Antibacterial potential and mechanism of action of botanicals and phytochemicals from Stachytarpheta cayennensis (Verbenaceae) against Gram-negative multidrug-resistant phenotypes expressing efflux pumps

Abstract

Background: The effectiveness of antibiotics in the fight against bacterial infections motivated their massive use which resulted in the appearance of resistant bacteria. This phenomenon is nowadays in constant evolution and represents a major problem of public health. In this respect, the search for new molecules for fighting against bacterial resistance is essential. This work was committed to studying the in vitro antibacterial activity of the ethanol crude extract, fractions and compounds of a medicinal plant of the Cameroonian pharmacopoeia, Stachytarpheta cayennensis (Verbenaceae), against some multidrug-resistant (MDR) Gram-negative bacterial strains including Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter aerogenes and Providencia stuartii,.

Methods: The antibacterial activity of the crude extract and fractions (botanicals) of this plant and their constituents (phytochemicals) in the presence and absence of an efflux pump inhibitor phenylalanine-arginine β-naphthylamide (PAβN) was evaluated using the serial micro-dilution method. The determination of the effect of the most active sample on the bacterial H⁺-ATPase proton pumps was carried out by a standard method using a pH-meter and the study of the effect of that same sample on the cell growth kinetic was done using a spectrophotometric method.

Results: The phytochemical composition of the crude extract (SC), evaluated using standard qualitative methods, showed a rather selective distribution of secondary metabolites (presence of polyphenols, tannins, steroids, triterpenes, anthocyanins and absence of flavonoids, anthraquinones, alkaloids, saponins). The crude extract (SC) moderately inhibited the growth of 80% of studied bacteria with minimal inhibitory concentrations (MICs) of 256 and 512 µg/mL. Fractions SCc and SCd showed antibacterial activity against 90% and 70% of strains respectively with MICs ranging from 64 to 512 µg/mL, with moderate activity of the crude extract (SC) than the derived fractions. Fraction SCc showed the highest activity spectrum by inhibiting the growth of 90% (9/10) of the studied bacteria, with the least MIC of 64 µg/mL on Escherichia coli ATCC8739. In the presence of PAβN, the activity of the extract and derived fractions has been increased to at least 70% with respect of the strains and isolates used. Fraction SCc caused a prolongation of the lag phase and a general decrease of the growth of E. coli ATCC8739 bacteria through the blockage of the H⁺-ATPase proton pumps of that bacteria, resulting to a decrease of the acidity of the medium.

Conclusion: The overall results obtained in the present work provide important data which constitute a trail to explore regarding the likely use of Stachytarpheta cayennensis in antibiotherapy.