Membrane permeability modifications are involved in antibiotic resistance in Klebsiella pneumoniae

The Gram-negative permeability barrier: tipping the balance of the in and the out

Gram-negative bacteria are intrinsically resistant to many antibiotics, due in large part to the permeability barrier formed by their cell envelope. The complex and synergistic interplay of the two Gram-negative membranes and active efflux prevents the accumulation of a diverse range of compounds that are effective against Gram-positive bacteria. A lack of detailed information on how components of the cell envelope contribute to this has been identified as a key barrier to the rational development of new antibiotics with efficacy against Gram-negative species. This review describes the current understanding of the role of the different components of the Gram-negative cell envelope in preventing compound accumulation and the state of efforts to describe properties that allow compounds to overcome this barrier and apply them to the development of new broad-spectrum antibiotics.

Molecular patterns of clinically important fluoroquinolone resistance in multidrug-resistant Klebsiella pneumoniae isolates during nosocomial outbreaks in Shanghai, PR China

Introduction. The soaring resistance of Klebsiella pneumoniae to fluoroquinolones in PR China has substantially limited the application of these antimicrobials, especially in those clinical settings that were threatened by persistent carbapenem-resistant K. pneumoniae (CRKP), necessitating strict implementation of antimicrobial stewardship and active enhanced surveillance of infection control.

Hypothesis. There is interplay between plasmid-mediated quinolone resistance (PMQR) determinants and quinolone resistance-determining region (QRDR) mutations during the acquisition of a clinically important fluoroquinolone resistance (CI-FR) profile in multidrug-resistant K. pneumoniae (MDR-KP) isolates.

Aim. To investigate the high-risk CRKP clones responsible for nosocomial spread and analyse the molecular patterns of CI-FR in MDR-KP isolates in a tertiary hospital in Shanghai, PR China.

Methodology. A total of 34 isolates, including 30 CRKPs, were molecularly characterized. Investigations included antimicrobial susceptibility tests, multilocus sequence typing (MLST) and wzi genotyping, PCR sequencing and phylogenetic analysis for resistance-associated genes, and clinical information retrieval from medical records.

Results. Two high-risk CRKP clones, ST11-wzi64 and ST15-wzi19/wzi24, were identified as being responsible for nosocomial outbreaks in the intensive care unit (ICU) and the neurosurgery department, potentially by the respiratory route. QRDR mutations of both gyrA and parC were detected in isolates of ST15 (S83F/D87A/S80I), ST11 (S83I/D87G/S80I) and ST218 (D87A/S80I), respectively. The PMQR genes, qnrS1, aac(6′)-Ib-cr and oqxAB, were present in 32 (94.1 %) of the isolates alone or in combination, co-occurring with genes (bla) encoding β-lactamases, 16S rRNA methylases and putrescine ABC permeases. AcrR, an AcrAB transcriptional repressor, was insertion-inactivated by the IS5-like element in ST11 isolates. The encoding sequences of OmpK35 and OmpK36 genes were associated with specific STs and wzi alleles. ST11, ST15-wzi19 and ST218 isolates had frameshift disruptions in OmpK35 and specific GD insertions at position 134–135 in OmpK36. The 27 isolates with clinically important ciprofloxacin resistance (MICs ≥2 mg l−1) included 25 isolates (ST15, ST11, ST218) with multiple QRDR mutations, plus 1 with only 2 PMQR determinants (ST290-wzi21) and another with an unknown resistance mechanism (ST65-wzi72). Ciprofloxacin-susceptible isolates maintained intact ompK36 genes, including two CRKPs each with ST13-wzi74 (KPC-2 and NDM-1 coproducers) and ST65-wzi72, plus carbapenem-susceptible isolates (ST15-wzi24, ST65-wzi72, ST107-wzi173).

Conclusions. Under selective pressures, the accumulation of mutations of three types (QRDR, acrR, ompK36) and the acquisition of resistance-conferring genes have continuously contributed to CI-FR in MDR-KP isolates.

Cephalosporin translocation across enterobacterial OmpF and OmpC channels, a filter across the outer membrane

Gram-negative porins are the main entry for small hydrophilic molecules. We studied translocation of structurally related cephalosporins, ceftazidime (CAZ), cefotaxime (CTX) and cefepime (FEP). CAZ is highly active on E. coli producing OmpF (Outer membrane protein F) but less efficient on cells expressing OmpC (Outer membrane protein C), whereas FEP and CTX kill bacteria regardless of the porin expressed. This matches with the different capacity of CAZ and FEP to accumulate into bacterial cells as quantified by LC-MS/MS (Liquid Chromatography Tandem Mass Spectrometry). Furthermore, porin reconstitution into planar lipid bilayer and zero current assays suggest permeation of ≈1,000 molecules of CAZ per sec and per channel through OmpF versus ≈500 through OmpC. Here, the instant killing is directly correlated to internal drug concentration. We propose that the net negative charge of CAZ represents a key advantage for permeation through OmpF porins that are less cation-selective than OmpC. These data could explain the decreased susceptibility to some cephalosporins of enterobacteria that exclusively express OmpC porins.

The translocation of cephalosporins across enterobacterial OmpF and OmpC channels is monitored in real-time, demonstrating differential permeation of some cephalosporins through OmpF and OmpC.

A New Chalcone and Antimicrobial Chemical Constituents of Dracaena stedneuri

Microbial infections are leading causes of death and morbidity all over the world due to the development of the resistance to antibiotics by certain microorganisms. In this study, the chemical exploration of the ethanol (EtOH) extract of the aerial part of Dracaena stedneuri (Dracaenaceae) led to the isolation of one previously unreported chalcone derivative, i.e., 2′,4′-dihydroxy-2,3′-dimethoxychalcone (1), together with 12 known compounds: 8-(C)-methylquercetagetin-3,6,3′-trimethyl ether (2), methylgalangine (3), quercetin (4), kaempferol (5), 6,8-dimethylchrysin (6), ombuine-3-O-rutinoside (4ʹ,7-dimethylquercetin-3-O-α-L-rhamnopyranosyl-(1 → 6) -β-D-glucopyranoside) (7), alliospiroside A (8), β-sitosterol 3-O-glucopyranoside (9), ishigoside (10), betulinic acid (11), oleanolic acid (12), and lupeol (13). The structures were determined by spectroscopic and spectrometric analysis including 1- and 2-Dimensional Nuclear Magnetic Resonance (1D- and 2D-NMR), High-Resolution Electrospray Ionization Mass Spectrometry (HRESIMS), and comparison with literature data. The isolated secondary metabolites and crude extract displayed antibacterial activity against some multidrug-resistant strains with minimal inhibitory concentration (MIC) values ranging from 32 to 256 μg/mL. The antibacterial activity of compound 13 against Enterobacter aerogenes ATCC13048 (MIC value: 32 μg/mL) was higher than that of chloramphenicol used as the reference drug (MIC = 64 μg/mL).

Microbial Efflux Pump Inhibitors: A Journey around Quinoline and Indole Derivatives

Antimicrobial resistance (AMR) is a complex threat to human health and, to date, it represents a hot topic in drug discovery. The use of non-antibiotic molecules to block resistance mechanisms is a powerful alternative to the identification of new antibiotics. Bacterial efflux pumps exert the early step of AMR development, allowing the bacteria to grow in presence of sub-inhibitory drug concentration and develop more specific resistance mechanisms. Thus, efflux pump inhibitors (EPIs) offer a great opportunity to fight AMR, potentially restoring antibiotic activity. Based on our experience in designing and synthesizing novel EPIs, herein, we retrieved information around quinoline and indole derivatives reported in literature on this topic. Thus, our aim was to collect all data around these promising classes of EPIs in order to delineate a comprehensive structure–activity relationship (SAR) around each core for different microbes. With this review article, we aim to help future research in the field in the discovery of new microbial EPIs with improved activity and a better safety profile.

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.

Antibacterial and antibiotic-modifying activities of fractions and compounds from Albizia adianthifolia against MDR Gram-negative enteric bacteria

Background

Albizia adianthifolia (Schum.) is medicinally used in Cameroon to manage bronchitis and skin diseases. Our previous study documented the antibacterial potential of its roots’ methanol extract. In this study, methanol roots extract was subjected to chromatography techniques and fractions (AARa and AARb), sub-fractions (AARa1–4, AARb1–2 and AARb11–14) together with isolated phytochemicals were assessed for their antimicrobial as well as their antibiotic-potentiating effects towards Gram-negative multidrug resistant (MDR) bacteria.

