Antibacterial potential and modes of action of methanol extracts of flowers and leaves of Vernonia glabra (Steetz) Vatke (Asteraceae) against multidrug-resistant Gram-negative bacteria overexpressing efflux pumps

Abstract

Background: Bacterial infections caused by multidrug-resistant (MDR) Gram-negative bacteria remain a public health problem and have contributed to a reduction in the range of antibiotics available for antibiotic therapy. The search for new antibacterial substances is becoming increasingly important, and plants represent an important reservoir of therapeutic molecules. In the present study, the antibacterial activity and modes of action of Vernonia glabra against Gram-negative and multi-resistant bacteria were evaluated. Methods: The antibacterial activity of Vernonia glabra extracts was assessed using the broth microdilution method, and the effects of the flower extract on bacterial growth kinetics and on the H+-proton pump/ATPase of Providencia stuartii ATCC29916 were carried out using standard experimental protocols; qualitative reference methods were used to identify the secondary metabolites present in the extracts. Results: Phytochemical screening of Vernonia glabra flower and leaf extracts revealed the presence of alkaloids, phenols, flavonoids, tannins, triterpenes, saponins and anthocyanins. The flower and leaf extracts showed antibacterial activity of 100% and 93.33% respectively against the bacteria tested. With minimum inhibitory concentrations (MICs) ranging from 32 µg/mL to 2048 µg/mL, the flower extract showed excellent activity against Escherichia coli AG100 and Providencia stuartii ATCC29916 with a MIC of 32 µg/mL, while the leaf extract showed good activity against Klebsiella pneumoniae ATCC11296 and Providencia stuartii PS2636 with a MIC of 256 µg/mL. The flower extract inhibited the growth of Providencia stuartii ATCC22916 at the exponential phase and inhibited its H+ proton pumps/ATPase. In the presence of the efflux pump inhibitor, phenylalanine-arginine β-naphthylamide (PAβN), the activity of the leaf extract was enhanced on 90.90% of the bacteria tested. With activity enhancement factors ranging from 2 to 128, both extracts potentiated the activity of antibiotics (imipenem, ampicillin, levofloxacin, tetracycline, vancomycin, ceftriaxone, ciprofloxacin, and doxycycline) against at least 70% of the bacteria tested. Conclusion: The results obtained in the present work show that Vernonia glabra is a source of antibacterial molecules that can be used against MDR Gram-negative bacteria. Keywords: Antibacterial; Asteraceae; efflux pumps; modes of action; multidrug resistance; Vernonia glabra.