The Impact of Efflux Pump Inhibitors on the Activity of Selected Non-Antibiotic Medicinal Products against Gram-Negative Bacteria

Exploiting thiol-functionalized benzosiloxaboroles for achieving diverse substitution patterns - synthesis, characterization and biological evaluation of promising antibacterial agents

Benzosiloxaboroles are an emerging class of medicinal agents possessing promising antimicrobial activity. Herein, the expedient synthesis of two novel thiol-functionalized benzosiloxaboroles 1e and 2e is reported. The presence of the SH group allowed for diverse structural modifications involving the thiol-Michael addition, oxidation, as well as nucleophilic substitution giving rise to a series of 27 new benzosiloxaboroles containing various polar functional groups, e.g., carbonyl, ester, amide, imide, nitrile, sulfonyl and sulfonamide, and pendant heterocyclic rings. The activity of the obtained compounds against selected bacterial and yeast strains, including multidrug-resistant clinical strains, was investigated. Compounds 6, 12, 20 and 22-24 show high activity against Staphylococcus aureus, including both methicillin-sensitive (MSSA) and methicillin-resistant (MRSA) strains, with MIC values in the range of 1.56-12.5 μg mL-1, while their cytotoxicity is relatively low. The in vitro assay performed with 2-(phenylsulfonyl)ethylthio derivative 20 revealed that, in contrast to the majority of known antibacterial oxaboroles, the plausible mechanism of antibacterial action, involving inhibition of the leucyl-tRNA synthetase enzyme, is not responsible for the antibacterial activity. Structural bioinformatic analysis involving molecular dynamics simulations provided a possible explanation for this finding.

Do NSAIDs and Other Pain Relief Drugs Can Inhibit the Growth of Lactobacillaceae?

Non-steroidal anti-inflammatory drugs (NSAIDs) commonly used in clinical practice may cause gastrointestinal injuries and influence the gut microbiota. This study investigated the effects of various NSAIDs and some analgesics on the viability of Lactobacillaceae strains (including probiotic strains) in vitro. It was found that diclofenac, ibuprofen, ketoprofen, dexketoprofen, flurbiprofen, and acetylsalicylic acid inhibited the growth of lactobacilli at a concentration of 0.05-3.2 mg/ml. These MICs of NSAIDs are well above therapeutic plasma concentrations achieved in humans, indicating that the tested drugs should not inhibit the growth of lactobacilli in the human digestive tract.

Aromatic Diboronic Acids as Effective KPC/AmpC Inhibitors

Over 30 compounds, including para-, meta-, and ortho-phenylenediboronic acids, ortho-substituted phenylboronic acids, benzenetriboronic acids, di- and triboronated thiophenes, and pyridine derivatives were investigated as potential β-lactamase inhibitors. The highest activity against KPC-type carbapenemases was found for ortho-phenylenediboronic acid 3a, which at the concentration of 8/4 mg/L reduced carbapenems' MICs up to 16/8-fold, respectively. Checkerboard assays revealed strong synergy between carbapenems and 3a with the fractional inhibitory concentrations indices of 0.1-0.32. The nitrocefin hydrolysis test and the whole cell assay with E. coli DH5α transformant carrying blaKPC-3 proved KPC enzyme being its molecular target. para-Phenylenediboronic acids efficiently potentiated carbapenems against KPC-producers and ceftazidime against AmpC-producers, whereas meta-phenylenediboronic acids enhanced only ceftazidime activity against the latter ones. Finally, the statistical analysis confirmed that ortho-phenylenediboronic acids act synergistically with carbapenems significantly stronger than other groups. Since the obtained phenylenediboronic compounds are not toxic to MRC-5 human fibroblasts at the tested concentrations, they can be considered promising scaffolds for the future development of novel KPC/AmpC inhibitors. The complexation of KPC-2 with the most representative isomeric phenylenediboronic acids 1a, 2a, and 3a was modeled by quantum mechanics/molecular mechanics calculations. Compound 3a reached the most effective configuration enabling covalent binding to the catalytic Ser70 residue.