Methods

The antibacterial activities of the samples (determination of Minimal Inhibitory « MIC » and Minimal Bactericidal Concentration « MBC ») were determined by the modified rapid p-iodonitrotetrazolium chloride (INT) colorimetric assay, as well as those of antibiotics in association with the compounds. Column chromatography was applied to isolate phytochemicals from roots extract and their chemical structures were determined using spectroscopic techniques.

Results

The phytochemicals isolated were stearic acid (1), a mixture (1:1) of stigmasterol and β-sitosterol (2 +  3), β-sitosterol 3-O-β-_D-glucopyranoside (4), palmatin (5), homomangiferin (6) and mangiferin (7). Fraction AARa exhibited selective inhibitory effects whilst all tested bacteria were inhibited by AARb in MIC ranges of 8 to 1024 μg/mL. Sub-fractions AARb1–2 had MIC values between 8 μg/mL and 1024 μg/mL on all tested bacteria. Phytochemicals 4, 2 +  3 and 7 inhibited the growth of 54.54% (6/11), 45.45% (5/11) and 27.27% (3/11) tested bacterial strains, respectively. When tested with an efflux pumps inhibitor (Phenylalanine-Arginine-β-Naphthylamide or PAβN), the inhibitory effects of compounds 2 + 3 and 4 increased towards all the tested bacteria. In association with erythromycin (ERY), streptomycin (STR) and tetracycline (TET), compounds 2 + 3 and 4 had the most significant synergistic activity on the seven selected bacteria.

Conclusion

The present study provides information on the possible use of Albizia adianthifolia and its constituents in the control of Gram-negative infections including MDR phenotypes.

Electronic supplementary material

The online version of this article (10.1186/s12906-019-2537-1) contains supplementary material, which is available to authorized users.

Antibacterial and Antibiotic Modifying Potential of Crude Extracts, Fractions, and Compounds from Acacia polyacantha Willd. against MDR Gram-Negative Bacteria

The present study aimed to assess the in vitro antibacterial and antibiotic modifying activities of methanol extracts prepared from the leaf (APL) and bark (APB) of Acacia polyacantha, fractions (APLa-d) and compounds isolated from APL against a panel of multidrug resistant (MDR) Gram-negative bacteria. Leaf extract was subjected to column chromatography for compounds isolation; antibacterial assays were performed on samples alone and with an efflux pump inhibitor (EPI), respectively, and several antibiotics on the tested bacteria. The phytochemical investigation of APL led to the isolation of stigmasterol (1), β-amyrin (2), 3-O-β-_D-glucopyranosylstigmasterol (3), 3-O-methyl-D-chiro-inositol (4), epicatechin (5), quercetin-3-O-glucoside (6), 3-O-[β-_D-xylopyranosyl-(1→4)-β-_D-galactopyranosyl]-oleanolic acid (7), and 3-O-[β-galactopyranosyl-(1→4)-β-_D-galactopyranosyl]-oleanolic acid (8). APL and APB had minimal inhibitory concentration (MIC) values ≤ 1024 μg/mL on 73.3% and 46.7% of the tested bacteria, respectively. APLb and APLd were effective against 88.9% of tested bacterial species with compound 8 showing the highest activity inhibiting 88.9% of tested bacteria. The EPI, phenylalanine-arginine-β-naphthylamide (PAßN), strongly improved the activity of APL, APLb, APLd, and compound 8 on all tested bacteria. Synergistic effects were obtained when APL and compounds 7 and 8 were combined with erythromycin (ERY), gentamycin (GEN), ciprofloxacin (CIP), and norfloxacin (NOR). The present study demonstrates the antibacterial potential of Acacia polyacantha and its constituents to combat bacterial infections alone or in combination with EPI.

Outer Membrane Porins

The transport of small molecules across membranes is essential for the import of nutrients and other energy sources into the cell and, for the export of waste and other potentially harmful byproducts out of the cell. While hydrophobic molecules are permeable to membranes, ions and other small polar molecules require transport via specialized membrane transport proteins . The two major classes of membrane transport proteins are transporters and channels. With our focus here on porins-major class of non-specific diffusion channel proteins , we will highlight some recent structural biology reports and functional assays that have substantially contributed to our understanding of the mechanism that mediates uptake of small molecules, including antibiotics, across the outer membrane of Enterobacteriaceae . We will also review advances in the regulation of porin expression and porin biogenesis and discuss these pathways as new therapeutic targets.

Syzygium jambos Displayed Antibacterial and Antibiotic-Modulating Activities against Resistant Phenotypes

The present study was designed to evaluate the antibacterial activities of methanol extracts of bark and leaves of Syzygium jambos, as well as their synergistic effects with selected antibiotics against drug-resistant Gram-positive and Gram-negative bacteria. The crude extracts were subjected to qualitative phytochemical screening; broth microdilution method was used for antibacterial assays. Phytochemical studies indicate that leaves and bark extracts contained polyphenols, anthraquinones, tannins, and steroids. Extract of the leaves was active against all the 26 strains of Staphylococcus aureus and all the 21 strains of Gram-negative bacteria tested, within the minimum inhibitory concentration (MIC) range of 32–512 μg/mL. The lowest MIC value of 32 μg/mL was obtained with extract of the leaves against Staphylococcus aureus MRSA9 strain. In Gram-negative bacteria, the lowest MIC value of 64 μg/mL was also obtained against Enterobacter aerogenes EA294 and Klebsiella pneumoniae K24 strains. Against S. aureus strains, antibiotic-modulating activity of extracts at MIC/2 towards more than 70% of the tested strains was obtained when leaves and bark extracts were tested in association with chloramphenicol (CHL). This was also the case when leaves extract was combined with CHL, kanamycin (KAN), tetracycline (TET), and erythromycin (ERY) and when bark extract was combined with ciprofloxacin (CIP), TET, and ERY against Gram-negative bacteria. In conclusion, this study demonstrated that Syzygium jambos has antibacterial and antibiotic-modulating activities.

Computational Study Reveals the Molecular Mechanism of the Interaction between the Efflux Inhibitor PAβN and the AdeB Transporter from Acinetobacter baumannii

Phenylalanine-arginine β-naphthylamide (PAβN) is a broad-spectrum efflux pump inhibitor that has shown to potentiate the activity of antibiotics in Gram-negative bacteria. AdeB is a part of the AdeABC tripartite pump that plays a pivotal role in conferring efflux-mediated resistance in Acinetobacter baumannii. To understand the molecular mechanism of efflux pump inhibition by PAβN, we investigated the interaction of PAβN with AdeB using different computational methods. We observed that PAβN does not have specific binding interactions with the proximal binding site and interacts strongly with the distal binding pocket. The Phe loop located between the proximal and distal binding pockets plays a key role in the PAβN-mediated inhibition and acts as a gate between the binding pockets. Molecular dynamics simulations suggested that PAβN behaved like a climber as we observed switching of the interaction energies between the ligand and the key Phe residues of the binding site during the course of the simulation. PAβN uses the hydrophobic microenvironment formed by Phe residues in the distal binding pocket to keep the binding monomer in the binding conformation. The simulation data suggests that this binding event should result in the inhibition of the peristaltic mechanism and prevent the exporter from extruding any other substrates leading to the inhibition of the tripartite pump.

Antibacterial activity of six medicinal Cameroonian plants against Gram-positive and Gram-negative multidrug resistant phenotypes

Background

Infectious diseases due to multidrug-resistant bacteria are one of the causes of treatment failures contributing to an increase in mortality and/or morbidity. In this study, we evaluated the antibacterial potential of different parts of six medicinal plants namely Alstonia boonei, Ageratum conyzoides, Croton macrostachys, Cassia obtusifolia, Catharanthus roseus and Paullinia pinnata against a panel of 36 multi-drug resistant (MDR) Gram-negative and Gram-positive bacteria.

Methods

Minimum Inhibitory Concentration (MIC) and Minimal Bactericidal Concentration (MBC) of the methanol extracts from different parts of the plants were determined using broth microdilution method; standard phytochemical methods were used for phytochemical screening.

