Abstract
Acacia polyacantha is a medicinal plant traditionally used to treat livestock diseases and gastrointestinal infections; our study was undertaken to evaluate the antistaphylococcal activities of the methanolic leaf, bark, and root extracts, fractions, and compounds from Acacia polyacantha against a panel of 14 multidrug-resistant Staphylococcus bacterial strains overexpressing efflux pumps. The study was also extended to investigate two possible modes of action, that is, influence on bacterial growth kinetics and influence on proton-ATPase pumps, of the most active compound against a reference strain. Materials and Methods. The crude extracts after extraction were subjected to column chromatography. Antibacterial assays of extracts, fractions, and compounds alone and in the presence of efflux pump inhibitors were carried out using the broth microdilution method and the study of two mechanisms of action achieved by standard methods with the most active compound. Results. The phytochemical study of Acacia polyacantha leaves leads to the isolation of stigmasterol (1), β-amyrin (2), 3-O-methyl-D-chiro-inositol (3), epicatechin (4), quercetin-3-O-galactoside (5), 3-O-[β-D-xylopyranosyl-(1 ⟶ 4)-β-D-galactopyranosyl]-oleanolic acid (6), 3-O-[β-galactopyranosyl-(1⟶ 4)-β-D-galactopyranosyl]-oleanolic acid (7) and that of leaves lead to the isolation of lupeol (8) 2,3-dihydroxypropyltetracosanoate (9), and methyl-gallate (10). Leaf, root, and bark extracts inhibited 92.85% (13/14), 92.85% (13/14), and 71.43 % (10/14) of the tested bacteria strains, respectively, with minimum inhibitory concentration (MIC) varying between 16 and 1024 μg/mL. Fractions exhibited better activities compared to those of their extracts of origin, as their MICs ranged from 16 to 512 μg/mL, with fractions from leaves being more active than those obtained from barks. Compounds had varying activities; MICs varied from 16 to 512 μg/mL with compound 4 presenting the best activity as MICs ≤100 μg/mL were obtained against 11 of the tested bacteria. The activities of extracts, fractions, and compounds were improved in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) as an efflux pump inhibitor to as much as >128 folds. Meanwhile, in the presence of chlorpromazine as an efflux pump inhibitor, only the activity of compound 10 was improved on 10 of the tested bacteria strains. Compound 4 prolonged the lag phase of the growth kinetic in a concentration-dependent manner and equally inhibited the proton-ATPase pumps of the tested bacteria strains. Conclusion. The present study demonstrates the antistaphylococcal potential of Acacia polyacantha and its constituents to combat bacterial infections alone or in combination with efflux pump inhibitors.
Highlights
Bacterial infections are a major burden to the public health sector, as they are responsible for an estimated 560,000 deaths yearly worldwide [1]. is is further complicated by the alarming rate of the emergence of drug-resistant strains [2]
Phytochemistry. e chemical structures (Figure 1) and properties of compounds isolated from leaves and barks ofAcacia polyacantha were determined using NMR (1H and 13C) data and in comparison with literature. ese compounds were identified from barks as Lupeol C30H50O (8; Wwite crystal; melting point (m.p.): 220–222°C; [M]+ at m/z 426), 2,3-dihydroxypropyltetracosanoate C27H54O4 (9; white powder; m.p.: 86–88°C; [M+Na]+ at m/z 465), and methyl gallate C8H8O5 (10; white powder; m.p.: 197, 6–199°C; [M]+ at m/z 184), and from leaves as stigmasterol C29H50O (1; white powder, m.p.:134–135°C; m/z 414) [22], β-amyrinC30H50O (2; white powder, m.p.:187–190°C; m/z 426) [23], 3-O-methyl-D-chiro-inositol C7H14O6 (3; white powder, m.p.:181°C; m/z 217; [α]2D5: +60,00) [24], epicatechinC15H14O6 (4; red powder, m.p.:345–350°C; m/z 270) [25], quercetin-3-O-galactoside C21H20O12 (5; yellow powder, m.p.:230–232°C; m/z 464) [26], 3-O-[β-D
Effect of Efflux Pump Inhibitors on the Activities of Plant Samples. e extracts, selected fractions, and compounds were tested in the presence of two efflux pump inhibitors: chlorpromazine (CPZ) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) for any improvement of their antistaphylococcal activities. e results obtained showed that, in the presence of CCCP as an efflux pump inhibitor, the activities of all samples were significantly improved; the degree of improvement varied from one bacterial strain to another (Table 4)
Summary
Bacterial infections are a major burden to the public health sector, as they are responsible for an estimated 560,000 deaths yearly worldwide [1]. is is further complicated by the alarming rate of the emergence of drug-resistant strains [2]. Is is further complicated by the alarming rate of the emergence of drug-resistant strains [2]. Among these infections, those caused by Staphylococcus aureus are globally responsible for 7–10% of deaths annually [3]. Is phenomenon of resistance propels the search for new antimicrobial agents with higher efficacy and low toxicity [6]. Plants and their secondary metabolites have long been used by humans in the treatment of ailments caused by these pathogens [7]. Acacia polyacantha is a deciduous, straight cylindrical, erect tree of about 10–15 m height found in Tropical Africa, occurring from Gambia to Ethiopia and southwards to Kenya and Zimbabwe [10, 11]. e root extract is used as a treatment for snake bites and applied to wash the skin of children who are agitated at nighttime [12]
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