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Antibacterial and antibiotic resistance modifying activity of the extracts from allanblackia gabonensis, combretum molle and gladiolus quartinianus against Gram-negative bacteria including multi-drug resistant phenotypes

Background Bacterial resistance to antibiotics is becoming a serious problem worldwide. The discovery of new and effective antimicrobials and/or resistance modulators is necessary to tackle the spread of resistance or to reverse the multi-drug resistance. We investigated the antibacterial and antibiotic-resistance modifying activities of the methanol extracts from Allanblackia gabonensis, Gladiolus quartinianus and Combretum molle against 29 Gram-negative bacteria including multi-drug resistant (MDR) phenotypes. Methods The broth microdilution method was used to determine the minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of the samples meanwhile the standard phytochemical methods were used for the preliminary phytochemical screening of the plant extracts. Results Phytochemical analysis showed the presence of alkaloids, flavonoids, phenols and tannins in all studied extracts. Other chemical classes of secondary metabolites were selectively presents. Extracts from A. gabonensis and C. molle displayed a broad spectrum of activity with MICs varying from 16 to 1024 μg/mL against about 72.41 % of the tested bacteria. The extract from the fruits of A. gabonensis had the best activity, with MIC values below 100 μg/mL on 37.9 % of tested bacteria. Percentages of antibiotic-modulating effects ranging from 67 to 100 % were observed against tested MDR bacteria when combining the leaves extract from C. molle (at MIC/2 and MIC/4) with chloramphenicol, kanamycin, streptomycin and tetracycline. Conclusion The overall results of the present study provide information for the possible use of the studied plant, especially Allanblackia gabonensis and Combretum molle in the control of Gram-negative bacterial infections including MDR species as antibacterials as well as resistance modulators.


Background
Infectious diseases caused by multi-drug resistant (MDR) Gram-negative bacteria are worldwide health concern, causing increasingly morbidity and mortality particularly in developing countries [1]. In Cameroon, previous studies showed high levels of resistance to commonly used antibiotics in Gram-negative bacilli [2]. Several reports also mentioned an increase in the hospital dissemination of bacterial strains specifically those expressing drug efflux mechanism [3,4]. Against Gram-negative bacteria, the discovery of efflux pump inhibitors (EPIs) is an attractive strategy to combat MDR phenotypes [5]. EPI generally interact with specific efflux pump proteins to restore the susceptibility of MDR bacteria to antibiotics [6]. Medicinal plants constitute an important source of chemotherapeutic molecules, in regards to the chemical diversity found in several species [7,8]. In recent years, some plants have been successfully evaluated for their direct antibacterial action, and for their antibiotic-modulation activity [9][10][11][12]. In the present work, we hypothesized that herbal medicines traditionally used for the treatment of infectious diseases could contain molecules acting as antibacterial and/ or antibiotic-resistance modulators. This study was therefore designed to investigate the in vitro antibacterial and antibiotic-resistance modifying activities of the methanol extracts from Allanblackia gabonensis Pellegr. (Clusiaceae), Gladiolus quartinianus A. Rich (Iridaceae) and Combretum molle R. Br. ex G. Don (Combretaceae) against Gramnegative bacteria including multi-drug phenotypes. These plants are traditionally used to manage various ailments including bacterial related infections.

Plant materials and extraction
Medicinal plants used in this work were collected in different areas of Cameroon between January and April 2012. The plants were identified at the National Herbarium (Yaoundé, Cameroon), where voucher specimens were deposited under the reference numbers ( Table 1). The air-dried and powdered plant material was weighed (300 g) and soaked in 1 L of methanol (MeOH) for 48 h at room temperature. The filtrate obtained through Whatman filter paper No. 1 was concentrated under reduced pressure in vacuum to obtain the crude extracts. All crude extracts were then kept at 4°C until further uses.

Preliminary phytochemical investigation
The plant extracts were screened for the presence of major secondary metabolite classes such as alkaloids, anthocyanins, anthraquinones, flavonoids, phenols, saponins, sterols and triterpenes according to common phytochemical methods previously described [38]. The tests were based on visual observation of the change in color or formation of precipitate after the addition of specific reagents.

