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In Vitro antibacterial and antibiotic-potentiation activities of four edible plants against multidrug-resistant gram-negative species
BMC Complementary and Alternative Medicinevolume 13, Article number: 190 (2013)
The present study was designed to investigate the antibacterial activities of the methanol extracts of four Cameroonian edible plants, locally used to treat microbial infections, and their synergistic effects with antibiotics against a panel of twenty nine Gram-negative bacteria including Multi-drug resistant (MDR) phenotypes expressing active efflux pumps.
The broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) of the extracts [alone and in the presence of the efflux pumps inhibitor (EPI) Phenylalanine-Arginine β- Naphtylamide (PAβN)], and those of antibiotics in association with the two of the most active ones, Piper nigrum and Telfairia occidentalis. The preliminary phytochemical screening of the extracts was conducted according to the standard phytochemical methods.
Phytochemical analysis showed the presence of alkaloids and flavonoids in all studied extracts. Other chemical classes of secondary metabolites were selectively present in the extracts. The results of the MIC determination indicated that the crude extracts from P. nigrum and V. amygdalina were able to inhibit the growth of all the twenty nine studied bacteria within a concentration range of 32 to 1024 μg/mL. At a similar concentration range (32 to 1024 μg/mL) the extract from T. occidentalis inhibited the growth of 93.1% of the tested microorganisms. At MIC/2 and MIC/5, synergistic effects were noted between the extracts from P. nigrum and T. occidentalis and seven of the tested antibiotics on more than 70% of the tested bacteria.
The overall results of the present study provide information for the possible use of the studied edible plants extracts in the control of bacterial infections including MDR phenotypes.
Despite the impressive scientific progress in vaccination and chemotherapy, infectious diseases remain a serious health issue. Following the massive and inappropriate use of antibiotics, bacteria have developed various mechanism of resistance; consequently, infectious diseases remain one of the leading causes of morbidity worldwide . Microbial infections constitute a major public health problem in developing countries  where the high cost of antibiotics makes them unaffordable to the majority of the population. Therefore, the discovery of new antimicrobial agents is still relevant nowadays. Among the bacterial resistance mechanisms, efflux of antibiotics plays an important role; In fact it is widely recognized that the expression of efflux pumps encoded by house-keeping genes in bacteria is largely responsible for the phenomenon of intrinsic antibiotic resistance . Also, the shortcomings of the drugs available today and the scarcity of novel antibiotics propel the discovery of new chemotherapeutic agents from medicinal plants . The medicinal properties of many phytochemicals have been demonstrated . In addition, promising new concepts such as the efflux pump inhibitors [6, 7], and synergy between antibiotics and phytochemicals are now been explored.
The present work was therefore designed to investigate the antibacterial potential of four Cameroonian edible plants used traditionally in the treatment of bacterial infections, namely the fruits of Piper nigrum L (Piperaceae), the leaves of Telfairia occidentalis Hook. F. (Cucurbitaceae) and Vernonia amygdalina Del. (Asteraceae) and the fruits of Syzygium aromaticum [L.] Merr & Perry (Myrtaceae) against MDR bacteria expressing active efflux via the Resistance-Nodulation Cell Division (RND)-type pumps.
Plant material and extraction
The four edible plants used in this work were purchased from Dschang local market, West Region of Cameroon in June 2010. The collected plants material were the fruits of Piper nigrum, the fruits of Syzygium aromaticum, the leaves of Telfairia occidentalis and the leaves of Vernonia amygdalina. These plants were identified by M. Victor Nana of the National Herbarium (Yaounde-Cameroon) where all the voucher specimens were available under the reference numbers (see Table 1). The air dried and powdered sample (1 kg) from each plant was extracted with methanol (MeOH) for 48 h at room temperature. The extracts were then filtered and concentrated under reduced pressure to give the crude extracts. All extracts were kept at 4°C until further investigations.
Preliminary phytochemical investigations
The major classes of secondary metabolites such as alkaloids, anthocyanins, anthraquinones, flavonoids, phenols, saponins, tannins, sterols and triterpenes were screened according to the common phytochemical methods described by Harbone .
