With steadily growing of the microbial resistance against the classical antibiotics, searching new and effective natural
antimicrobial agents become crucial. The study introduces new herbal extracts as antibacterial agents against both Gram-positive and
Gram-negative bacteria. In vitro antibacterial activity of 15 weed plant species, collected from Aseer region, Saudi Arabia and their
effective minimum inhibitory concentrations (MIC) were determined. The possible mode of action of the most effective plant extracts
was investigated. Crude extracts of the investigated species were prepared using three organic solvents. Bacillus subtilis and Proteus
vulgaris were used as test organisms to evaluate the antibacterial effect of the crude extracts and their (MIC). Examination with
scanning electron microscopy of the treated bacteria as well as analysis of the plant extract using GC-MS were performed to explore the
possible mode of action. Foeniculum vulgare, Xanthium spinosum, Abutilon pannosum, Solanum incanum, Forsskaolea tenacissima
and Brassica deflexa were the most effective plants among the fifteenth tested species. Scan electron micrographs (SEM) revealed a
clear morphological malformation in the bacterial shapes. This supports our hypothesis that extracts could alter the bacterial cell
membrane permeability causing plasmolysis. GC-MS analysis of plant extracts proved the presence of aliphatic and aromatic
compounds, whereas, the number and types of these compounds depended on the type of the organic solvent. The chloroform extracts of
Xanthium spinosum, Abutilon pannosum are very promising antibacterial agents against both Gram-positive and Gram-negative
bacteria. Further investigation is recommended to scale up and validate the application of the test plant extracts