Synergistic combination of carvedilol, amlodipine, amitriptyline, and antibiotics as an alternative treatment approach for the susceptible and multidrug-resistant A. baumannii infections via drug repurposing

We evaluated in vitro activity of 13 drugs used in the treatment of some non-communicable diseases via repurposing to determine their potential use in the treatment of Acinetobacter baumannii infections caused by susceptible and multidrug-resistant strains. A. baumannii is a multidrug-resistant Gram-negative bacteria causing nosocomial infections, especially in intensive care units. It has been identified in the WHO critical pathogen list and this emphasises urgent need for new treatment options. As the development of new therapeutics is expensive and time consuming, finding new uses of existing drugs via drug repositioning has been favoured. Antimicrobial susceptibility tests were conducted on all 13 drugs according to CLSI. Drugs with MIC values below 128 μg/mL and control antibiotics were further subjected to synergetic effect and bacterial time-kill analysis. Carvedilol-gentamicin (FICI 0.2813) and carvedilol-amlodipine (FICI 0.5625) were determined to have synergetic and additive effect, respectively, on the susceptible A. baumannii strain, and amlodipine-tetracycline (FICI 0.75) and amitriptyline-tetracycline (FICI 0.75) to have additive effect on the multidrug-resistant A. baumannii strain. Most remarkably, both amlodipine and amitriptyline reduced the MIC of multidrug-resistant, including some carbapenems, A. baumannii reference antibiotic tetracycline from 2 to 0.5 μg/mL, for 4-folds. All these results were further supported by bacterial time-kill assay and all combinations showed bactericidal activity, at certain hours, at 4XMIC. Combinations proposed in this study may provide treatment options for both susceptible and multidrug-resistant A. baumannii infections but requires further pharmacokinetics and pharmacodynamics analyses and in vivo re-evaluations using appropriate models.

Efflux-Related Carbapenem Resistance in Acinetobacter baumannii Is Associated with Two-Component Regulatory Efflux Systems' Alteration and Insertion of ΔAbaR25-Type Island Fragment

The efflux pumps, beside the class D carbapenem-hydrolysing enzymes (CHLDs), are being increasingly investigated as a mechanism of carbapenem resistance in Acinetobacter baumannii. This study investigates the contribution of efflux mechanism to carbapenem resistance in 61 acquired bla_CHDL-genes-carrying A. baumannii clinical strains isolated in Warsaw, Poland. Studies were conducted using phenotypic (susceptibility testing to carbapenems ± efflux pump inhibitors (EPIs)) and molecular (determining expression levels of efflux operon with regulatory-gene and whole genome sequencing (WGS)) methods. EPIs reduced carbapenem resistance of 14/61 isolates. Upregulation (5-67-fold) of adeB was observed together with mutations in the sequences of AdeRS local and of BaeS global regulators in all 15 selected isolates. Long-read WGS of isolate no. AB96 revealed the presence of AbaR25 resistance island and its two disrupted elements: the first contained a duplicate ISAba1-bla_OXA-23, and the second was located between adeR and adeA in the efflux operon. This insert was flanked by two copies of ISAba1, and one of them provides a strong promoter for adeABC, elevating the adeB expression levels. Our study for the first time reports the involvement of the insertion of the ΔAbaR25-type resistance island fragment with ISAba1 element upstream the efflux operon in the carbapenem resistance of A. baumannii.

TCA and SSRI Antidepressants Exert Selection Pressure for Efflux-Dependent Antibiotic Resistance Mechanisms in Escherichia coli