Results

Several phytochemical classes such as polyphenols, sterols, triterpenes, alkaloids, flavonoids and saponins were identified in the plant extracts. MIC values obtained ranged from 64 to 1024 μg/mL. Leaves extract of Catharanthus roseus (86.11 %), Croton macrostachys (83.33 %) and Paullinia pinnata (80.55 %) displayed the best antibacterial spectra. The lowest MIC value of 64 μg/mL was obtained with the Paullinia pinnata stems extract and Cassia obtusifolia extract against the strain of Staphylococcus aureus MRSA8. Results also showed that the tested samples generally displayed bacteriostatic effects with MBC values obtained in only 3.35 % of the cases where plant extracts were active.

Conclusion

The results obtained at the end of this study demonstrate for the first time the antibacterial activity of the studied medicinal plants against MDR bacteria. The tested plants could be a reservoir of molecules to fight against MDR bacterial infections.

Antibacterial activities of the methanol extracts of Canarium schweinfurthii and four other Cameroonian dietary plants against multi-drug resistant Gram-negative bacteria

Bacterial infections are among the major cause of morbidity and mortality worldwide. The present study was designed to evaluate the in vitro antibacterial activities of the methanol extracts of five Cameroonian edible plants namely Colocasia esculenta, Triumfetta pentandra, Hibiscus esculentus, Canarium schweinfurthii and Annona muricata against a panel of 19 multidrug resistant Gram-negative bacterial strains. The liquid broth microdilution was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the extracts. The preliminary phytochemical screening of the extracts was conducted according to the standard phytochemical methods. Results showed that all extracts contained compounds belonging to the classes of polyphenols, triterpenes and steroids, other classes of chemicals being selectively distributed. Canarium schweinfurthii extract showed the best activity with MIC values ranging from 64 to 1024 μg/mL against 89.5% of the 19 tested bacteria strains. MIC values below or equal to 1024 μg/mL were also recorded with Triumfetta pentandra, Annona muricata, Colocasia esculenta and Hibiscus esculentus extracts respectively against 15/19 (78.9%), 11/19 (57.9%), 10/19 (52.6%) and 10/19 (52.6%) tested bacteria. Extract from C. schweinfurthii displayed the lowest MIC value (64 μg/mL) against Escherichia coli AG100ATet. Finally, the results of this work provide baseline information for the use of C. esculenta, T. pentandra, H. esculentus, C. schweinfurthii and A. muricata in the treatment of bacterial infections including multidrug resistant phenotypes.

Antibacterial and antibiotic-resistance modifying activity of the extracts and compounds from Nauclea pobeguinii against Gram-negative multi-drug resistant phenotypes

BACKGROUND

Multi-drug resistance of Gram-negative bacteria constitutes a major obstacle in the antibacterial fight worldwide. The discovery of new and effective antimicrobials and/or resistance modulators is necessary to combat the spread of resistance or to reverse the multi-drug resistance. In this study, we investigated the antibacterial and antibiotic-resistance modifying activities against 29 Gram-negative bacteria including multi-drug resistant (MDR) phenotypes of the methanol extracts from Nauclea pobeguiinii leaves (NPL), Nauclea pobeguiinii bark (NPB) and six compounds from the bark extract, identified as 3-acetoxy-11-oxo-urs-12-ene (1), p-coumaric acid (2), citric acid trimethyl ester (3), resveratrol (4), resveratrol β- D -glucopyranoside (5) and strictosamide (6).

METHODS

The broth microdilution method was used to determine the minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of crude extracts and compounds as well as the antibiotic-resistance modifying effects of MPB and 4.

RESULTS

MIC determinations indicate values ranging from 32-1024 μg/mL for NPB and NPL on 89.7 % and 69.0 % of the tested bacterial strains respectively. MIC values below 100 μg/mL were obtained with NPB against Escherichia coli ATCC10536, AG100 and Enterobacter aerogenes CM64 strains. The lowest MIC value for crude extracts of 32 μg/mL was obtained with NPB against E. coli ATCC10536. Compound 4 was active all tested bacteria, whilst 1, 3 and 6 displayed weak and selective inhibitory effects. The corresponding MIC value (16 μg/mL) was obtained with 4 against Klebsiella pneumoniae KP55 strain. Synergistic effects of the combination of NPB with chloramphenicol (CHL), kanamycin (KAN) as well as that of compound 4 with streptomycin (STR) and ciprofloxacin (CIP) were observed.

CONCLUSION

The present study provides information on the possible use of Nauclea pobeguinii and compound 4 in the control of Gram-negative bacterial infections including MDR phenotypes. It also indicates that NPB and 4 can be used as naturally occurring antibiotic-resistance modulators to tackle MDR bacteria.

Antibacterial activities and structure-activity relationships of a panel of 48 compounds from Kenyan plants against multidrug resistant phenotypes

In the current study forty eight compounds belonging to anthraquinones, naphthoquinones, benzoquinones, flavonoids (chalcones and polymethoxylated flavones) and diterpenoids (clerodanes and kauranes) were explored for their antimicrobial potential against a panel of sensitive and multi-drug resistant Gram-negative and Gram-positive bacteria. The minimal inhibitory concentration (MIC) determinations on the tested bacteria were conducted using modified rapid INT colorimetric assay. To evaluate the role of efflux pumps in the susceptibility of Gram-negative bacteria to the most active compounds, they were tested in the presence of phenylalanine arginine β-naphthylamide (PAβN) (at 30 µg/mL) against selected multidrug resistance (MDR) bacteria. The anthraquinone, emodin, naphthaquinone, plumbagin and the benzoquinone, rapanone were active against methicillin resistant Staphylococcus aureus (MRSA) strains of bacteria with MIC values ranging from 2 to 128 μg/mL. The structure activity relationships of benzoquinones against the MDR Gram-negative phenotype showed antibacterial activities increasing with increase in side chain length. In the chalcone series the presence of a hydroxyl group at C3′ together with a methoxy group and a second hydroxyl group in meta orientation in ring B of the chalcone skeleton appeared to be necessary for minimal activities against MRSA. In most cases, the optimal potential of the active compounds were not attained as they were extruded by bacterial efflux pumps. However, the presence of the PAβN significantly increased the antibacterial activities of emodin against Gram-negative MDR E. coli AG102, 100ATet; K. pneumoniae KP55 and KP63 by >4–64 g/mL. The antibacterial activities were substantially enhanced and were higher than those of the standard drug, chloramphenicol. These data clearly demonstrate that the active compounds, having the necessary pharmacophores for antibacterial activities, including some quinones and chalcones are substrates of bacterial efflux pumps and therefore should be combined to efflux pump inhibitors in the fight against MDR bacterial infections.

Bacterial resistance and immunological profiles in HIV-infected and non-infected patients at Mbouda AD LUCEM Hospital in Cameroon

This study investigated the variations in some cells of the immune system, as well as the antibiotic resistance of the bacteria responsible for enteric infections among HIV+ patients compared to HIV- patients in Mbouda AD LUCEM Hospital, Cameroon. A cross-sectional study was performed from September 2014 to February 2015 in 67 human immunodeficiency virus (HIV)-seropositive (HIV+) and 37 HIV-seronegative (HIV-) patients. Blood collected from these patients was used to perform cluster of differentiation 4 (CD4) and cluster of differentiation 8 (CD8) lymphocyte blood counts and a white blood cell count, as well as to measure C-reactive protein (CRP) blood by flow cytometry and perform optical and immuno-turbidimetric detection. Enteric bacteria were isolated from the stool of patients, and their antibiotic susceptibility profiles were determined using agar diffusion methods. The results showed that Escherichia coli was the main pathogenic bacteria in the digestive tracts of HIV+ (85.3%) and HIV- (81.1%) patients, and infections with Klebsiella sp. were also predominant among HIV- patients (29.4%). Resistance of Klebsiella sp. to ceftriaxone (CRO; P=0.001), gentamicin (GEN; P=0.005), chloramphenicol (CHL; P=0.0004), ciprofloxacin (CIP; P=0.005) and doxycycline (DOX; P<0.0001) was significantly higher in HIV+ patients than in HIV- patients. Enterobacter sp. showed high resistance to GEN (P=0.009) and CIP (P=0.001) in HIV+ patients compared to HIV- patients. Citrobacter sp. was resistant to GEN (P=0.009) in HIV+ patients compared to HIV- patients. Salmonella sp. showed high resistance to CHL (P<0.0001) and DOX (P<0.0001) in HIV+ patients compared to HIV- patients. Resistance of Serratia sp. to AMO (P=0.005), AMC (P=0.005) and CHL (P=0.005) was significantly higher in HIV+ patients than in HIV- patients. Lymphopenia was higher in HIV+ patients (36.8%) than in HIV- patients (2.7%). In 45.9% of the HIV- patients, the CRP rate was higher than 6mg/L compared to 16.2% in HIV+ patients. In general, bacterial multi-drug resistance in HIV+ patients (79.4%) was significantly higher (P<0.0001) than in HIV- patients (29.7%). The present study revealed that the resistance profiles of bacteria should be considered in HIV-infected patients to improve their health care.

