Antibacterial activities of the methanol extract, fractions and compounds from Elaeophorbia drupifera (Thonn.) Stapf. (Euphorbiaceae)

Despite the progress in antibiotherapy, the fight against bacterial infections still constitute a major concern worldwide. The antibacterial fight is seriously challenged by the development of of multi-drug resistant (MDR) phenotypes [1]. Scientists should take into account the resistance issue when investigating natural products for their antimicrobial potential. In regards of the biodiversity of plant kingdom, evidenced-based botanicals appear to be undeniable source of medicine to fight bacterial resistance [2, 3]. Recently, several bioactive plants against MDR bacteria were reported. Some of these include Erythrina sigmoidea [4], Cinnamomum zeylanicum, Aframomum citratum, Paullinia pinnata [5, 6], Allanblackia gabonensis, Combretum molle [7] and Harungana madagascariensis [8]. In our continous search of antibacterials from plants used traditionnally to manage microbial infections, we targeted Elaeophorbia drupifera (Thonn.) Stapf. (Euphorbiaceae). The plant is used in traditional medicine to treat skin infections, Guinea worm [9] as well as hypertension and diabetes [10]. Leaves extract was reported to moderately inhibit HIV-1 and HIV-2 proviral DNA copying [11], and to have relaxant effect on vascular smooth muscle on rats [12]. The leaves methanol extract of this plant also showed good cytotoxic activity towards leukemia CCRF-CEM and MDA-MB231 breast cancer cell lines [13]. Previous phytochemical investigations of the plant led to the isolation of triterpenoids and steroids [14, 15]. In the present study, the bioguided fractionation was undertaken for depth investigation of the antibacterial activity of methanolic extract of Elaeophorbia drupifera leaves’.

In the present study, we identified 9 compounds amongst which were 4 terpenoids (1–4), 2 flavonoid glycosides (6 and 7), 3 ellagic acid derivatives (5, 8 and 9). Among terpenoids were steroids (1 and 2), a steroid glucoside (4) and a triterpenoid (3). The isolation of compounds such as euphol (3), tirucallol, euphorbol, ingenol elaeophorbate, epitaraxerol, taraxerone, friedelin, lup-20(29)-en-3-one or lupenone, lupeol, olean-12-ene-3-one, olean-12-ene-3-ol, elaeophorbate in E. drupifera was previously reported [14, 15]. However, in the present work, fewer compounds as well as other not previously isolated ones were isolated, probably due to the fact that the purification was guided by the antibacterial activity and hence all fractions and sub-fractions were not explored. According to established criteria, the antibacterial activity of a plant extract is considered to be significant when MIC is below 100 μg/mL, moderate when 100 μg/mL < MIC < 625 μg/mL or low when MIC > 100 μg/mL [30, 31]. Therefore, the antibacterial activity of the leaves extract (EDL) of E. drupifera could be considered as moderate, as MIC values below 625 μg/mL were obtained on the majority of the tested bacteria (Table 1). However, fractionation of EDL afforded more active samples, and the antibacterial effects of EDLc, EDLc3, EDLc4, EDLc32 to EDLc35 could be considered important. In fact, the lowest MIC value of 32 μg/mL was obtained with fractions EDLa against Escherichia coli AG100, EDLc against Enterobacer aerogenes ATCC13048and EA298. For sub-fractions obtained from EDLc, the lowest MIC value of 16 μg/mL was noted with EDLc3 against Staphylococcus aureus MRSA11. A corresponding value of 8 μg/mL against Providencia stuartii NAE16 was recorded with EDLc33 obtained from further fractionation of EDLc3. These data highlight the increase of activity with consecutive fractionation of extracts and also demonstrate the good antibacterial potential of E. drupifera. Imporantly, the MIC values obtained with the best sub-fractions EDLc3 against Pseudomonas aeruginosa PA124, E. aerogenes EA289, Providencia stuartii PS29964, Enterobacter cloacae BM47, BM67, BM94, S. aureus MRSA4 (Table 2) and EDLc33 against P. stuartii NAE16 (Table 2) were lower than those of chormaphenicol. It should also be highlighted that EDLc3 had MIC values below 100 μg/mL against all the 26 tested bacteria (Table 2). The ratio MBC/MIC obtained were generally above 4, indicating that the studied extracts as well as the active fractions mostly exerted bacteriostatic effects [32‐34]. Also, MIC and MBC values of the reference drug chloramphenicol were also very high (>64 μg/mL) on several pathogens, confirming that most of the bacterial strains used were MDR phenotypes. The activity of compounds is significant when MIC < 10 μg/mL, moderate when 10 < MIC < 100 μg/mL and low when MIC > 100 μg/mL [30, 31]. On this basis, none of the compound or mixtures displayed significant antibacterial activity. Also the lowest MIC value of 64 μg/mL obtained with compound 5 was much more higher than the corresponding values for the most active fraction EDLc3 where it were isolated. This suggests that constituents of this fraction may exert synergistic effects. This also indicates that combating the tested bacteria with fractions and mostly EDLc3 could be more efficient than with isolated compounds. When regarding the structure-activity relationship, it appears that terpenoids were poorly or not active against the tested bacteria. This result is not suprising, as terpenoids are known to generally have poor antibacterial activity [30]. Amongst the three ellagic acid derivatives, compound 5 and the glucoside 8 had antibacterial activities contrary to 9. Hence, it may be deduced that the substitution of a glucosyl- (8) group by the xylopyranosyl- group (9) cancels the antibacterial effect of the ellagic acid derivative 9.

To the best of our knowledge, the identification of the antibacterial constituents of E. drupifera is being reported for the first time. However, the antibacterial activity of the leave of extract of the plant was reported on Staphylococcus aureus and Streptococcus pyogenes [9]. The present study therefore provides more information on the antibacterial potential of E. drupifera and identified the bioactive components of plant. Also, compound 5 was previously reported against a panel of sensitive Gram-positive and Gram-negative bacteria, with MIC values ranged from 9.76 to 156.25 μg/mL [18]. Data obtained in the present study (MIC ranged from 64 to 256 μg/mL) are in consistence with previous and also confirm the activity of compound 5 against MDR bacteria.

The results of the present investigation are very interesting, taking in account the medical importance of the studied microorganisms. The most active fraction of the plant, identified as EDLc3 displayed significant antibacterial activity on a panel of 26 tested bacteria including both sensitive and MDR Gram-negative and Gram-positive bacteria. This fraction could therefore be useful in the management of bacterial infection including MDR phenotypes. The bioactive constituents of the plant include 3,3′,4′-tri-O-methylelleargic acid, 3-O-rhamnopyranosyl kaempherol, 3-O-rhamnopyranosyl quercetin and 3,3′,4′-tri-O-méthylelleargic acid 4-O-β-_D-glucopyranoside.