Antimicrobial Activity and Synergistic Effects of Libyan Extracts from Artemisia Herba-alba, Thymus Vulgaris, and Zingiber Officinale Against ATCC Strains of Bacillus Subtilis and Escherichia Coli

Abstract

The increasing prevalence of antibiotic-resistant bacteria has intensified interest in medicinal plants as a promising source of natural antimicrobial agents. This study aimed  to evaluate the antibacterial activity of ethanol, methanol, and aqueous extracts of Artemisia herba-alba, Thymus vulgaris, and Zingiber officinale against Bacillus subtilis and Escherichia coli. Antibacterial activity was assessed using the agar well diffusion, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and checkerboard assays to determine synergistic interactions with Streptomycin. In addition, phytochemical screening and high-performance liquid chromatography (HPLC) analyses were performed. The results demonstrated that Gram-positive bacteria were more susceptible to the plant extracts than Gram-negative bacteria. The ethanolic extract of A. herba-alba showed the strongest antibacterial activity, producing the largest inhibition zone (19.5 ± 0.6 mm) against B. subtilis and the lowest MIC  (62.5 mg/mL). The strongest synergistic interaction with Streptomycin was observed for A. herba-alba (FICI = 0.41). HPLC analysis identified caffeic acid as the major phenolic compound (19.3 mg/g), together with other bioactive constituents, including thymol, carvacrol, and gingerol, which may contribute to the observed antibacterial activity. These findings highlight the potential of these plant extracts as natural antimicrobial agents and support further investigations into their pharmaceutical applications.