Bacterial strains and culture media
The studied microorganisms included the reference (from the American Type Culture Collection) and clinical (Laboratory collection) strains of Providencia stuartii, Pseudomonas aeruginosa, K. pneumoniae, Escherichia coli, Enterobacter aerogenes and Enterobacter cloacae (See supporting information Additional file 1: Table S1 for their features). They were maintained in a Nutrient Broth at 4°C and activated on a fresh appropriate Mueller Hinton Agar plates 24 h prior to antimicrobial test. The Mueller Hinton Broth (MHB) was also used for all the antibacterial assays.
Chemicals for antimicrobial assays
Tetracycline (TET), cefepime (FEP), cloxacillin (CLX), streptomycine (STR), ciprofloxacine (CIP), norfloxacine (NOR), chloramphenicol (CHL), cloxacillin (CLX), ampicillin (AMP), erythromycin (ERY), kanamycin (KAN) and streptomycin (STR) (Sigma-Aldrich, St Quentin Fallavier, France) were used as reference antibiotics. p-Iodonitrotetrazolium chloride (INT) and Phenylalanine Arginine β-naphthylamide (PAβN) were used as microbial growth indicator and efflux pumps inhibitor (EPI) respectively.
Bacterial susceptibility determination
The MICs were determined using the rapid INT colorimetric assay [45, 46]. Briefly, the test samples were first dissolved in DMSO/MHB. The solution obtained was then added to MHB, and serially diluted two fold (in a 96- wells microplate). One hundred microlitres (100 μL) of inoculum (1.5 × 106 CFU/mL) prepared in MHB was then added. The plates were covered with a sterile plate sealer, then agitated to mix the contents of the wells using a shaker and incubated at 37°C for 18 h. The final concentration of DMSO was set at 2.5% (a concentration at which DMSO does not affect the microbial growth). Wells containing MHB, 100 μL of inoculum and DMSO at a final concentration of 2.5% served as negative control (this internal control was systematically added). Chloramphenicol was used as reference antibiotic. The MICs of samples were detected after 18 h incubation at 37°C, following addition of 40 μL of a 0.2 mg/mL INT solution and incubation at 37°C for 30 minutes. Viable reduce the yellow dye to pink. MIC was defined as the lowest sample concentration that exhibited complete inhibition of microbial growth and then prevented this change .
Samples were tested alone and the best three extracts (those from the seeds of P. nigrum, T. occidentalis and V. amygdalina) were also tested in the presence of PAβN at 30 mg/L final concentration. After a preliminary assay on one of the MDR bacteria, P. aeruginosa PA124 (See supporting information Additional file 1: Table S2), the two best extracts were those from P. nigrum and T. occidentalis. They were then selected and tested at MIC/2 and MIC/5 in association with antibiotics. Fractional inhibitory concentration (FIC) was calculated as the ratio of MICAntibiotic in combination/MICAntibiotic alone and the results were discussed as follows: synergy (≤ 0.5), indifferent (0.5 to 4), or antagonism (> 4) [48, 49]. All assays were performed in triplicate.
Phytochemical composition and antibacterial activity of the extracts
The results of the qualitative phytochemical analysis showed that each of the tested plant extract contains at least 3 classes of secondary metabolites (Table 2). The antibacterial activities of the extracts alone and in some cases in combination with PAβN on a panel of 29 Gram-negative bacteria are depicted in Table 3. It appears that extracts from P. nigrum and V. amygdalina inhibited the growth of all the twenty nine tested bacterial strains within a concentration range from 32 to 1024 μg/mL. A good spectrum of antibacterial activity was also recorded with the extract of T. occidentalis, its inhibitory effects being observed against 27/29 (93.1%) of the tested microorganisms. The lowest MIC value (32 μg/mL) was obtained with the extract of P. nigrum on P. aeruginosa PA01.
Role of efflux pumps in the susceptibility of Gram-negative bacteria to the tested plant extracts
Fourteen of the studied MDR bacteria were also tested for their susceptibility to the most active plant extracts (P. nigrum, V. amygdalina and T. occidentalis) in the presence PAβN at 30 μg/mL. When combined with extracts, PAβN improved the activity (decrease of MIC values) of P. nigrum on almost all of the tested MDR strains [13/14 (92.9%)]. The EPI also improved the activity of T. occidentalis against E. aerogenes CM64, EA 289 and E. cloacae BM67 as well as that of V. amygdalina against E. coli AG100 and E. aerogenes EA 289 (Table 3).