Antibacterial activities of the methanol extracts of Albizia adianthifolia, Alchornea laxiflora, Laportea ovalifolia and three other Cameroonian plants against multi-drug resistant Gram-negative bacteria

In the last 10 years, resistance in Gram-negative bacteria has been increasing. The present study was designed to evaluate the in vitro antibacterial activities of the methanol extracts of six Cameroonian medicinal plants Albizia adianthifolia, Alchornea laxiflora, Boerhavia diffusa, Combretum hispidum, Laportea ovalifolia and Scoparia dulcis against a panel of 15 multidrug resistant Gram-negative bacterial strains. The broth microdilution was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the extracts. The preliminary phytochemical screening of the extracts was conducted according to the reference qualitative phytochemical methods. Results showed that all extracts contained compounds belonging to the classes of polyphenols and triterpenes, other classes of chemicals being selectively distributed. The best antibacterial activities were recorded with bark and root extracts of A. adianthifolia as well as with L. ovalifolia extract, with MIC values ranging from 64 to 1024 μg/mL on 93.3% of the fifteen tested bacteria. The lowest MIC value of 64 μg/mL was recorded with A. laxiflora bark extract against Enterobacter aerogenes EA289.

Finally, the results of this study provide evidence of the antibacterial activity of the tested plants and suggest their possible use in the control of multidrug resistant phenotypes.

Antibacterial and antibiotic-modifying activities of three food plants (Xanthosoma mafaffa Lam., Moringa oleifera (L.) Schott and Passiflora edulis Sims) against multidrug-resistant (MDR) Gram-negative bacteria

Background

The present study was designed to investigate the antibacterial activities of the methanol extract of three edible plants, namely Xanthosoma mafaffa, Moringa oleifera and Passiflora edulis and their synergistic effects with some commonly used antibiotics against MDR Gram-negative bacteria expressing active efflux pumps.

Methods

Broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) and the minimum bactericidal concentrations (MBCs) of the extracts, as well as those of antibiotics in association with the extracts.

Results

The phytochemical test indicate that all tested crude extracts contained polyphenols, triterpenes and steroids whilst other phytochemical classes were selectively distributed. Extracts showed antibacterial activities with minimum inhibitory concentrations ranging from 128-1024 μg/mL on the majority of the 19 tested Gram-negative bacterial strains. Extract from the pericarp of P. edulis inhibited the growth of 89.5 % of the 19 tested bacterial strains, the lowest minimal inhibitory concentration (MIC) value of 128 μg/mL being recorded against Escherichia coli AG100 strain. In the presence of Phenylalanine-Arginine β-Naphtylamide (PAβN)], an efflux pump inhibitor (EPI), the activity of the extract from X. mafaffa increased on 40 % of tested strains. In combination with antibiotics, extracts of X. mafaffa, M. oleifera and pericarp of P. edulis showed synergistic effects with some antibiotics against more than 75 % of the tested bacteria.

Conclusion

The results of the present study indicate that the tested plants may be used in the treatment of bacterial infections including the multi-resistant bacteria.

Ngadjui B, Kuete V. Antibacterial activities of the methanol extracts and compounds from Erythrina sigmoidea against Gram-negative multi-drug resistant phenotypes

Background

In the present study, the methanol extracts from the leaves, as well as compounds namely sigmoidin I (1), atalantoflavone (2), bidwillon A (3), neocyclomorusin (4), 6α-hydroxyphaseollidin (5) and neobavaisoflavone (6) (from the bark extract) were tested for their activities against a panel of Gram-negative bacteria including multi-drug resistant (MDR) phenotypes.

Methods

Broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) and the minimum bactericidal concentrations (MBCs) of the extracts as well as compounds 1–6.

Results

The MIC results indicated that the crude extracts from the leaves and bark of this plant were able to inhibit the growth of 96.3 % of the 27 tested bacteria. Compounds 2–6 displayed selective activities, their inhibitory effects being obtained on 8.3 %, 41.7 %, 58.3 %, 58.3 % and 66.7 % of tested bacteria respectively for 2, 3, 5, 6 and 4. The lowest MIC value of 8 μg/mL was obtained with 6 against Escherichia coli ATCC8739, Enterobacter cloacae ECCI69, Klebsiella pneumoniae KP55, Providencia stuartii NAE16 and Pseudomonas aeruginosa PA01.

Conclusion

The present study demonstrates that Erythrina sigmoidea is a potential source of antibacterial drugs to fight against MDR bacteria. Neobavaisoflavone (6) is the main antibacterial consituents of the bark crude extract.

Role of the Outer Membrane and Porins in Susceptibility of β-Lactamase-Producing Enterobacteriaceae to Ceftazidime-Avibactam

This study examined the activity of the novel antimicrobial combination ceftazidime-avibactam against Enterobacteriaceae exhibiting different outer membrane permeability profiles, specifically with or without porins and with or without expression of the main efflux pump (AcrAB-TolC). The addition of the outer membrane permeabilizer polymyxin B nonapeptide increased the antibacterial activities of avibactam alone, ceftazidime alone, and ceftazidime-avibactam against the characterized clinical isolates of Escherichia coli, Enterobacter aerogenes, and Klebsiella pneumoniae. This enhancement of activities was mainly due to increased passive penetration of compounds since inhibition of efflux by the addition of phenylalanine-arginine β-naphthylamide affected the MICs minimally. OmpF (OmpK35) or OmpC (OmpK36) pores were not the major route by which avibactam crossed the outer membranes of E. coli and K. pneumoniae. In contrast, Omp35 and Omp36 allowed diffusion of avibactam across the outer membrane of E. aerogenes, although other diffusion channels for avibactam were also present in that species. It was clear that outer membrane permeability and outer membrane pore-forming proteins play a key role in the activity of ceftazidime-avibactam. Nevertheless, the MICs of ceftazidime-avibactam (with 4 mg/liter avibactam) against the ceftazidime-resistant clinical isolates of the three species of Enterobacteriaceae studied were ≤ 8 mg/liter, regardless of outer membrane permeability changes resulting from an absence of defined porin proteins or upregulation of efflux.

In vitro antibacterial and antibiotic-potentiation activities of the methanol extracts from Beilschmiedia acuta, Clausena anisata, Newbouldia laevis and Polyscias fulva against multidrug-resistant Gram-negative bacteria

Background

The present study was designed to investigate the antibacterial activities of the methanol extracts from different parts of Beilschmedia acuta Kosterm (Lauraceae), Clausena anisata (Willd) Hook (Rutaceae), Newbouldia laevis Seem (Bignoniaceae) and Polyscias fulva (Hiern) Harms (Araliaceae) as well as their synergistic effects with antibiotics against a panel of Gram-negative bacteria, including multi-drug resistant (MDR) phenotypes expressing active efflux pumps.

Methods

Broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) and the minimum bactericidal concentrations (MBCs) of the extracts, as well as those of antibiotics in association with the most active ones, B. acuta, N. laevis and P. fulva.

Results

MIC values obtained indicate that extracts from the bark of B. acuta were active on all the 26 tested Gram-negative bacteria, with MICs ranging from values below 8 to 256 μg/mL. Other samples displayed selective activities, their inhibitory effects being observed on 9 (34.62 %) of the 26 bacterial strains for N. laevis leaves extract, 6 (23.10 %) for both C. anisata leaves and roots extracts, 7 (26.9 %) and 4 (15.4 %) for leaves and roots extracts of P. fulva respectively. Extract from B. actua bark displayed the best antibacterial activity with MIC values below 100 μg/mL against 16 (61.5 %) of the 26 tested microorganisms. The lowest MIC values (below 8 μg/mL) were obtained with this extract against Escherichia coli W3110 and Klebsiella pneumoniae ATCC11296. The MIC values of this extract were lower than those of ciprofloxacin against E. coli W3110, Enterobacter aerogenes ATCC13048, CM64 and Providencia stuartii NAE16. At MIC/2, the best percentages of synergistic effects (100 %), were obtained with B. acuta bark extract and tetracycline (TET) as well as with P. fulva leaves extract and TET and kanamycin (KAN).