Microbial diversity is reduced in the gut microbiota of animals and humans treated with selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs). The mechanisms driving the changes in microbial composition, while largely unknown, is critical to understand considering that the gut microbiota plays important roles in drug metabolism and brain function. Using Escherichia coli, we show that the SSRI fluoxetine and the TCA amitriptyline exert strong selection pressure for enhanced efflux activity of the AcrAB-TolC pump, a member of the resistance-nodulation-cell division (RND) superfamily of transporters. Sequencing spontaneous fluoxetine- and amitriptyline-resistant mutants revealed mutations in marR and lon, negative regulators of AcrAB-TolC expression. In line with the broad specificity of AcrAB-TolC pumps these mutants conferred resistance to several classes of antibiotics. We show that the converse also occurs, as spontaneous chloramphenicol-resistant mutants displayed cross-resistance to SSRIs and TCAs. Chemical-genomic screens identified deletions in marR and lon, confirming the results observed for the spontaneous resistant mutants. In addition, deletions in 35 genes with no known role in drug resistance were identified that conferred cross-resistance to antibiotics and several displayed enhanced efflux activities. These results indicate that combinations of specific antidepressants and antibiotics may have important effects when both are used simultaneously or successively as they can impose selection for common mechanisms of resistance. Our work suggests that selection for enhanced efflux activities is an important factor to consider in understanding the microbial diversity changes associated with antidepressant treatments. IMPORTANCE Antidepressants are prescribed broadly for psychiatric conditions to alter neuronal levels of synaptic neurotransmitters such as serotonin and norepinephrine. Two categories of antidepressants are selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs); both are among the most prescribed drugs in the United States. While it is well-established that antidepressants inhibit reuptake of neurotransmitters there is evidence that they also impact microbial diversity in the gastrointestinal tract. However, the mechanisms and therefore biological and clinical effects remain obscure. We demonstrate antidepressants may influence microbial diversity through strong selection for mutant bacteria with increased AcrAB-TolC activity, an efflux pump that removes antibiotics from cells. Furthermore, we identify a new group of genes that contribute to cross-resistance between antidepressants and antibiotics, several act by regulating efflux activity, underscoring overlapping mechanisms. Overall, this work provides new insights into bacterial responses to antidepressants important for understanding antidepressant treatment effects.

Antimicrobial Activity of Azithromycin Encapsulated into PLGA NPs: A Potential Strategy to Overcome Efflux Resistance

Antimicrobial resistance represents a public health problem with a major negative impact on health and socioeconomic development, and is one of the biggest threats in the modern era. This requires the discovery of new approaches to control microbial infections. Nanomedicine could be one of the promising strategies to improve the treatment of microbial infections. Polymer nanoparticles (PNPs) were reported to overcome the efflux-resistant mechanism toward chemotherapeutic agents. However, to the best of our knowledge, no studies were performed to explore their ability to overcome the efflux-resistant mechanism in bacteria. In the current study, azithromycin (AZI), a macrolide antibiotic, was encapsulated into a biocompatible polymer, poly (lactic-co-glycolic acid) (PLGA) using the nano-precipitation method. The effect of the drug to polymer ratio, surfactant, and pH of the aqueous medium on particle size and drug loading percentage (DL%) were investigated in order to maximize the DL% and control the size of NPs to be around 100 nm. The antibacterial activity of AZI-PLGA NPs was investigated against AZI-resistant bacteria; Methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus faecalis (E. faecalis), where the efflux mechanism was demonstrated to be one of the resistant mechanisms. AZI-PLGA NPs were safer than free AZI, as revealed from the cytotoxicity test, and were able to overcome the efflux-resistant mechanism, as revealed by decreasing the MIC of AZI-PLGA NPs by four times than free AZI. The MIC value reduced from 256 to 64 µg/mL and from >1000 to 256 µg/mL for MRSA and E. faecalis, respectively. Therefore, encapsulation of AZI into PNPs was shown to be a promising strategy to overcome the efflux-resistant mechanism towards AZI and improve its antibacterial effect. However, future investigations are necessary to explore the effect (if any) of particle size, surface charge, and material composition of PNPs on antibacterial activity. Moreover, it is essential to ascertain the safety profiles of these PNPs, the possibility of their large-scale manufacture, and if this concept could be extended to other antibiotics.