Effect of the association of extracts with antibiotics
A preliminary study (See supporting information; Additional file 1: Table S2) was performed against P. aeruginosa PA124 using the three most active plant extracts. The results permitted the selection of the extracts from P. nigrum and T. occidentalis with the appropriate sub-inhibitory concentrations of MIC/2 and MIC/5 for further studies. Therefore, the extracts from P. nigrum and T. occidentalis were combined with eleven antibiotics [TET, DOX, CIP, NOR, STR, KAN, CHL, ERY, FEP, CLX and AMP] separately to evaluate their possible synergistic effects. As results, synergistic effects were observed with the two extracts and most of the tested antibiotics except ß-lactams (AMP, FEP and CLX) (Tables 4 and 5). At MIC/2 and MIC/5 of the extract from T. occidentalis, synergistic effects were observed with 7 of the 11 antibiotics (TET, DOX, CIP, NFX, KAN, CHL, ERY) against the tested MDR bacteria (Table 5).
Antibacterial activities and chemical composition of the tested extracts
Many secondary metabolites belonging to alkaloids, anthocyanins, anthraquinons, flavonoids, phenols, saponins, sterols, tannins and triterpenes were detected in the tested plant extracts. Several compounds from the investigated classes of phytochemicals were reported for their antibacterial activities [50, 51], and their presence in the tested extracts could explain their antibacterial effects. The differences in bacterial susceptibility to the extracts may be either due to the differences in cell wall composition and/or genetic content of their plasmids  or to the differences in the composition and the mechanism of action of the bioactive compounds . As shown in Table 3, the three most active plants (P. nigrum, T. occidentalis and V. amygdalina) possess more classes of phytochemicals than the extract from S. aromaticum. Each of the three most active plant extracts contains at least four classes of secondary metabolites namely alkaloids, phenols, flavonoids and tannins. However, it should be noted that the activity does not depend on the number of classes of detected bioactive compounds, but mostly on their concentration. The inhibitory activity of P. nigrum was previously reported against some bacteria such as Staphylococcus aureus, Bacillus cereus, Streptococcus faecalis, Pseudomonas aeruginosa, Salmonella typhi and Escherichia coli, and the data reported in this study confirms the anti-infective potential of this plant. It has also been demonstrated that the acetone-ethanol extract of the leaves from V. amygdalina was weakly active against K. pneumoniae, E. coli, S. aureus, B. cereus, S. dysentriae and S. typhimurium with MIC values ranged from 7.5 mg/mL to 25 mg/mL . These activities are in accordance with the results obtained in the present work, but we observed higher antibacterial activity of this plant on all 29 bacteria including MDR phenotypes (with MIC values ranging between 256 and 1024 μg/mL).
Role of efflux pumps in the susceptibility of Gram-negative bacteria to the tested extracts and effects of the association of some extracts with antibiotics
All the bacterial strains tested with a combination of plant extract and PAβN were proven to possess multidrug resistance efflux pumps [55–59]. Tripartite efflux systems, mainly those clinically described such as AcrAB-TolC in Enterobacteriaceae or MexAB-OprM in P. aeruginosa play a central role in multidrug resistance of pathogenic Gram-negative bacteria [55, 56]. PAßN, a potent inhibitor of the RND efflux systems is especially active on AcrAB-TolC and MexAB-OprM [57, 58] and does not present any intrinsic effect on the bacteria at the concentration of 30 μg/mL used in this work . In the presence of PAßN at this concentration, significant increase of the activity of the extract from P. nigrum was noted against 13/14 of the tested MDR bacteria. This shows that at least one active compound from this plant, acting inside the bacteria cell could be the substrate of efflux pumps. From this observation, it can be suggested that the association of the extract of P. nigrum and efflux pump inhibitors could be helpful in the fight against infections due to MDR bacteria .
