Sandrine Alibert-Franco

Strategies for bypassing the membrane barrier in multidrug resistant Gram-negative bacteria

Regarding bacterial susceptibility towards antibacterial agents, membrane permeability is part of the early bacterial defense. The bacterium manages the translocation process, influx and efflux, to control the intracellular concentration of various molecules. Antibiotics and biocides are substrates of these mechanisms and the continuing emergence of multidrug resistant isolates is a growing worldwide health concern. Different strategies could be proposed to bypass the bacterial membrane barrier, comprising influx and efflux mechanisms, in order to restore the activity of antibiotics against resistant bacteria.

Antibacterial activity of some natural products against bacteria expressing a multidrug-resistant phenotype

The present study assessed the antimicrobial activities of various natural products belonging to the terpenoids, alkaloids and phenolics against a collection of Gram-negative multidrug-resistant (MDR) bacteria. The results demonstrated that most of the compounds were extruded by bacterial efflux pumps. In the presence of the efflux pump inhibitor phenylalanine arginine β-naphthylamide (PAβN), the activities of laurentixanthone B (xanthone), plumbagin (naphthoquinone), 4-hydroxylonchocarpin (flavonoid) and MAB3 (coumarin) increased significantly against all studied MDR bacteria. Laurentixanthone B, 4-hydroxylonchocarpin and MAB3 contained the same pharmacophoric moiety as plumbagin. This study indicates that the AcrAB-TolC (Enterobacteriaceae) and MexAB-OprM (Pseudomonas aeruginosa) efflux pumps are involved in resistance of Gram-negative bacteria to most of the natural products.

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

ABSTRACT 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 μg/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.

Membrane permeation by multidrug-resistance-modulators and non-modulators: effects of hydrophobicity and electric charge.

This study was designed to test the hypothesis that lipophilic cationic drugs with only roughly similar structures mediate the reversal of multidrug‐resistance (MDR) by interacting with membrane phospholipids. The permeation properties of MDR‐modulators and non‐modulators were studied by quantifying their ability to induce the leakage of Sulphan blue through the membrane of negatively charged unilamellar liposomes.

Efflux pumps of gram-negative bacteria, a new target for new molecules.

Antibiotic resistance mechanisms reported in Gram-negative bacteria are a worldwide health problem. The continuous dissemination of multi-drug resistant (MDR) bacteria drastically reduces the efficacy of our antibiotic “arsenal” and consequently increases the frequency of therapeutic failure. In MDR bacteria the over-expression of efflux pumps expel structurally-unrelated antibiotics decreasing their intracellular concentration. It is necessary to clearly define the molecular and genetic bases of the efflux pump in order to combat this mechanism. The characterization of efflux pumps, from genetic to structural studies, allows the definition of a new, original antiresistance bullet, the efflux pump inhibitor (EPI). This new class of antibacterial molecules may act conjointly to the usual antibiotic in order to restore its activity. Several families of EPIs have been now reported and described. The use of these EPIs promotes a significant increase of susceptibility to one or more antibiotics in strains or clinical isolates which were initially resistant. These EPIs may target different efflux targets, (i) the expression of genes that induces MDR, the transporters that pump the antibiotic out of bacterium, (ii) the assembly of membrane transporter complex involved in drug efflux, (iii) the energy involved in this active transport, (iv) the inhibition of the flux of molecules inside the efflux channel by competition or blocking (via steric hindrances). With the recent thorough characterization of the efflux pump AcrB at its structural and physiological level including the identification of drug affinity sites and kinetic parameters for some antibiotics, it is now possible to rationally develop an improved new generation of EPIs.

Efflux mechanism, an attractive target to combat multidrug resistant Plasmodium falciparum and Pseudomonas aeruginosa.

Chemoresistance is a general health problem concerning infectious diseases and cancer treatments. In this context, the worldwide dissemination of << pandrug >> and << multidrug>> resistant pathogens has severely compromised the efficacy of our antimicrobial weapons and dramatically increased the occurence of therapeutic failure. To efficiently combat multi-resistant pathogens, it is necessary to clearly define the molecular basis of the general resistance mechanism associated with the expression of active efflux pumps, which strongly restrict the intracellular concentration of antimicrobial drugs. Several families of efflux systems capable of multiple drug extrusion have been described. The activity of some efflux systems requires ATP hydrolysis for drug transport while others require a sodium or proton antiport. In this review we focus on two important human pathogens, Plasmodium falciparum and Pseudomonas aeruginosa, which exhibit a high level of antimicrobial resistance associated with the expression of efflux mechanisms. The efflux mechanisms and the development of efflux pump inhibitors (EPIs) are discussed regarding these two pathogens.

The crystal and molecular structures of 9,10-dihydro-9, 10-ethano- and etheno-anthracenes

As part of our investigation on histamine receptor antagonists, the structures of two 9-10-dihydroanthracene derivatives are reported: 4: C22H28N2; hexagonal, P61; a = 10.014(1), c = 34.556(7) Å; V = 3001.0(7)Å3; and Z = 6; 9:C19H19N: triclinic, P-1; a = 8.427(2), b = 12.806(3), c = 15.292(3) Å, α = 70.27(3), β = 80.83(3), γ = 72.98(3)°; V = 1482.0(6) Å3; and Z = 4. All non-aromatic rings in both molecules adopt a similar conformation. However, the three six-membered rings in the etheno-anthracene structure adopt a more regular boat form in spite of two sp2 carbon bridged atoms.

Synthesis and X-ray structure of 11-N-benzylamido-9,10-dihydro-9,10-ethenoanthracene

As a part of studies on MDR reversing agents, the structure of a benzylamido-9,10-dihydro-9,10-ethenoanthracene is reported: C24 H19 N O; orthorhombic; space group: Pca21; a = 9.214(2), b = 18.624(4), c = 10.170(2) Å; 1745.2(6) Å3; Z = 4. The three nonplanar rings from the 9,10-dihydro-9,10-ethenoanthracene skeleton of the molecule adopt a boat conformation. The benzamide side-chain is extended (conformation, trans-trans-gauche). The molecules in the crystal are joined by infinite chains of H-bonds N14–H14···O13.