and MexEF-OprN efflux pumps, and a strain with all three of these pumps deleted, were used. In vitro, the effect of combinations of steroidal alkaloids and conessine with antibiotics was compared with antibiotic treatment alone via MIC determination and time-kill assays. Efficacy of combinations of the steroidal alkaloids and conessine with levofloxacin were compared with monotherapies against infections in G. mellonella larvae by measuring larval mortality and bacterial burden.

Results

Combination therapies of conessine or steroidal alkaloids with levofloxacin enhanced bacterial inhibition in vitro and restored antibiotic efficacy in vivo compared to the constituent monotherapies. Neither conessine nor the steroidal alkaloids induced any detectable toxicity in G. mellonella larvae. The enhanced efficacy of the combination treatments was most pronounced with conessine and correlated with reduced larval burden of infecting P. aeruginosa. Notably, the enhanced efficacy of conessine/levofloxacin combinations was only detected in the parent strain and strains that overexpressed the MexAB-OprM or MexEF-OprN efflux systems.

Conclusions

Steroidal alkaloids from Holarrhena antidysenterica, and particularly the principal active ingredient conessine, restored levofloxacin efficacy against resistant P. aeruginosa strains possessing efflux-mediated MDR phenotypes. The compounds should be investigated further as a potential novel therapy.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2348-9) contains supplementary material, which is available to authorized users.