Abstract
Background:
Eupatorium odoratum L (Siam weed) is a highly invasive species that contains various beneficial active compounds. This study was conducted to explore the antibacterial properties of E. odoratum ethanol extract against gram-positive and gram-negative bacteria.

Aim:
The aim of this study was to evaluate the antibacterial activity of E. odoratum ethanolic extract against various gram-positive and gram-negative bacteria to assess its potential as an antimicrobial agent.

Methods:
The study employed the agar well diffusion method to measure the antimicrobial effectiveness of the E. odoratum ethanolic extract. Additionally, minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using the broth microdilution assay for different bacterial strains.

Results:
The results showed that the E. odoratum ethanol extract could not inhibit Escherichia coli and Klebsiella pneumoniae. It presented weak to moderate inhibition against Bacillus subtilis, Enterococcus faecalis, and Proteus vulgaris, and exhibited moderate to strong inhibition against Staphylococcus aureus. The MIC and MBC of the E. odoratum ethanolic extract against B. subtilis, S. aureus, E. faecalis, E. coli, P. vulgaris, and K. pneumoniae were 12.00 mg/ml and 12.00 mg/ml; 5.00 mg/ml and 7.50 mg/ml; 15.46 mg/ml and 18.98 mg/ml; 75.00 mg/ml and 87.50 mg/ml; 25.00 mg/ml and 25.00 mg/ml; and 77.50 mg/ml and 100.00 mg/ml.

Conclusion:
It was concluded that while E. odoratum ethanolic extract showed moderate to strong effectiveness against S. aureus, it was less effective against other gram-positive bacteria and showed no activity against the tested gram-negative bacteria. Therefore, the ethanolic extract of E. odoratum was considered to have potential as an antimicrobial agent, particularly against S. aureus.

Key words: Antimicrobial, Ethanolic extract, E. odoratum