Conclusion

The overall results of the present study provide information for the possible use of the studied plants and mostly Beilschmedia acuta in the control of bacterial infections including MDR phenotypes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-015-0944-5) contains supplementary material, which is available to authorized users.

Antibacterial activities of Fagara macrophylla, Canarium schweinfurthii, Myrianthus arboreus, Dischistocalyx grandifolius and Tragia benthamii against multi-drug resistant Gram-negative bacteria

Bacterial infections caused by multidrug resistant phenotypes constitute a worldwide health concern. The present study was designed to evaluate the in vitro antibacterial activities of the methanol extracts of five medicinal plants: Fagara macrophylla, Canarium schweinfurthii, Myrianthus arboreus, Dischistocalyx grandifolius and Tragia benthamii against a panel of 28 multidrug resistant Gram-negative bacterial strains. The liquid broth microdilution was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the extracts. The best activity was recorded with Canarium schweinfurthii bark extract, MIC values ranging from 32 to 1024 µg/mL being recorded against 85.7 % tested bacteria. Broad spectra of antibacterial activities were also obtained with both bark and leaf extracts from Myrianthus arboreus (78.6 %) as well as the bark extract from Fagara macrophylla (75.0 %). The lowest MIC value of 32 µg/mL was obtained with Canarium schweinfurthii bark extract against Klebsiella pneumoniae KP63 strain. The results of this work provide baseline information for the use of the studied plants, and mostly Fagara macrophylla, Canarium schweinfurthii and Myrianthus arboreus in the treatment of bacterial infections including multidrug resistant phenotypes.

Antibacterial activities of the methanol extracts, fractions and compounds from Fagara tessmannii

ETHNOPHARMACOLOGICAL RELEVANCE

Fagara tessmannii is a shrub of the African rainforests used to treat bacterial infections, cancers, swellings and inflammation. In the present study, the methanol extract from the leaves (FTL), bark (FTB), and roots (FTR) of this plant as well as fractions (FTR1-5) and compounds isolated from FTR namely β-sitosterol-3-O-β-d-glucopyranoside (1), nitidine chloride (2) and buesgenine (3), were tested for their antimicrobial activities against a panel of Gram-negative bacteria including multidrug resistant (MDR) phenotypes.

MATERIALS AND METHODS

The broth microdilution method was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the above samples; Column chromatography was used for the fractionation and purification of the roots extract whilst the chemical structures of compounds were determined using spectroscopic techniques.

RESULTS

Results of the MIC determinations indicated that the crude extracts from the roots as well as fraction FTRa4 were active on all the 26 tested bacterial strains. MIC values below 100µg/mL were obtained with roots, leaves and bark extract respectively against 30.8%, 15.4% and 11.5% tested bacteria. The lowest MIC value below of 8µg/mL was obtained with extract from the roots against Escherichia coli MC100 strain. The lowest MIC value of 4µg/mL was also obtained with compound 3 against E. coli AG102 and Klebsiella pneumoniae ATCC11296 CONCLUSIONS: The present study demonstrates that F. tessmannii is a potential source of antimicrobial drugs to fight against MDR bacteria. Benzophenanthrine alkaloids 2 and 3 are the main antibacterial consituents of the roots of the plant.

The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria

The global emergence of multidrug-resistant Gram-negative bacteria is a growing threat to antibiotic therapy. The chromosomally encoded drug efflux mechanisms that are ubiquitous in these bacteria greatly contribute to antibiotic resistance and present a major challenge for antibiotic development. Multidrug pumps, particularly those represented by the clinically relevant AcrAB-TolC and Mex pumps of the resistance-nodulation-division (RND) superfamily, not only mediate intrinsic and acquired multidrug resistance (MDR) but also are involved in other functions, including the bacterial stress response and pathogenicity. Additionally, efflux pumps interact synergistically with other resistance mechanisms (e.g., with the outer membrane permeability barrier) to increase resistance levels. Since the discovery of RND pumps in the early 1990s, remarkable scientific and technological advances have allowed for an in-depth understanding of the structural and biochemical basis, substrate profiles, molecular regulation, and inhibition of MDR pumps. However, the development of clinically useful efflux pump inhibitors and/or new antibiotics that can bypass pump effects continues to be a challenge. Plasmid-borne efflux pump genes (including those for RND pumps) have increasingly been identified. This article highlights the recent progress obtained for organisms of clinical significance, together with methodological considerations for the characterization of MDR pumps.

Tetracycline improved the efficiency of other antimicrobials against Gram-negative multidrug-resistant bacteria

Treatment of infectious diseases with antimicrobials constituted a great achievement in the history of medicine. Unfortunately, the emergence of resistant strains of bacteria to all classes of antimicrobials limited their efficacy. The present study was aimed at evaluating the effect of combinations of antibiotics on multi-drug resistant Gram-negative (MDRGN) bacteria. A liquid micro-broth dilution method was used to evaluate the antibacterial activity of 10 different classes of antimicrobials on 20 bacterial strains belonging to six different species. The antimicrobials were associated with phenylalanine β-naphthylamide (PAβN), an efflux pump inhibitor, and with other antimicrobials at their sub-inhibitory concentrations. The effectiveness of each combination was monitored using the minimal inhibitory concentration (MIC) and the fractional inhibitory concentration (FIC). Most of the antimicrobials tested showed low antibacterial activity with a MIC value of 128 mg/L on a majority of the bacterial strains, justifying their multidrug-resistant (MDR) profile. Synergistic effects were mostly observed (FIC≤0.5) when ampicillin (AMP), cloxacillin (CLX), erythromycin (ERY), chloramphenicol (CHL), kanamycin (KAN) and streptomycin (STR) were combined with tetracycline (TET) at the sub-inhibitory concentration of MIC/5 or MIC/10. The results of the present work suggest that the association of several antimicrobials with TET could improve the fight against MDRGN bacterial species.

Activities of selected medicinal plants against multi-drug resistant Gram-negative bacteria in Cameroon

BACKGROUND

Medicinal plants are used worldwide for several human ailments including bacterial infections. The present work was designed to assess the in vitro antibacterial activities of some Cameroonian medicinal plants including Entada abyssinica, Entada africana, Pentaclethra macrophylla, Allexis cauliflora, Anthocleista leibrechtsiana, Carapa procera, Carica papaya and Persea americana against Gram-negative bacteria expressing multidrug resistant (MDR) phenotypes.

METHODS

The microbroth dilution was used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the samples against eight bacterial strains belonging to four species, Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae and Providencia stuartii.

RESULTS

The extracts displayed selective antibacterial activities with the minimal inhibitory concentrations (MIC) values ranges of 64 to 1024 µg/mL. The most active extract was that from Pentaclethra macrophylla (TPM) that showed inhibitory activities against five of the eight (62.5%) tested bacteria. The lowest MIC value (64 µg/mL) was recorded with the crude extract of Entada africana against E. coli AG100A whilst the best MBC (256 µg/mL) value was also obtained with methanol extract of Persea americana against this bacterial strain.

CONCLUSION

The results of the present work provide baseline information on the possible use of Pentaclethra macrophylla, Entada africana and Entada abyssinica in the treatment of selected bacterial infections.