Oxazoline scaffold in synthesis of benzosiloxaboroles and related ring-expanded heterocycles: diverse reactivity, structural peculiarities and antimicrobial activity

Two isomeric benzosiloxaborole derivatives 3a and 5a bearing fluorine and 4,4-dimethyl-2-oxazolin-2-yl substituents attached to the aromatic rings were obtained. Both compounds were prone to hydrolytic cleavage of the oxazoline ring after initial protonation or methylation of the nitrogen atom. The derivative 3c featuring N-methylammoniumalkyl ester functionality was successfully subjected to N-sulfonylation and N-acylation reactions to give respective derivatives which demonstrates its potential for modular synthesis of structurally extended benzosiloxaboroles. Compound 5c bearing N-ammoniumalkyl ester underwent conversion to a unique macrocyclic dimer due to siloxaborole ring opening. Furthermore, an unexpected 4-electron reduction of the oxazoline ring occurred during an attempted synthesis of 5a. The reaction gave rise to an unprecedented 7-membered heterocyclic system 4a comprising a relatively stable B-O-B-O-Si linkage and stabilized by an intramolecular N-B coordination. It could be cleaved to derivative 4c bearing BOH and SiMe2OH groups which acts as a pseudo-diol as demonstrated by formation of an adduct with Tavaborole. Apart from the multinuclear NMR spectroscopy characterization, crystal structures of the obtained products were determined in many cases by X-ray diffraction. Investigation of biological activity of the obtained compounds revealed that derivatives 3e and 3f with pendant N-methyl arylsulfonamide groups exhibit high activity against Gram-positive cocci such as methicillin-sensitive Staphylococcus aureus ATCC 6538P, methicillin-resistant S. aureus (MRSA) ATCC 43300 as well as the MRSA clinical strains, with MIC values in the range of 3.12-6.25 mg L-1. These two compounds also showed activity against Enterococcus faecalis ATCC 29212 and Enterococcus faecium ATCC 6057 (with MICs of 25-50 mg L-1). The results of the antimicrobial activity and cytotoxicity studies indicate that 3e and 3f can be considered as potential antibacterial agents, especially against S. aureus MRSA.

The Contribution of Efflux Systems to Levofloxacin Resistance in Stenotrophomonas maltophilia Clinical Strains Isolated in Warsaw, Poland

Simple Summary

Fluoroquinolones, mainly levofloxacin, are considered an alternative treatment option of Stenotrophomonas maltophilia infections to trimethoprim/sulfamethoxazole. However, an increase in the number of levofloxacin-resistant strains is observed worldwide. The fluoroquinolone resistance in S. maltophilia is usually caused by an overproduction of various multidrug efflux pumps, which are able to extrude antibiotics and chemotherapeutics from the bacterial cells. The purpose of the study was to analyze the contribution of efflux systems to levofloxacin resistance in S. maltophilia clinical strains, isolated in Warsaw, by phenotypic and molecular methods. Previously, the occurrence of genes encoding various ten efflux pumps was shown in 94 studied isolates. Additionally, 44 of 94 isolates demonstrated reduction in susceptibility to levofloxacin. In this study, in the presence of efflux pump inhibitors, an increase in levofloxacin susceptibility was observed in 13 isolates. The overexpression of genes encoding two efflux pump system, such as SmeDEF and Sme VWX (in five and one isolate, respectively), was demonstrated. Sequencing analysis revealed an amino acid change in the local regulators of these efflux pump operons. Our data indicate that the overproduction of the SmeVWX efflux system, unlike SmeDEF, plays a significant role in the levofloxacin resistance of the clinical isolates.

Abstract

Levofloxacin is considered an alternative treatment option of Stenotrophomonas maltophilia infections to trimethoprim/sulfamethoxazole. The fluoroquinolone resistance in S. maltophilia is usually caused by an overproduction of efflux pumps. In this study, the contribution of efflux systems to levofloxacin resistance in S. maltophilia clinical isolates was demonstrated using phenotypic (minimal inhibitory concentrations, MICs, of antibiotics determination ± efflux pump inhibitors, EPIs) and molecular (real-time polymerase-chain-reaction and sequencing) methods. Previously, the occurrence of genes encoding ten efflux pumps was shown in 94 studied isolates. Additionally, 44/94 isolates demonstrated reduction in susceptibility to levofloxacin. Only 5 of 13 isolates (with ≥4-fold reduction in levofloxacin MIC) in the presence of EPIs showed an increased susceptibility to levofloxacin and other antibiotics. The overexpression of smeD and smeV genes (in five and one isolate, respectively) of 5 tested efflux pump operons was demonstrated. Sequencing analysis revealed 20–35 nucleotide mutations in local regulatory genes such as smeT and smeRv. However, mutations leading to an amino acid change were shown only in smeT (Arg123Lys, Asp182Glu, Asp204Glu) for one isolate and in smeRv (Gly266Ser) for the other isolate. Our data indicate that the overproduction of the SmeVWX efflux system, unlike SmeDEF, plays a significant role in the levofloxacin resistance.

