Background
Pseudomonas aeruginosa is an emerging global opportunistic multidrug-resistant (MDR) pathogen associated with high morbidity and mortality rates. The organism causes a number of infections such as pneumonia, urinary tract infection, and sepsis [
1]. Broad spectrum antimicrobial resistance in MDR
P. aeruginosa seriously limits effective therapeutic options. MDR phenotype can be mediated by a variety of resistance mechanisms including chromosomally encoded enzymes, expression of efflux pumps, and low membrane permeability. Various chromosomally encoded efflux systems and outer membrane porins have been identified as important contributors to resistance [
1]. The most relevant multidrug efflux systems in MDR pathogens are members of resistance–nodulation–division (RND) family. A number of MDR RND efflux pumps have been characterized in clinical isolates of
P. aeruginosa, namely MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM. Among these pumps, only MexAB-OprM is constitutively expressed at a level sufficient to confer intrinsic MDR in wild-type
P. aeruginosa strains [
2]. MexAB-OprM transports a number of antibiotics including fluoroquinolones, β-lactams, tetracycline, macrolides, chloramphenicol, novobiocin, trimethoprim, and sulphonamides [
3]. Mutations in
nalB or
mexR resulted in overexpression of MexAB-OprM efflux pump [
4].
Combination therapy may be beneficial for controlling MDR
P. aeruginosa that could restore susceptibility to various antibiotics [
5‐
7]. A number of potent efflux pump inhibitors including phenylalanyl arginyl β-naphthylamide (PAβN), carbonyl cyanide
m-chlorophenylhydrazone (CCCP), quinoline derivatives, and 1-(1-Naphthylmethyl)-piperazine (NMP) have been reported to enhance antibiotic activity against antibiotic-resistant Gram-negative bacteria. In addition, various compounds such as PAβN, ethylenediaminetetraacetic acid (EDTA), and polymyxin B nonapeptide (PMBN) have been documented to permeabilize the bacterial outer membrane. However, none has reached potential clinical applications because of its toxicity [
8].
A number of plant extracts and phytochemical products have demonstrated their potential as synergists or potentiators of other antibacterial agents [
9]. Curcumin derived from
Curcuma longa inhibited efflux pump systems in
P. aeruginosa, resulting in restoring gentamicin and ciprofloxacin activity [
10]. Extract from
Holarrhena antidysenterica displayed resistance modifying ability to enhance novobiocin and rifampicin activity against
Acinetobacter baumannii [
11,
12]
. It has been demonstrated that the extract potentiated the effect of antibiotics by acting as a permeabilizer [
13]. Moreover, a recent study indicated that both
Holarrhena antidysenterica extract and conessine, a steroidal alkaloid compound, could restore antibiotic activity due to interference with AdeIJK pump in
A. baumannii [
14]. Previous study documented that AdeIJK pump and MexAB-OprM pump are functionally equivalent pumps in both organisms [
15].
Holarrhena antidysenterica belonging to family Apocynaceae has been employed as an ethnobotanical plant for the treatment of dysentery, diarrhoea, fever, and bacterial infections. Biological activities of the plant including antimalarial, anti-diabetic, anti-oxidant, anti-urolithic, anti-mutagenic, CNS-stimulating, angiotensin-converting-enzyme inhibitory, and acetylcholinesterase inhibitory activity were documented [
16]. In contrast, anti-diarrhoea and anti-plasmodial effects of conessine were briefly mentioned [
17].
This study aimed to investigate (i) whether conessine, a steroidal alkaloid compound, could act as a resistance modifying agent against multidrug-resistant Pseudomonas aeruginosa, and (ii) whether MexAB-OprM efflux pump is involved in the mechanism.
Methods
Bacterial strains
P. aeruginosa PAO1 strain K767 (wild-type), MexAB-OprM overexpressed strain K1455 (PAO1-nalB), and MexB deletion strain K1523 (PAO1-∆mexB) were generously provided by Professor Dr. R. Keith Poole, Queen’s University, Kingston, Ontario, Canada.
Phenylalanine-arginine β-naphthylamide (PAβN), 1-N-phenylnaphthylamine (NPN), Hoechst 33,342 (H33342), conessine, and antibiotics were purchased from Sigma–Aldrich (St Louis, MO, USA). Dimethylsulfoxide (DMSO) and ethylenediaminetetraacetic acid (EDTA) were obtained from Merck (Merck, Germany).
Mueller-Hinton broth (MHB) and Tryptic soy agar (TSA) were purchased from Becton Dickinson Microbiology Systems (Sparks, MD, USA).