Moreover, we demonstrated in this study that the beneficial effect of the combination of two of the tested plant extracts namely those from P. nigrum and T. occidentalis, with the first line antibiotics could be achieved. Their synergistic effects with antibiotics were noted on more than 70% of the tested MDR bacteria (with seven antibiotics), also suggesting that some of their constituents can act as efflux pump inhibitor . This hypothesis is emphasized by the fact that these extracts were more synergistic with antibiotics acting inside the bacteria cells. Besides, it has already been proved that the extract from P. nigrum can also act by improving the penetration of antibiotics in cells via membrane alteration . However, further phytochemical investigations will be done to isolate the active constituents of P. nigrum, T. occidentalis and V. amygdalina. Besides, toxicological studies will be carried out to evaluate their safety.
The overall results of the present study provide baseline information for the possible use of the tested plants and mostly P. nigrum, T. occidentalis and V. amygdalina in the control of infections due to MDR Gram-negative bacteria. In addition, the extracts from P. nigrum and T. occidentalis could be used in association with antibiotics to combat multidrug resistant pathogens.
American type culture collection
Colony forming unit
Efflux pump inhibitor
Fractional inhibitory concentration
Mueller hinton broth
Minimal inhibitory concentration
Phenylalanine arginine ß-Naphthylamide
Resistance nodulation-cell division
Ahluwalia G, Sharma SK: Philanthropy and medical science: at last a new dawn for tuberculosis also!. Indian J Chest Dis Allied Sci. 2007, 49: 71-73.
Adwan G, Abu-Shanab B, Adwan K: Antibacterial activities of some plant extracts alone and in combination with different antimicrobials against multidrug-resistant Pseudomonas aeruginosa strains. Asian Pac J Trop Med. 2010, 3 (4): 266-269. 10.1016/S1995-7645(10)60064-8.
Poole K: Efflux-mediated antimicrobial resistance. J Antimicrob Chemother. 2005, 56: 20-51. 10.1093/jac/dki171.
Ates DA, Erdogrul OT: Antimicrobial activity of various medicinal and commercial plants extracts. Turk J Biol. 2003, 27: 157-162.
Rahman S, Parvez AK, Islam R, Khan MH: Antibacterial activity of natural spices on multiple drug resistant Escherichia coli isolated from drinking water Bangladesh. Ann Clin Microbiol Antimicrob. 2011, 10: 10-10.1186/1476-0711-10-10.
Newman DJ, Cragg GM: Natural products as sources of new drugs over the last 25 years. J Nat Prod. 2007, 70 (3): 461-477. 10.1021/np068054v.
Coutinho HDL, Siqueira-Júnior JG, JP : Additive effects of Hyptis martiusii Benth with aminoglycosides against Escherichia coli. Indian J Med Res. 2010, 131: 106-108.
Nisar A, Fazal H, Abbasi BH, Farooq S, Ali M, Khan MA:Biological role ofPiper nigrumL. (Black pepper): a review. Asian Pac J Trop Biomed. 2012, 2 (3): S1945-S1953. 10.1016/S2221-1691(12)60524-3.
Menon AN, Padmakumari KP, Jayalekshmy A: Essential oil composition of four major cultivars of Black Pepper (Piper nigrum L) III. J Essent Oil Res. 2003, 15 (3): 155-157. 10.1080/10412905.2003.9712099.
Kang MJ, Cho JYS, Duk K, Kim BH, Lee J: Bioavailability enhancing activities of natural compounds from medicinal plants. J Med Plant Res. 2009, 3 (13): 1204-1211.
Ee GCL, Lim CM, Rahmani M, Shaari K, Bong CFJ: Pellitorine, a potential Anti-cancer lead compound against HL60 and MCT-7 cell lines and microbial transformation of piperine from Piper nigrum. Molecules. 2010, 15 (4): 2398-2404. 10.3390/molecules15042398.
Vijayakumar RS, Surya D, Nalini N: Antioxidant efficacy of black pepper (Piper nigrum L.) and piperine in rats with high fat diet induced oxidative stress. Redox Rep. 2004, 9 (2): 105-110. 10.1179/135100004225004742.