Antibacterial activities of the methanol extracts of seven Cameroonian dietary plants against bacteria expressing MDR phenotypes

The morbidity and mortality caused by bacterial infections significantly increased with resistance to commonly used antibiotics. This is partially due to the activation of efflux pumps in Gram-negative bacteria. The present work designed to assess the in vitro antibacterial activities of seven Cameroonian dietary plants (Sesamum indicum, Sesamum radiatum, Cinnamomum zeylanicum, Corchous olitorius, Cyperus esculentus, Adansonia digitata, Aframomum kayserianum), against multidrug resistant (MDR) Gram-negative bacteria over expressing active efflux pumps. The standard phytochemical methods were used to detect the main classes of secondary metabolites in the extracts. The antibacterial activities of the studied extracts in the absence or presence of an efflux pump inhibitor (PAβN) were evaluated using liquid microbroth dilution method. The results obtained indicated that apart from the extract of C. esculentus, all other samples contained alkaloids, phenols and polyphenols meanwhile other classes of chemicals were selectively present. The studied extracts displayed antibacterial activities with minimal inhibitory concentrations (MICs) values ranged from 64 to 1024 μg/mL on the majority of the 27 tested microbial strains. The extract of S. indicum was active against 77.77% of the tested microorganisms whilst the lowest MIC value (64 μg/mL) was recorded with that of A. kayserianum against E. aerogenes EA294. The results of the present work provide baseline information on the possible used of the tested Cameroonian dietary plants in the treatment of bacterial infections including multi-drug resistant phenotypes.

Antibacterial activities of the extracts, fractions and compounds from Dioscorea bulbifera

Background

Dioscorea bulbifera is an African medicinal plant used to treat microbial infections. In the present study, the methanol extract, fractions (DBB1 and DBB2) and six compounds isolated from the bulbils of D. bulbifera, namely bafoudiosbulbins A (1), B (2), C (3), F (4), G (5) and 2,7-dihydroxy-4-methoxyphenanthrene (6), were tested for their antimicrobial activities against Mycobacteria and Gram-negative bacteria involving multidrug resistant (MDR) phenotypes expressing active efflux pumps.

Methods

The microplate alamar blue assay (MABA) and the broth microdilution methods were used to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of the above samples.

Results

The results of the MIC determinations indicated that when tested alone, the crude extract, fractions DBB1 and DBB2 as well as compounds 2 to 5 were able to prevent the growth of all the fifteen studied microorganisms, within the concentration range of 8 to 256 μg/mL. The lowest MIC value for the methanol extract and fractions (16 μg/mL) was obtained with DBB1 and DBB2 on E, coli AG100A and DBB2 on Mycobacterium tuberculosis MTCS2. The lowest value for individual compounds (8 μg/mL) was recorded with compound 3 on M. smegmatis and M. tuberculosis ATCC and MTCS2 strains respectively. The activity of the samples on many MDR bacteria such as Enterobacter aerogenes EA289, CM64, Klebsiella pneumoniae KP63 and Pseudomonas aeruginosa PA124 was better than that of chloramphenicol. When tested in the presence of the efflux pump inhibitor against MDR Gram-negative bacteria, the activity of most of the samples increased. MBC values not greater than 512 μg/mL were recorded on all studied microorganisms with fraction DBB2 and compounds 2 to 5.

Conclusions

The overall results of the present investigation provided evidence that the crude extract D. bulbifera as well as some of the compounds and mostly compounds 3 could be considered as potential antimicrobial drugs to fight against MDR bacteria.

Antimicrobial activities of the methanol extract and compounds from the twigs of Dorstenia mannii (Moraceae)

BACKGROUND

Dorstenia mannii (Moraceae) is a medicinal herb used traditionally for the treatment of many diseases. In the present study, the methanol extract of D. mannii and nine of its isolated compounds, namely dorsmanin A (1), B (2), C (3), D (4), E (6), F (7), G (8) dorsmanin I (9) and 6,8-diprenyleriodictyol (5), were tested for their antimicrobial activities against yeast, Mycobacteria and Gram-negative bacteria.

METHODS

The microplate alamar blue assay (MABA) and the broth microdilution method were used to determine the minimal inhibitory concentration (MIC) and minimal microbicidal concentration (MMC) of the above extract and compounds on a panel of bacterial species.

RESULTS

The results of the MIC determinations demonstrated that the methanol extract as well as compounds 3 and 8 were able to prevent the growth of all the fourteen studied microorganisms within the concentration range of 4 to 1024 μg/ml. The lowest MIC value for the methanol extract (64 μg/ml) was obtained on Candida albicans. The lowest value for individual compounds (4 μg/ml) was recorded with compounds 3 on Pseudomonas aeruginosa PA01 and 7 on Eschericia coli ATCC strain. The MIC values recorded with compounds 3 on P. aeruginosa PA01, 6 on C. albicans,7 on P. aeruginosa PA01 and K. pneumoniae ATCC strain and C. albicans,and 8 on P. aeruginosa PA01, PA124, P. stuartii, M. tuberculosis MTCS1 were lower than or equal to those of the reference drugs. MMC values not greater than 1024 μg/ml were recorded on all studied microorganisms with compounds 3 and 8.

CONCLUSION

The overall results of the present investigation provided evidence that the crude extract of D. mannii as well as some of its compounds such compounds 3 and 8 could be a potential source of natural antimicrobial products.

Antibacterial Activities of Selected Cameroonian Plants and Their Synergistic Effects with Antibiotics against Bacteria Expressing MDR Phenotypes

The present work was designed to assess the antibacterial properties of the methanol extracts of some Cameroonian medicinal plants and the effect of their associations with currently used antibiotics on multidrug resistant (MDR) Gram-negative bacteria overexpressing active efflux pumps. The antibacterial activities of twelve methanol extracts of medicinal plants were evaluated using broth microdilution. The results of this test showed that three extracts Garcinia lucida with the minimal inhibitory concentrations (MIC) varying from 128 to 512 μg/mL, Garcinia kola (MIC of 256 to 1024 μg/mL), and Picralima nitida (MIC of 128 to 1024 μg/mL) were active on all the twenty-nine studied bacteria including MDR phenotypes. The association of phenylalanine arginine β-naphthylamide (PAβN or efflux pumps inhibitor) to different extracts did not modify their activities. At the concentration of MIC/2 and MIC/5, the extracts of P. nitida and G. kola improved the antibacterial activities of some commonly used antibiotics suggesting their synergistic effects with the tested antibiotics. The results of this study suggest that the tested plant extracts and mostly those from P. nitida, G. lucida and G. kola could be used alone or in association with common antibiotics in the fight of bacterial infections involving MDR strains.

Antibacterial activities of selected Cameroonian spices and their synergistic effects with antibiotics against multidrug-resistant phenotypes

Background

The emergence of multi-drug resistant (MDR) phenotypes is a major public health problem today in the treatment of bacterial infections. The present study was designed to evaluate the antibacterial activities of the methanol extracts of eleven Cameroonian spices on a panel of twenty nine Gram negative bacteria including MDR strains.

Methods

The phytochemical analysis of the extracts was carried out by standard tests meanwhile the liquid micro-broth dilution was used for all antimicrobial assays.

Results

Phytochemical analysis showed the presence of alkaloids, phenols and tannins in all plants extracts. The results of the antibacterial assays indicated that all tested extracts exert antibacterial activities, with the minimum inhibitory concentration (MIC) values varying from 32 to 1024 μg/ml. The extracts from Dichrostachys glomerata, Beilschmiedia cinnamomea, Aframomum citratum, Piper capense, Echinops giganteus, Fagara xanthoxyloïdes and Olax subscorpioïdea were the most active. In the presence of efflux pump inhibitor, PAßN, the activity of the extract from D. glomerata significantly increased on 69.2% of the tested MDR bacteria. At MIC/5, synergistic effects were noted with the extract of D. glomerata on 75% of the tested bacteria for chloramphenicol (CHL), tetracycline (TET) and norfloxacin (NOR). With B. cinnamomea synergy were observed on 62.5% of the studied MDR bacteria with CHL, cefepime (FEP), NOR and ciprofloxacin (CIP) and 75% with erythromycin (ERY).

Conclusion

The overall results provide information for the possible use of the studied extracts of the spices in the control of bacterial infections involving MDR phenotypes.

Genetic regulation of the ramA locus and its expression in clinical isolates of Klebsiella pneumoniae

Tigecycline resistance has been attributed to ramA overexpression and subsequent acrA upregulation. The ramA locus, originally identified in Klebsiella pneumoniae, has homologues in Enterobacter and Salmonella spp. In this study, we identify in silico that the ramR binding site is also present in Citrobacter spp. and that Enterobacter, Citrobacter and Klebsiella spp. share key regulatory elements in the control of the romA-ramA locus. RACE (rapid amplification of cDNA ends) mapping indicated that there are two promoters from which romA-ramA expression can be regulated in K. pneumoniae. Correspondingly, electrophoretic binding studies clearly showed that purified RamA and RamR proteins bind to both of these promoters. Hence, there appear to be two RamR binding sites within the Klebsiella romA-ramA locus. Like MarA, RamA binds the promoter region, implying that it might be subject to autoregulation. We have identified changes within ramR in geographically distinct clinical isolates of K. pneumoniae. Intriguingly, levels of romA and ramA expression were not uniformly affected by changes within the ramR gene, thereby supporting the dual promoter finding. Furthermore, a subset of strains sustained no changes within the ramR gene but which still overexpressed the romA-ramA genes, strongly suggesting that a secondary regulator may control ramA expression.