Antibacterial assays
MICs and MBCs of the plant extracts and chloramphenicol were determined by microdilution method using rapid INT colorimetric assay [25,39]. Briefly, the samples were first dissolved in 10 % Dimethyl-sulfoxide (DMSO)/Mueller Hinton Broth (MHB). The solution obtained was then added to MHB and serially diluted two fold (in a 96-well microplate). One hundred microliters of inoculum (1.5× 10 6 CFU/mL) prepared in MHB were then added. The plates were covered with a sterile plate sealer and then agitated with a shaker to mix the contents of the wells and incubated at 37°C for 18 h. The final concentration of DMSO was less than 2.5 %, and did not affect the microbial growth. Wells containing MHB, 100 μL of inoculum, and DMSO at a final concentration of 2.5 % served as the negative control. The MIC of each sample was detected after 18 h of incubation at 37°C following addition of 40 μL INT (0.2 mg/mL) and incubation at 37°C for 30 min. Viable bacteria reduced the yellow dye to a pink. The MIC was defined as the lowest sample concentration that prevented this change and that resulted in the complete inhibition of bacterial growth. The MBC of the sample was determined by sub-culturing 50 μL of the suspensions from the wells which did not show any growth after incubation during MIC assays to 150 μl of fresh broth, and re-incubated at 37°C for 48 h before revelation. The MBC was defined as the lowest concentration of sample which completely inhibited the growth of bacteria [40]. Each assay was performed in three independent tests in triplicate. The samples were also tested in the presence of phenylalanine arginine β-naphthylamide (PAβN) at a final concentration of 20 μg/mL as previously described [41] on nine MDR bacteria. All assays were performed three time in duplicate.

Phytochemical Screening of the plant extracts
The main classes of secondary metabolites for each extract were screened and the results are summarized in Table 3. Tannins, flavonoids, alkaloids and phenols were present in all tested extracts. Others classes of botanicals were selectively distributed in different plant extracts.

Antibacterial activity of the extracts in the presence of an Efflux Pumps Inhibitors
In the present work, extracts were combined with PAβN; However, no significant increase of the activities of the tested plant extracts was generally observed. Only AGL showed 4 times decrease of MICs against E. coli AG102 and E. cloacae BM67. In contrast, PAβN significantly improved the activity of the reference drug, CHL (more than 16 times) on MDR bacteria used (Table 5).

Antibiotic resistance modifying activities of the plant extracts
Tables 2, 6 and 7 highlights the potentiating effects of the extracts on the activity of eight commonly used antibiotics. The most important modulating effects were observed of association CML with aminoglycosides (kanamycin and streptomycin), the potentiation effects varying from 77.78 to 88.89 % and from 66.67 to 77.78 % at MIC/2 and MIC/4 respectively; and with tetracycline (100 % and 77.78 % at MIC/2 and MIC/4 respectively) ( Table 6). The modulating effects also ranged between 50 to 67 %, with the extract from A. gabonensis fruits (AGF) when combined at (MIC/2) with the some antibiotics. At MIC/4, AGF showed synergy less than 50 % on the tested bacteria with all antibiotics ( Table 7). The most significant increases of antibiotic activity in the presence of plant extracts were noted with the association of streptomycin and CML and AGF on E. coli AG100A Tet, with more than 128 fold and 64-fold decreases of MIC respectively. No increase of activity was noted with ampicillin, a β-lactamine when it was combined with plant extracts.

Discussion
Medicinal plants are potential source of antimicrobial agents used in the treatment of infectious diseases [43,44]. According to Kuete et al. [45,46], the antibacterial activity of a plant extract is considered significant when the MICs are below 100 μg/mL, moderate when 100 ≤ MIC ≤ 625 μg/mL and weak when MIC are above 625 μg/mL. Consequently, the antibacterial activities of the tested extracts particularly those from A. gabonensis (AGF, AGR, AGB and AGFl) and C. molle (CML) were generally moderated (Table 4). Significant activities were recorded with AGF, AGR, AGB, and AGFl respectively on 37.93 %, 24.14 %, 20.70 % and 17.24 %. This highlights the good antibacterial potential of the tested extracts. The overall activity recorded with most of the studied extracts could be considered significant, especially those from A. gabonensis and C. molle [47]. When analyzing carefully the MIC and MBC results for the extract, it can be noted that MBC ≤ 4 were obtained with these samples on most of the tested bacterial species, suggesting their killing effects [48]. PAβN is a potent inhibitor of RND systems like AcrAB-TolC in Enterobacteriaceae or MexAB-OprM in P. aeruginosa used in the present work [49,50]; The activity observed when chloramphenicol was tested in the presence of PAβN increased significantly, confirming that the tested bacteria are good models of efflux pumpsexpressing bacteria.

Conclusion
This study provides informative data on the antimicrobial potential of the tested plant extracts and suggests that extracts from Allanblackia gabonensis could be a source of natural antibacterial products whilst Combretum molle leaves extract could contain both antibacterial substances and antibiotic-modulation agents. These data indicate that these plants can be used to fight bacterial infections and especially those involving MDR phenotypes.