Neha JMR: Antioxidant activity of Trikatu megaExt. Inter J Res Pharm Biomed Sci. 2011, 2 (2): 624-628.
Erturk O: Antibacterial and antifungal activity of ethanolic extracts from eleven spice plants. Biologia Bratisl. 2006, 61 (3): 275-278. 10.2478/s11756-006-0050-8.
Li S, Wang C, Li W, Koike K, Nikaido T, Wang MW: Antidepressant-like effects of Piperine and its derivative, antiepilepsirine. J Asian Nat Prod Res. 2007, 9: 421-430. 10.1080/10286020500384302.
Umit AK, Akgun KO: Antifungal activity of aqueous extracts of spices against bean rust (Uromyces appendiculatus). Allelopathy J. 2009, 24: 0973-5046.
Parmar VS, Bracke ME, Philippe J, Wengel J, Jain SC, Olsen CE, Bisht KS, Sharma NK, Courtens A, Sharma SK, Vennekens K, Van Marck V, Singh SK, Kumar N, Kumar A, Malhotra S, Kumar R, Rajwanshi VK, Jain R, Mareel MM: Anti-invasive activity of alkaloids and polyphenolics in vitro. Bioorg Med Chem. 1997, 5 (8): 1609-1619. 10.1016/S0968-0896(97)00091-6.
Kumar S, Singhal V, Roshan R, Sharma A, Rembhotkar GW, Ghosh B: Piperine inhibits TNF- α induced adhesion of neutrophils to endothelial monolayer through suppression of NF-κ and IκB kinase activation. Eur J Pharmacol. 2007, 575: 177-186. 10.1016/j.ejphar.2007.07.056.
Pathak N, Khandelwal S: Cytoprotective and immunomodulating properties of piperine on murine splenocytes: an in vitro study. Eur J Pharmacol. 2007, 576 (1–3): 160-170.
Sunila ES, Kuttan G: Immunomodulatory and antitumor activity of Piper longum Linn. and piperine. J Ethnopharmacol. 2004, 90 (2–3): 339-346.
Upadhyay RK, Jaiswal G: Evaluation of biological activities of Piper nigrum oil against Tribolium castaneum. Bull Insectology. 2007, 60: 57-61.
Tajuddin Ahmad S, Latif A, Qasmi I: Effect of 50% ethanolic extract of Syzygium aromaticum (L.) Merr. & Perry. (clove) on sexual behaviour of normal male rats. BMC Complement Altern Med. 2004, 4 (1): 17-10.1186/1472-6882-4-17.
Sharma A, Kumar M, Kaur S: Modulatory effects of Syzygium aromaticum (L.) Merr. & Perry and Cinnamomum tamala Nees & Ebrem. on toxicity induced by chromium trioxide. Phytopharmacology. 2011, 1 (4): 71-81.
Moleyar V, Narasimham P: Antibacterial activity of essential oil components. Int J Food Microbiol. 1992, 16 (4): 337-342. 10.1016/0168-1605(92)90035-2.
Palombo E: Traditional medicinal plant extracts and natural products with activity against oral bacteria: potential application in the prevention and treatment of oral diseases. eCAM. 2011, 2011: 680354-
Pinto E, Vale-Silva L, Cavaleiro C, Salgueiro L: Antifungal activity of the clove essential oil from Syzygium aromaticum on Candida, Aspergillus and dermatophyte species. J Med Microbiol. 2009, 58: 1454-1462. 10.1099/jmm.0.010538-0.
Wagner H, Jurcic K, Deininger R: Über die spasmolytische Wirkung von Eugenolestern und -äthern. Planta Med. 1979, 37 (09): 9,14-
Banerjee S, Das S: Anticarcinogenic Effects of an aqueous infusion of cloves on skin carcinogenesis. Asia Pacific J Cancer Prev. 2005, 6 (3): 304-308.
Fregni V, Casadio R: Kinetic characterization of the ATP-dependent proton pumps in bacterial photosynthetic membranes: a study with the fluorescent probe 9-amino-6-chloro-2-methoxyacridine. BBA - Bioenergetics. 1993, 1143 (2): 215-222. 10.1016/0005-2728(93)90146-7.