An alkylaminoquinazoline restores antibiotic activity in Gram-negative resistant isolates

To date, various bacterial drug efflux pump inhibitors (EPIs) have been described. They exhibit variability in their activity spectrum with respect to antibiotic structural class and bacterial species. Among the various 4-alkylaminoquinazoline derivatives synthesized and studied in this work, one molecule, 1167, increased the susceptibility of important human-pathogenic, resistant, Gram-negative bacteria towards different antibiotic classes. This 4-(3-morpholinopropylamino)-quinazoline induced an increase in the activity of chloramphenicol, nalidixic acid, norfloxacin and sparfloxacin, which are substrates of the AcrAB-TolC and MexAB-OprM efflux pumps that act in these multidrug-resistant isolates. In addition, 1167 increased the intracellular concentration of chloramphenicol in efflux pump-overproducing strains. The rate of restoration depended on the structure of the antibiotic, suggesting that different sites in the efflux pumps may be involved. A molecule exhibiting a morpholine functional group and a propyl extension of the side chain was more active.

Efflux pumps are involved in the defense of Gram-negative bacteria against the natural products isobavachalcone and diospyrone

The activities of two naturally occurring compounds, isobavachalcone and diospyrone, against documented strains and multidrug-resistant (MDR) Gram-negative bacterial isolates were evaluated. The results indicated that the two compounds exhibited intrinsic antibacterial activity against several Gram-negative bacteria, and their activities were significantly improved in the presence of an efflux pump inhibitor (MIC values decreased to below 10 microg/ml). In addition, the activities of isobavachalcone and diospyrone against various strains exhibiting deletions of the major efflux pump components (AcrAB, TolC) were significantly increased. The overall results indicate that isobavachalcone and diospyrone could be candidates for the development of new drugs against MDR strains and that their use in combination with efflux pump inhibitors reinforces their activity.

New antibiotic molecules: bypassing the membrane barrier of gram negative bacteria increases the activity of peptide deformylase inhibitors

Background

Multi-drug resistant (MDR) bacteria have become a major concern in hospitals worldwide and urgently require the development of new antibacterial molecules. Peptide deformylase is an intracellular target now well-recognized for the design of new antibiotics. The bacterial susceptibility to such a cytoplasmic target primarily depends on the capacity of the compound to reach and accumulate in the cytosol.

Methodology/Principal Findings

To determine the respective involvement of penetration (influx) and pumping out (efflux) mechanisms to peptide deformylase inhibitors (PDF-I) activity, the potency of various series was determined using various genetic contexts (efflux overproducers or efflux-deleted strains) and membrane permeabilizers. Depending on the structure of the tested molecules, two behaviors could be observed: (i) for actinonin the first PDF-I characterized, the AcrAB efflux system was the main parameter involved in the bacterial susceptibility, and (ii), for the lastest PDF-Is such as the derivatives of 2-(5-bromo-1H-indol-3-yl)-N-hydroxyacetamide, the penetration through the membrane was a important limiting step.

Conclusions/Significance

Our results clearly show that the bacterial membrane plays a key role in modulating the antibacterial activity of PDF-Is. The bacterial susceptibility for these new antibacterial molecules can be improved by two unrelated ways in MDR strains: by collapsing the Acr efflux activity or by increasing the uptake rate through the bacterial membrane. The efficiency of the second method is associated with the nature of the compound.

Efflux pump, the masked side of beta-lactam resistance in Klebsiella pneumoniae clinical isolates

Background

β-lactamase production and porin decrease are the well-recognized mechanisms of acquired ß-lactam resistance in Klebsiella pneumoniae isolates. However, such mechanisms proved to be absent in K. pneumoniae isolates that are non susceptible to cefoxitin (FOX) and succeptible to amoxicillin+clavulanic acid in our hospital. Assessing the role of efflux pumps in this β-lactam phenotype was the aim of this study.

Methodology/Findings

MICs of 9 β-lactams, including cloxacillin (CLX), and other antibiotic families were tested alone and with an efflux pump inhibitor (EPI), then with both CLX (subinhibitory concentrations) and EPI against 11 unique bacteremia K. pneumoniae isolates displaying the unusual phenotype, and 2 ATCC strains. CLX and EPI-dose dependent effects were studied on 4 representatives strains. CLX MICs significantly decreased when tested with EPI. A similar phenomenon was observed with piperacillin+tazobactam whereas MICs of the other β-lactams significantly decreased only in the presence of both EPI and CLX. Thus, FOX MICs decreased 128 fold in the K. pneumoniae isolates but also16 fold in ATCC strain. Restoration of FOX activity was CLX dose-dependent suggesting a competitive relationship between CLX and the other β-lactams with regard to their efflux. For chloramphenicol, erythromycin and nalidixic acid whose resistance was also due to efflux, adding CLX to EPI did not increase their activity suggesting differences between the efflux process of these molecules and that of β-lactams.

Conclusion

This is the first study demonstrating that efflux mechanism plays a key role in the β-lactam susceptibility of clinical isolates of K. pneumoniae. Such data clearly evidence that the involvement of efflux pumps in ß-lactam resistance is specially underestimated in clinical isolates.

Phenotypic characterization of pore mutants of the Vibrio cholerae porin OmpU

General-diffusion porins form large beta-barrel channels that control the permeability of the outer membrane of gram-negative bacteria to nutrients, some antibiotics, and external signals. Here, we have analyzed the effects of mutations in the OmpU porin of Vibrio cholerae at conserved residues that are known to affect pore properties in the Escherichia coli porins OmpF and OmpC. Various phenotypes were investigated, including sensitivity to beta-lactam antibiotics, growth on large sugars, and sensitivity to and biofilm induction by sodium deoxycholate, a major bile component that acts as an external signal for multiple cellular responses of this intestinal pathogen. Overall, our results indicate that specific residues play different roles in controlling the passage of various compounds. Mutations of barrel wall arginine residues that protrude in the pore affect pore size and growth in the presence of large sugars or sodium deoxycholate. Sensitivity to large cephalosporins is mostly affected by D116, located on the L3 loop, whose homolog in E. coli, OmpF, is a known binding determinant for these drugs. L3 loop residues also affect biofilm induction. The results are interpreted in terms of a homology model based on the structures of E. coli porins.

Interaction of zwitterionic penicillins with the OmpF channel facilitates their translocation

To study translocation of beta-lactam antibiotics of different size and charge across the outer bacterial membrane, we combine an analysis of ion currents through single trimeric outer membrane protein F (OmpF) porins in planar lipid bilayers with molecular dynamics simulations. Because the size of penicillin molecules is close to the size of the narrowest part of the OmpF pore, penicillins occlude the pore during their translocation. Favorably interacting penicillins cause time-resolvable transient blockages of the small-ion current through the channel and thereby provide information about their dynamics within the pore. Analyzing these random fluctuations, we find that ampicillin and amoxicillin have a relatively high affinity for OmpF. In contrast, no or only a weak interaction is detected for carbenicillin, azlocillin, and piperacillin. Molecular dynamics simulations suggest a possible pathway of these drugs through the OmpF channel and rationalize our experimental findings. For zwitterionic ampicillin and amoxicillin, we identify a region of binding sites near the narrowest part of the channel pore. Interactions with these sites partially compensate for the entropic cost of drug confinement by the channel. Whereas azlocillin and piperacillin are clearly too big to pass through the channel constriction, dianionic carbenicillin does not find an efficient binding region in the constriction zone. Carbenicillin's favorable interactions are limited to the extracellular vestibule. These observations confirm our earlier suggestion that a set of high-affinity sites at the narrowest part of the OmpF channel improves a drug's ability to cross the membrane via the pore.