The effects of antidepressants fluoxetine, sertraline, and amitriptyline on the development of antibiotic resistance in Acinetobacter baumannii

This study investigates the effects of antidepressants fluoxetine, sertraline, and amitriptyline on the development of antibiotic resistance in clinical Acinetobacter baumannii isolates. The isolates were exposed to fluoxetine, sertraline, and amitriptyline for 30 days, respectively. The bacteria that developed resistance to gentamicin, imipenem, colistin, and ciprofloxacin were isolated and expression levels of some antibiotic-resistance genes were determined by quantitative reverse-transcriptase PCR. Before and after the exposure, minimum inhibitory concentration (MIC) values of the bacteria were determined by the microdilution method. The statistical analysis was performed using Student's t test. A time-dependent increase was observed in the number of bacteria that developed resistance and increased the MIC value. After exposure to fluoxetine and sertraline, decreases were observed for efflux and outer membrane porin genes in isolates that developed colistin resistance, and increases were observed in isolates that developed ciprofloxacin resistance. These observations suggest that these antidepressants have similar effects on the development of resistance. While the exposure to fluoxetine did not result in the development of resistance to imipenem, it was observed after exposure to sertraline and amitriptyline, and a common decrease in ompA gene expression was determined in these isolates. To our knowledge, the comparative effects of selected antidepressants on the development of antibiotic resistance in A. baumannii are reported and presented in the literature here for the first time.

Fumigation practice combined with organic fertilizer increase antibiotic resistance in watermelon rhizosphere soil

Chemical fumigants and organic fertilizer are commonly used in facility agriculture to control soil-borne diseases and promote soil health. However, there is a lack of evidence for the effect of non-antibiotic fumigants on the distribution of antibiotic resistance genes (ARGs) in plant rhizosphere soils. Here, the response of a wide spectrum of ARGs and mobile genetic elements (MGEs) to dazomet fumigation practice in the rhizosphere soil of watermelon was investigated along its branching, flowering and fruiting growth stages in plastic shelters using high-throughput quantitative PCR approach. Our results indicated that soil fumigation combined with organic fertilizer application significantly increased the relative abundance of ARGs and MGEs in the rhizosphere soil of watermelon plant. The positive correlations between the relative abundance of ARGs and MGEs suggested that soil fumigation might increase the horizontal gene transfer (HGT) potential of ARGs. This result was further confirmed by the enhanced associations between ARG and MGE subtypes in the networks of fumigation treatments. Moreover, bipartite associations between ARGs/MGEs and microbial communities (bacteria and fungi) revealed a higher percentage of linkage between MGEs and microbial taxa in the fumigated soils. Structural equation model analysis further suggested that the increases in antibiotic resistance after fumigation and organic fertilizer application were mainly driven by MGEs and fungal community. Together, our results provide vital evidence that dazomet fumigation process combined with organic fertilizer in plastic shelters has the great potential to promote ARGs' dissemination in the rhizosphere, and raise cautions of the acquired resistance by soil-borne fungal pathogen and the potential spreading of ARGs along soil-plant continuum.

Botanicals and phytochemicals from the bark of Hypericum roeperianum (Hypericaceae) had strong antibacterial activity and showed synergistic effects with antibiotics against multidrug-resistant bacteria expressing active efflux pumps

ETHNOPHARMACOLOGICAL RELEVANCE

Infections due to multidrug-resistant (MDR) bacteria constitute a real problem in the public health worldwide. Hypericum roeperianum Schimp. ex A. Rich (Hypericaceae) is used traditionally for treatment of various ailments such as abdominal pains, constipation, diarrhea, indigestion, nausea, and bacterial diseases.