Pandey A, Singh P: Antibacterial activity of Syzygium aromaticum (clove) with metal ion effect against food borne pathogens. Asia J Plant Sci. 2011, 1 (2): 69-80.
Eseyin OA, Ebong P, Ekpo A, Igboasoiyi A, Oforah E: Hypoglycemic effect of the seed extract of Telfairia occidentalis in rat. Pakistan J Biol Sci. 2007, 10 (3): 498-501. 10.3923/pjbs.2007.498.501.
Oboh G, Nwanna EE, Elusiyan CA: Antioxidant and antimicrobial properties of Telfairia occidentalis (Fluted pumpkin) leaf extracts. J Pharmacol Toxicol. 2006, 1 (2): 167-175.
Kayode AAA, Kayode OT, Odetola AA: Telfairia occidentalis ameliorates oxidative brain damage in Malnorished rats. nt J Biol Sci. 2010, 4 (1): 10-18.
Iwalokun BB, SB , Durojaiye OO: An Antimicrobial Evaluation of Vernonia amygdalina (Compositae) against Gram - positive and Gram-Negative bacteria from Lagos, Nigeria. W Afr J Pharmacol Drug Res. 2003, 19: 9-15.
Uzoigwe CA, OK : Antimicrobial activity of Vernonia amygdalina on selected urinary tract pathogens. Afr J Microbiol Res. 2011, 5 (12): 1467-1472.
Akinpelu DA: Antimicrobial activity of Vernonia amygdalina leaves. Fitoterapia. 1999, 70 (4): 432-434. 10.1016/S0367-326X(99)00061-1.
Nwanjo HU, Nwokoro EA: Antidiabetic and biochemical effects of aqueous extract of Vernonia amygdalina leaf in normoglycaemic and and diabetic rats. J Innov Life Sci. 2004, 7: 6-10.
Kupchan SM, Hemingway RJ, Karim A, Werner D: Tumor inhibitors. XLVII. Vernodalin and vernomygdin, two new cytotoxic sesquiterpene lactones from Vernonia amygdalina Del. J Organic Chem. 1969, 34 (12): 3908-3911. 10.1021/jo01264a035.
Izevbigie EB: Discovery of water soluble anticancer agents from a vegetable found in Benin City, Nigeria. Exp Biol Med. 2003, 228: 293-298.
Tadesse A, Gebre-Hiwot A, Asres K, Djote M, Frommel D: The in-vitro activity of Vernonia amygdalina on Leishmania acthiopica. Ethiop Med J. 1993, 31: 183-189.
Fasola TO, Okeocha PC, Odetola A: Screening for hypoglycaemic potential of Vernonia amygdalina. Ethnobotanical Leaflets. 2010, 14: 759-765.
Ibrahim TA, Ajala L, Adetuyi FO, Jude-Ojei B: Assessment of the antibacterial activity of Vernonia amygdalina and Occimum gratissimum leaves on selected food borne pathogens. Elec J Env Agricult Food Chem. 2009, 8 (11): 1212-1218.
Nwanjo HU: Efficacy of aqueous leaf extract of Vernonia amygdalina on plasma lipoprotein and oxidative status in diabetic rat models. Niger J Physiol Sci. 2005, 20 (1–2): 39-42.
Harbone JB: Phytochemical methods: A guide to modern techniques of plant analysis. 1973, London: Chapman and Hall
Eloff JN: A sensitive and quick microplate method to determine the minimal inhibitory concentration of plant extracts for bacteria. Planta Med. 1998, 64: 711-713. 10.1055/s-2006-957563.
Mativandlela SPN, Lall N, Meyer JJM: Antibacterial, antifungal and antitubercular activity of Pelargonium reniforme (CURT) and Pelargonium sidoides (DC) (Geraniaceae) root extracts. S Afr J Bot. 2006, 72: 232-237. 10.1016/j.sajb.2005.08.002.
Kuete V, Ngameni B, Simo CCF, Tankeu RK, Ngadjui BT, Meyer JJM, Lall N, Kuiate JR: Antimicrobial activity of the crude extracts and compounds from Ficus chlamydocarpa and Ficus cordata (Moraceae). J Ethnopharmacol. 2008, 120 (1): 17-24. 10.1016/j.jep.2008.07.026.