Inhibitors of efflux pumps in Gram-negative bacteria

In Gram-negative bacteria, efflux complexes, consisting of an inner-membrane pump, a periplasmic adaptor protein and outer-membrane channel, provide an efficient means for the export of structurally unrelated drugs, causing the multidrug-resistance phenotype. Resistance due to this antibiotic efflux is an increasing problem worldwide. A new molecular challenge is to combat this transport by searching for new molecules to block efflux and thus restore drug susceptibility to resistant clinical strains. Recent data shed new light on the structure and activity of the archetypal efflux pumps AcrAB-TolC and MexAB-OprM. Here, we describe recent insights into the molecular mechanisms of bacterial efflux pumps and their inhibitors. Current progress for the clinical use of efflux-pump inhibitors and new strategies to combat the drug-efflux mechanisms will be discussed.

Detection and prevalence of active drug efflux mechanism in various multidrug-resistant Klebsiella pneumoniae strains from Turkey

The prevalence of active drug efflux pump and porin alterations was investigated in Turkish nosocomial strains of Klebsiella pneumoniae exhibiting a multidrug-resistant phenotype. MICs of various antibiotics, including quinolones, chloramphenicol, tetracycline, and beta-lactams, for those strains were determined either with or without the efflux pump inhibitor phenylalanine arginine beta-naphthylamide (PAbetaN). Thirty-nine percent of the strains exhibited a PAbetaN-modulated resistance for quinolones, chloramphenicol, and tetracycline. In these strains, a significant increase of chloramphenicol accumulation was gained in the presence of the efflux pump inhibitor PAbetaN or with the energy uncoupler carbonyl cyanide m-chlorophenylhydrazone. Moreover, high-level expression of the membrane fusion protein AcrA, which was immunodetected in most of those isolates, suggests that the AcrAB/TolC efflux machinery contributed to their antibiotic resistance. Studies of K. pneumoniae porins indicated that the majority of the strains, including extended-spectrum beta-lactamase producers and efflux-positive ones, presented an alteration in their sorbitol-sensitive porin (OmpK35) expression. This is the first report showing the prominent role of active drug efflux in the antibiotic resistance of nosocomial K. pneumoniae strains from Turkey.

Antibiotic-resistant gram-negative bacteria in hospitalized children

Antibiotic-resistant Gram-negative bacilli are a prominent and growing problem among hospitalized children. Epidemics caused by these organisms have been implicated in many outbreaks in children's hospitals, primarily in neonatal intensive care units. These epidemics are characterized by efficient patient-to-patient transmission of the outbreak clone via the hands of caregivers and through exposure of contaminated inanimate sources. The epidemiology of these resistant organisms in pediatric hospitals during endemic periods is more complex. The isolates cultured from hospitalized individuals in the absence of an outbreak usually are unique to each individual and are derived from the patient's endogenous flora or other disparate sources. As in adults, chronic care facilities for children represent significant reservoirs of antibiotic-resistant bacilli that are circulated back into the acute care hospital environment when the child becomes ill.

Inhibitors of antibiotic efflux in resistant Enterobacter aerogenes and Klebsiella pneumoniae strains

In Enterobacter aerogenes and Klebsiella pneumoniae, efflux provides efficient extrusion of antibiotics and contributes to the multidrug resistance phenotype. One of the alkoxyquinoline derivatives studied here, 2,8-dimethyl-4-(2'-pyrrolidinoethyl)-oxyquinoline, restores noticeable drug susceptibility to resistant clinical strains. Analyses of energy-dependent chloramphenicol efflux indicate that this compound inhibits the efflux pump mechanism and improves the activity of structurally unrelated antibiotics on multidrug-resistant E. aerogenes and K. pneumoniae isolates.

Cefotaximases (CTX-M-ases), an expanding family of extended-spectrum β-lactamases

Among the extended-spectrum β-lactamases, the cefotaximases (CTX-M-ases) constitute a rapidly growing cluster of enzymes that have disseminated geographically. The CTX-M-ases, which hydrolyze cefotaxime efficiently, are mostly encoded by transferable plasmids, and the enzymes have been found predominantly in Enterobacteriaceae, most prevalently in Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, and Proteus mirabilis. Isolates of Vibrio cholerae, Acinetobacter baumannii, and Aeromonas hydrophila encoding CTX-M-ases have also been reported. The CTX-M-ases belong to the molecular class A β-lactamases, and the enzymes are functionally characterized as extended-spectrum β-lactamases. This group of β-lactamases confers resistance to penicillins, extended-spectrum cephalosporins, and monobactams, and the enzymes are inhibited by clavulanate, sulbactam, and tazobactam. Typically, the CTX-M-ases hydrolyze cefotaxime more efficiently than ceftazidime, which is reflected in substantially higher MICs to cefotaxime than to ceftazidime. Phylogenetically, the CTX-M-ases are divided into four subfamilies that seem to have descended from chromosomal β-lactamases of Kluyvera spp. Insertion sequences, especially ISEcp1, have been found adjacent to genes encoding enzymes of all four subfamilies. The class I integron-associated orf513 also seems to be involved in the mobilization of blaCTX-M genes. This review discusses the phylogeny and the hydrolytic properties of the CTX-M-ases, as well as their geographic occurrence and mode of spread.Key words: extended-spectrum β-lactamases, cefotaximases, phylogeny, dissemination, hydrolytic properties.

Role of AcrR and ramA in fluoroquinolone resistance in clinical Klebsiella pneumoniae isolates from Singapore

The MICs of ciprofloxacin for 33 clinical isolates of K. pneumoniae resistant to extended-spectrum cephalosporins from three hospitals in Singapore ranged from 0.25 to >128 microg/ml. Nineteen of the isolates were fluoroquinolone resistant according to the NCCLS guidelines. Strains for which the ciprofloxacin MIC was >or=0.5 microg/ml harbored a mutation in DNA gyrase A (Ser83-->Tyr, Leu, or IIe), and some had a secondary Asp87-->Asn mutation. Isolates for which the MIC was 16 microg/ml possessed an additional alteration in ParC (Ser80-->IIe, Trp, or Arg). Tolerance of the organic solvent cyclohexane was observed in 10 of the 19 fluoroquinolone-resistant strains; 3 of these were also pentane tolerant. Five of the 10 organic solvent-tolerant isolates overexpressed AcrA and also showed deletions within the acrR gene. Complementation of the mutated acrR gene with the wild-type gene decreased AcrA levels and produced a two- to fourfold reduction in the fluoroquinolone MICs. None of the organic solvent-tolerant clinical isolates overexpressed another efflux-related gene, acrE. While marA and soxS were not overexpressed, another marA homologue, ramA, was overexpressed in 3 of 10 organic solvent-tolerant isolates. These findings indicate that multiple target and nontarget gene changes contribute to fluoroquinolone resistance in K. pneumoniae. Besides AcrR mutations, ramA overexpression (but not marA or soxS overexpression) was related to increased AcrAB efflux pump expression in this collection of isolates.

Mechanism of quinolone resistance in anaerobic bacteria

Several recently developed quinolones have excellent activity against a broad range of aerobic and anaerobic bacteria and are thus potential drugs for the treatment of serious anaerobic and mixed infections. Resistance to quinolones is increasing worldwide, but is still relatively infrequent among anaerobes. Two main mechanisms, alteration of target enzymes (gyrase and topoisomerase IV) caused by chromosomal mutations in encoding genes, or reduced intracellular accumulation due to increased efflux of the drug, are associated with quinolone resistance. These mechanisms have also been found in anaerobic species. High-level resistance to the newer broad-spectrum quinolones often requires stepwise mutations in target genes. The increasing emergence of resistance among anaerobes may be a consequence of previous widespread use of quinolones, which may have enriched first-step mutants in the intestinal tract. Quinolone resistance in the Bacteroides fragilis group strains is strongly correlated with amino acid substitutions at positions 82 and 86 in GyrA (equivalent to positions 83 and 87 of Escherichia coli). Several studies have indicated that B. fragilis group strains possess efflux pump systems that actively expel quinolones, leading to resistance. DNA gyrase seems also to be the primary target for quinolones in Clostridium difficile, since amino acid substitutions in GyrA and GyrB have been detected in resistant strains. To what extent other mechanisms, such as mutational events in other target genes or alterations in outer-membrane proteins, contribute to resistance among anaerobes needs to be further investigated.