AIM OF THE STUDY

This study was aimed at investigating the antibacterial and antibiotic-modifying activity of the crude methanol extracts (HRB), ethyl-acetate soluble fraction (HRBa), residual material (HRBb), and 11 compounds from the bark of Hypericum roeperianum against multi-drug resistant (MDR) bacteria expressing active efflux pumps.

MATERIALS AND METHODS

The antibacterial activity, the efflux pump effect using the efflux pump inhibitor (EPI), phenylalanine-arginine-ß-naphthylamide (PAβN), as well as the antibiotic-modifying activity of samples were determined using the broth micro-dilution method. Spectrophotometric methods were used to evaluate the effects of HRB and 8,8-bis(dihydroconiferyl) diferulate (11) on bacterial growth, and bacterial membrane damage, whereas follow-up of the acidification of the bacterial culture was used to study their effects on bacteria proton-ATPase pumps.

RESULTS

The crude extract (HRB), HRBa, and HRBb had selective antibacterial activity with MICs ranging from 16 to 512 μg/mL. Phytochemical 11 displayed the best antibacterial activity (0.5 ≤ MIC ≤ 2 μg/mL). The activity of HRB and 11 in the presence of EPI significantly increased on the tested bacteria strains (up to 32-fold). The activity of cloxacillin (CLO), doxycycline (DOX), and tetracycline (TET), was considerably improved (up to 64-fold) towards the multidrug-resistant Enterobacter aerogenes EA-CM64 strain. The crude extract (HRB) and 11 induced the leakage of bacterial intracellular components and inhibited the proton-ATPase pumps.

CONCLUSIONS

The crude extract (HRB) and 8,8-bis(dihydroconiferyl)diferulate from the bark of Hypericum roeperianum are good antibacterial candidates that deserve further investigations to achieve antibacterial drugs to fight infections involving MDR bacteria.

Development of structurally extended benzosiloxaboroles - synthesis and in vitro biological evaluation

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed. This compound proved useful in various derivatizations resulting in a wide range of O-functionalized benzosiloxaboroles. Reactions of 5b with selected substituted benzoyl chlorides gave rise to a series of respective derivatives with 6-benzoate side groups attached to the benzosiloxaborole core. Furthermore, treatment of 5b with substituted benzenesufonyl chlorides afforded several benzosiloxaboroles bearing functionalized benzenesulfonate moieties at the 6 position. The synthesis of related chloropyridine-2-yloxy substituted benzosiloxaboroles was accomplished by a standard approach involving silylation/boronation of appropriate heterodiaryl ethers. Investigation of biological activity of obtained compounds revealed that some benzoate and most benzenesulfonate derivatives exhibit high activity against Gram-positive cocci such as methicillin-sensitive Staphylococcus aureus ATCC 6538P as well as methicillin-resistant S. aureus ATCC 43300 with the MIC values in the range of 0.39–3.12 mg L⁻¹. Some benzenesulfonate derivatives showed also potent activity against Enterococcus faecalis ATCC 29212 and E. faecium ATCC 6057 with MIC = 6.25 mg L⁻¹. Importantly, for the most promising cocci-active benzenesulfonate derivatives the obtained MIC values were far below the cytotoxicity limit determined with respect to human normal lung fibroblasts (MRC-5). For those derivatives, the obtained IC₅₀ values were higher than 12.3 mg L⁻¹. The results of antimicrobial activity and cytotoxicity indicate that the tested compounds can be considered as potential antibacterial agents.

The synthesis of potassium 6-hydroxy-7-chloro-1,1-dimethyl-3,3-difluorobenzo-1,2,3-siloxaborolate 5b from readily available 4-bromo-2-chlorophenol was developed.

