Aim: This research investigates the antibacterial activities of methanol extracts from six Cameroonian edible plants and their synergistic effects with some commonly used antibiotics against multidrug-resistant (MDR) Gram-negative bacteria expressing active efflux pumps.
Material and method: The plant materials included P. guajava; leaves and bark of P. americana and nuts of C. edulis. The major phytochemical classes such as alkaloids (Dragendorff’s and Mayer’s tests), triterpenes (Liebermann-Burchard’s test), flavonoids (aluminium chloride test), anthraquinones (Borntrager’s test), polyphenols (ferric chloride test), sterols (Salkowski’s test), coumarins (lactone test), saponins (foam test), and tannins (gelatin test) were investigated through the phytochemical methods. The studied bacteria strains included and clinical strains of Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, Providencia stuartii, and Pseudomonas aeruginosa. Their bacterial features were previously reported. Mueller-Hinton Agar (MHA) and Mueller-Hinton Broth (MHB) were used. The reference antibiotics used included tetracycline (TET), cefepime (CEP), ciprofloxacin (CIP), streptomycin (STP), chloramphenicol (CHL), ampicillin (AMP), erythromycin (ERY), and kanamycin (KAN). P-Iodonitrotetrazolium (INT) chloride and phenylalanine-arginine-ß-naphthylamide (PAßN) were used as microbial growth indicator and efflux pumps inhibitor (EPI), respectively. The antibacterial activities of plants’ extracts alone and in some cases, in the presence of the PAβN on a panel of 29 Gram-negative bacteria, was investigated. Fifteen of the studied MDR bacteria were also tested for their susceptibility to plant extracts in the presence of the PAβN at 30 μg/ml. A preliminary study performed against P. aeruginosa PA124 allowed us to select six out of ten extracts at the appropriate subinhibitory concentrations of MIC/2 and MIC/4 for further studies. All the six extracts were combined separately to eight antibiotics (CIP, STP, CHL, ERY, KAN, TET, CEF, and AMP) to evaluate their possible synergetic effects.
Results: The results of phytochemical tests indicate that all tested crude extracts contained polyphenols, flavonoids, triterpenes, and steroids. Extracts displayed selective antibacterial activities with the minimal inhibitory concentration (MIC) values ranging from 32 to 1024 μg/mL. The lowest MIC value (32 μg/mL) was recorded with Coula edulis extract against E. coli AG102 and K. pneumoniae K2 and with Mangifera indica bark extract against P. aeruginosa PA01 and Citrus sinensis extract against E. coli W3110 which also displayed the best MBC (256 μg/mL) value against E. coli ATCC8739. In combination with antibiotics, extracts from M. indica leaves showed synergistic effects with 75% (6/8) of the tested antibiotics against more than 80% of the tested bacteria.
Conclusions: The findings of the present work indicate that the tested plants may be used alone or in combination in the treatment of bacterial infections including the multidrug-resistant bacteria. The results of this study can provide some explanations on the traditional use of certain parts of the plants tested herein to combat infections, particularly those caused by enteric bacteria. Evidence of the antibacterial activity of the studied plants, especially edible ones, as well as the ability of some of them to improve the activity of commonly used antibiotics, is an indication that there is a possibility of discovering alternative antibacterial in these plants.