Antibacterial activity assays
Minimum inhibitory concentration was tested by broth microdilution assay in accordance with the Clinical and Laboratory Standards Institute (CLSI) recommendation [
18]. Antibiotics used in this study were selected based on substrate specificity of Ade efflux pump in
A. baumannii: cefotaxime for AdeDE pump, novobiocin for AdeIJK pump, rifampicin for AdeDE and AdeIJK pump, erythromycin, levofloxacin, and tetracycline for AdeABC, AdeDE, and AdeIJK pump. In addition, cefotaxime, levofloxacin, novobiocin, and tetracycline have been reported as substrates for MexAB-OprM in
P. aeruginosa. Stock solution of novobiocin (50 mg/L), rifampicin (1 mg/L), levofloxacin (18 mg/L), erythromycin (2 mg/L), cefotaxime (10 mg/L), and PAβN (10 mg/L) were prepared in sterile deionized water. Tetracycline (4 mg/L) and conessine (1 mg/L) were dissolved in 95% ethanol and 100%DMSO, respectively. Serial dilutions of conessine, PAβN, and antibiotics were prepared in MHB. In order to investigate the effect of each agent, 100 μL bacterial culture (1 × 10
6 cfu/mL) was mixed with 100 μL each conessine, PAβN, or antibiotics. Synergistic effects of conessine (20 mg/L) or PAβN (25 mg/L) and antibiotics were assessed using checkerboard assay by adding 100 μL culture into a well containing 50 μL conessine or PAβN and 50 μL antibiotics. DMSO at a final concentration of 1% used as a negative control and PAβN, an efflux pump inhibitor was used as a positive control. Plates were then read after 18 h of incubation at 37 °C. Each assay with three biological triplicates was repeated at least twice. A 4-fold or greater reduction in MIC values after addition of conessine or PAβN was considered significant. Fractional inhibitory concentration index (FICI) value was calculated for each combination according to the following formula [
19]: FICI = (MIC of efflux pump inhibitors in combination/MIC of efflux pump inhibitors alone) + (MIC of antibiotics in combination/MIC of antibiotics alone). Synergy, additivity, and antagonism were defined as FICI <1, =1, and >1, respectively.
H33342 accumulation assay
H33342 accumulation assay was performed to evaluate the effect of efflux pump inhibitors on the activity of MexAB-OprM efflux pump [
20]. Briefly, overnight bacterial cultures were inoculated into MHB and rotated at 250 rpm at 37 °C for 4–5 h. Bacterial cells were harvested by centrifugation (3000 rpm for 15 min) and the cells were washed with phosphatebuffered saline containing 1 mM MgSO
4 and 20 mM glucose. After centrifugation, the pellets were resuspended in the same buffer and OD
600 of each suspension was adjusted to 0.4. An aliquot of 100 μL of the bacterial suspension was added into a well in black microtiter plate containing each of 50 μL conessine (20 mg/L) or an efflux pump inhibitor, PAβN (25 mg/L).
The final concentration of DMSO in all assays was ≤1%. Plates were incubated at 37 °C for 15 min and 50 μL H33342 (2.5 μM) was added to each assay well. Fluorescence (excitation 355 nm, emission 460 nm) was measured at 37 °C every 2.30 min for 1 h using a Varioskan Flash spectral scanning multimode reader. Each assay was repeated at least twice. Differences in accumulation in the presence of efflux pump inhibitors compared with the absence of efflux pump inhibitors were analysed for statistical significance using Student’s t-test. P value ≤0.05 was considered significant.
NPN uptake assay
Ability of conessine to permeabilize
P. aeruginosa outer membrane was assessed by NPN uptake assay [
21]. NPN, an uncharged lipophilic molecule, fluoresces weakly in aqueous environments but becomes strongly fluorescent in nonpolar environments such as cell membranes. Briefly, overnight bacterial cultures were inoculated into MHB and rotated at 250 rpm at 37 °C for 4–5 h. Bacterial cells were harvested at 3000 rpm for 15 min, washed with 100 mM NaCl and 50 mM sodium phosphate buffer (pH 7.0), and resuspended in the same buffer at A 600 = 0.1 in the presence of 0.05% of glucose. An aliquot of 100 μL of the bacterial suspension was added into a well in black microtiter plate containing each of 50 μL conessine (20 mg/L) or EDTA (100 μM) as a permeabilizer followed by adding 50 μL of NPN (40 μM). The final concentration of DMSO in all assays was ≤1%. NPN fluorescence intensity (excitation 322 nm, emission 424 nm) was monitored at 37 °C after 2.30 min for 1 h using a Varioskan Flash spectral scanning multimode reader (Thermo Fisher Scientific, Finland). Each assay was repeated at least twice. Differences in accumulation in the presence of efflux pump inhibitors compared with the absence of efflux pump inhibitors were analysed for statistical significance using Student’s
t-test.
P value ≤0.05 was considered significant.
Discussion
Inhibition of MexAB-OprM efflux pump appears to be an attractive approach to restore efficacy of antibiotics that were substrates of this pump. Herein, we describe efficacy of conessine, an inhibitor of RND class MexAB-OprM efflux pump, which is the major efflux pump and plays a vital role in MDR phenotype in
P. aeruginosa. The present study demonstrated that conessine displayed characteristics of an efflux pump inhibitor as previously documented by Lomovskaya et al. [
5].
A mechanism of efflux pump inhibition by conessine was possibly through competitive inhibition and/or blockage of access to the substrate binding site of MexB. In comparison with MexB-specific PAβN, the compounds might interact with “G-loop” or “switch loop”, which separates the distal and the proximal binding sites. G-loop has been proposed to be involved in movement of substrates from the proximal to the distal site. Therefore, efflux pump inhibitors inhibited MexB extrusion of various substrates through binding to G-loop [
26]. Differences in spectrum of antibiotics enhanced by conessine versus PAβN suggested that conessine may bind to a different site in MexB binding pocket.