Repositioning of non-antibiotic drugs as an alternative to microbial resistance: a systematic review

The global spread of microbial resistance coupled with high costs and slow pace in the discovery of a new antibiotic have made drug repositioning an attractive and promising alternative in the treatment of infections caused by multidrug resistant (MDR) microorganisms. The reuse involves the production of compounds with lower costs and development time, using diversified production technologies. The present systematic review aimed to present a selection of studies published in the last 20 years, which report the antimicrobial activity of non-antibiotic drugs that are candidates for repositioning, which could be used against the current microbial multidrug resistance. A search was performed in the PubMed, SciELO and Google Scholar databases using the following search strategies: [(drug repurposing) OR (drug repositioning) OR (repositioning) AND (non-antibiotic) AND (antibacterial activity) AND (antimicrobial activity)]. Overall, 112 articles were included, which explored the antimicrobial activity in antidepressants, antihypertensives, anti-inflammatories, antineoplastics, hypoglycemic agents, among other drugs. It was concluded that they have significant antimicrobial activity in vitro and in vivo, against standard strain and clinical isolates (Gram-negative and Gram-positive) and fungi. When associated with antibacterials, most of these drugs had their antibacterial activity enhanced. It was also a consensus of the studies included in this review that the presence of aromatic rings in the molecular structure contributes to antimicrobial activity. This review highlights the potential repositioning of several classes of non-antibiotic drugs as promising candidates for repositioning in the treatment of severe bacterial infections of MDR bacteria, extensively resistant (XDR) and pan-resistant (PDR) to drugs.

Antibiotic Resistance in the Collembolan Gut Microbiome Accelerated by the Nonantibiotic Drug Carbamazepine

The effects of pharmaceuticals as emerging contaminants in soil on the gut microbiome and antibiotic resistome in nontarget soil fauna are largely elusive. In this study, we explored the composition of the bacterial community and the presence of antibiotic resistance genes (ARGs) in the gut of the model soil collembolan (Folsomia candida) upon antiepileptic drug carbamazepine (CBZ) and antibiotic tetracycline (TC) exposure. Results showed that, individually or in combination, exposure to TC or CBZ significantly altered the gut community structure of F. candida, causing some enrichment of the bacteria associated with xenobiotic metabolism, such as Arthrobacter, Achromobacter, Gordonia, and Shinella. More importantly, oral exposure to the nonantibiotic drug CBZ enhanced the abundance and diversity of ARGs in the gut of F. candida, especially for the beta-lactams and multidrug resistance genes. Our results revealed that the most likely hosts of ARGs in the gut of F. candida were Proteobacteria and Actinobacteria. The significant positive correlation between mobile genetic elements (MGEs) and ARGs indicated the potential risk of ARGs transmission in the gut of F. candida. Overall, the nonantibiotic CBZ is likely to disturb the gut microbiota of nontarget soil fauna such as collembolans, thereby enhancing the dissemination of ARGs.

Non-antibiotics, Efflux Pumps and Drug Resistance of Gram-negative Rods

Non-antibiotic medicinal products consist of drugs with diverse activity against bacteria. Many non-antibiotics demonstrate direct anti-bacterial activity against Gram-positive cocci. The activity observed against Gram-negative rods is much lower and non-antibiotics primarily from the following groups: non-steroidal anti-inflammatory drugs, cardiovascular and antidepressant medicinal products demonstrate this activity. It has been shown that the low activity of some non-antibiotics or the absence of activity against Gram-negative rods is related, among other things, to the extrusion of these compounds from bacterial cells by multi-drug resistance efflux pumps. Substrates for the resistance-nodulation-division efflux systems include the following non-antibiotics: salicylate, diclofenac, ibuprofen, mefenamic acid, naproxen, amitriptyline, alendronate sodium, nicergoline, and ticlopidine. In addition, interactions between non-antibiotics and multi-drug resistance efflux pumps have been observed. It has also been revealed that depending on the concentration, salicylate induces expression of multi-drug resistance efflux pumps in Escherichia coli, Salmonella enterica subsp. enterica serotype Typhimurium, and Burkholderia cenocepacia. However, salicylate does not affect the expression of the resistance-nodulation-division efflux systems in Stenotrophomonas maltophilia and Acinetobacter baumannii. Most importantly, there were no effects of medicinal products containing some non-antibiotic active substances, except salicylate, as substrates of multi-drug resistance efflux pumps, on the induction of Gram-negative rod resistance to quinolones.