Shahverdi AR, Monsef-Esfahani HR, Tavasoli F, Zaheri A, Mirjani R: Trans-Cinnamaldehyde from Cinnamomum zeylanicum Bark Essential Oil Reduces the Clindamycin Resistance of Clostridium difficile in vitro. J Food Sci. 2007, 72 (1): S055-S058. 10.1111/j.1750-3841.2006.00204.x.
Braga LC, Leite AAM, Xavier KGS, Takahashi JA, Bemquerer MP, Chartone-Souza E, Nascimento AMA: Synergic interaction between pomegranate extract and antibiotics against Staphylococcus aureus. Can J Microbiol. 2005, 51 (7): 541-547. 10.1139/w05-022.
Cowan MM: Plant products as antimicrobial agents. Clin Microbiol Rev. 1999, 12 (4): 564-582.
Kuete V: Potential of Cameroonian plants and derived products against microbial infections: a review. Planta Med. 2010, 76 (EFirst): 1479-1491.
Karaman İ, Şahin F, Güllüce M, Öǧütçü H, Şengül M, Adıgüzel A: Antimicrobial activity of aqueous and methanol extracts of Juniperus oxycedrus L. J Ethnopharmacol. 2003, 85 (2–3): 231-235.
Ono T, Kashimura M, Suzuki K, Oyauchi R, Miyachi J, Ikuta H, Kawauchi H, Akashi T, Asaka T, Morimoto S: In vitro and in vivo antibacterial activities of the tricyclic ketolide te-802 and its analogues. J Antibiot (Tokyo). 2004, 57 (8): 518-527. 10.7164/antibiotics.57.518.
Pavithra VK, O Bhagya L: Antibacterial activity of black pepper (Piper nigrum Linn.) with special reference to its mode of action bacteria. Indian J Comp Microbiol Immunol Infect Dis. 2009, 30 (1): 65-66.
Blot S, Depuydt P, Vandewoude K, De-Bacquer D: Measuring the impact of multidrug resistance in nosocomial infection. Curr Opin Infect Di. 2007, 20: 391-396. 10.1097/QCO.0b013e32818be6f7.
Papadopoulos CJCC, Chang BJ: Role of the MexAB-OprM efflux pump of Pseudomonas aeruginosa in tolerance to tea tree (Melaleuca alternifolia) oil and its monoterpene components terpinen-4-ol, 1,8-cineole and α-terpineol. Appl Environ Microbiol. 2008, 74: 1932-1935. 10.1128/AEM.02334-07.
Lomovskaya O, Bostian KA: Practical applications and feasibility of efflux pump inhibitors in the clinic–a vision for applied use. Biochem Pharmacol. 2006, 71: 910-918. 10.1016/j.bcp.2005.12.008.
Pages J-M, Lavigne J-P, Leflon-Guibout V, Marcon E, Bert F, Noussair L, Nicolas-Chanoine M-H: Efflux Pump, the Masked Side of ß-Lactam Resistance in Klebsiella pneumoniae clinical isolates. PLoS One. 2009, 4 (3): e4817-10.1371/journal.pone.0004817.
Lorenzi V, Muselli A, Bernardini AF, Berti L, Pagès JM, Amaral L, Bolla JM: Geraniol restores antibiotic activities against multidrug-resistant isolate from Gram-negative species. Antimicrob Agents Chemother. 2009, 53: 2209-2211. 10.1128/AAC.00919-08.
The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1472-6882/13/190/prepub
Authors are thankful to Romanian Government and The Agence Universitaire de la Francophonie for travel grant to JAKN, and also to Professor Jean-Marie Pagès, Chair of the UMR-MD1 Unit, Université de la Mediterranée, France for providing us with some MDR bacteria. Authors are also thankful to Dr Gerald Ngo Teke for the language editing.
The authors declare that there are no conflict of interest.
JAKN, MM and MS carried out the study; VK designed the experiments. JAKN, MM, JPD and VK wrote the manuscript; VK, JRK and DC supervised the work; VK provided the bacterial strains; all authors read and approved the final manuscript.