Antimicrobial properties of ethanolic and methanolic extracts of finger millet (Eleusine coracana (L.) Gaertn.) varieties cultivated in Sri Lanka

Abstract

The emerging significance of natural antimicrobial agents creates an imperative need to identify novel plant sources with antimicrobial activities. The objective of the present study was to evaluate antibacterial, antifungal and β-lactamase enzyme inhibitory activities of ethanolic and methanolic extracts of Ravi, Rawana and Oshadha finger millet varieties. Flours of whole grains of the finger millet varieties were extracted with absolute ethanol and methanol separately. Antibacterial activities against six antibiotic-sensitive and four antibiotic-resistant pathogenic bacterial strains were evaluated using the resazurin reduction assay. Antifungal activities against six antimicrobial-sensitive pathogenic fungal strains were evaluated using the agar tube dilution method. β-Lactamase enzyme inhibitory activity was evaluated using a standard method. Both ethanolic and methanolic extracts of the three finger millet varieties showed dose-dependent inhibitory activities against the tested antibiotic-sensitive and antibiotic-resistant bacterial strains while exhibiting high inhibitions against Gram-positive antibiotic-sensitive bacterial strains when compared to Gram-negative antibiotic-sensitive bacterial strains. The findings revealed the antibacterial potential of both ethanolic and methanolic extracts of the three finger millet varieties against antibiotic-sensitive Staphylococcus aureus (ATCC ® 6538™) and Bacillus subtilis (ATCC ® 23857™) strains and the minimum inhibitory concentrations of the extracts against S. aureus and B. subtilis were found to be 2.1 and 1.8 mg/ml, respectively. However, none of the extracts can be considered as significantly active against the tested antibiotic-sensitive and antibiotic-resistant bacterial strains when compared to the standard drugs. In addition, none of the extracts can be considered as active against the tested fungal strains at the tested concentrations. Nevertheless, all extracts showed more activities against the tested bacterial strains when compared to the tested fungal strains. Since all extracts showed less than 40% β-lactamase inhibitory activities even at 2 mg/ml concentration, they do not qualify as promising sources of β-lactamase inhibitors at the tested concentration.