Proof of an Outer Membrane Target of the Efflux Inhibitor Phe-Arg-β-Naphthylamide from Random Mutagenesis

Phe-Arg-β-naphthylamide (PAβN) has been characterized as an efflux pump inhibitor (EPI) acting on the major multidrug resistance efflux transporters of Gram-negative bacteria, such as AcrB in Eschericha coli. In the present study, in vitro random mutagenesis was used to evolve resistance to the sensitizing activity of PAβN with the aim of elucidating its mechanism of action. A strain was obtained that was phenotypically similar to a previously reported mutant from a serial selection approach that had no efflux-associated mutations. We could confirm that acrB mutations in the new mutant were unrelated to PAβN resistance. The next-generation sequencing of the two mutants revealed loss-of-function mutations in lpxM. An engineered lpxM knockout strain showed up to 16-fold decreased PAβN activity with large lipophilic drugs, while its efflux capacity, as well as the efficacy of other EPIs, remained unchanged. LpxM is responsible for the last acylation step in lipopolysaccharide (LPS) synthesis, and lpxM deficiency has been shown to result in penta-acylated instead of hexa-acylated lipid A. Modeling the two lipid A types revealed steric conformational changes due to underacylation. The findings provide evidence of a target site of PAβN in the LPS layer, and prove membrane activity contributing to its drug-sensitizing potency.

Statins: antimicrobial resistance breakers or makers?

Introduction

The repurposing of non-antibiotic drugs as adjuvant antibiotics may help break antimicrobial resistance (AMR). Statins are commonly prescribed worldwide to lower cholesterol. They also possess qualities of AMR “breakers”, namely direct antibacterial activity, synergism with antibiotics, and ability to stimulate the host immune system. However, statins’ role as AMR breakers may be limited. Their current extensive use for cardiovascular protection might result in selective pressures for resistance, ironically causing statins to be AMR “makers” instead. This review examines statins’ potential as AMR breakers, probable AMR makers, and identifies knowledge gaps in a statin-bacteria-human-environment continuum. The most suitable statin for repurposing is identified, and a mechanism of antibacterial action is postulated based on structure-activity relationship analysis.

Methods

A literature search using keywords “statin” or “statins” combined with “minimum inhibitory concentration” (MIC) was performed in six databases on 7th April 2017. After screening 793 abstracts, 16 relevant studies were identified. Unrelated studies on drug interactions; antifungal or antiviral properties of statins; and antibacterial properties of mevastatin, cerivastatin, antibiotics, or natural products were excluded. Studies involving only statins currently registered for human use were included.

Results

Against Gram-positive bacteria, simvastatin generally exerted the greatest antibacterial activity (lowest MIC) compared to atorvastatin, rosuvastatin, and fluvastatin. Against Gram-negative bacteria, atorvastatin generally exhibited similar or slightly better activity compared to simvastatin, but both were more potent than rosuvastatin and fluvastatin.

Discussion

Statins may serve as AMR breakers by working synergistically with existing topical antibiotics, attenuating virulence factors, boosting human immunity, or aiding in wound healing. It is probable that statins’ mechanism of antibacterial activity involves interference of bacterial cell regulatory functions via binding and disrupting cell surface structures such as wall teichoic acids, lipoteichoic acids, lipopolysaccharides, and/or surface proteins. The widespread use of statins for cardiovascular protection may favor selective pressures or co-selection for resistance, including dysbiosis of the human gut microbiota, sublethal plasma concentrations in bacteremic patients, and statin persistence in the environment, all possibly culminating in AMR.

Conclusion

Simvastatin appears to be the most suitable statin for repurposing as a novel adjuvant antibiotic. Current evidence better supports statins as potential AMR breakers, but their role as plausible AMR makers cannot be excluded. Elucidating the mechanism of statins’ antibacterial activity is perhaps the most important knowledge gap to address as this will likely clarify statins’ role as AMR breakers or makers.