Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 8 1 2016 03 01 1 7 EN Mahdieh Neghabi-Hajiagha Department of Microbiology, School of Medicine, Tehran University of Medical Sciences, Poorsina street, Tehran, Iran. Atousa Aliahmadi Department of Biology, Medicinal Plants and Drugs Research Institute, Shahid Beheshti University, Tehran, Iran. Mohammad Reza Taheri Department of Phytochemistry, Medicinal Plants and Drug Research Institute, Shahid Beheshti University, Tehran, Iran. Alireza Ghassempour Department of Phytochemistry, Medicinal Plants and Drug Research Institute, Shahid Beheshti University, Tehran, Iran. Gholamreza Irajian Department of Microbiology, School of Medicine, Iran University of Medical Sciences, Tehran, Iran. Hassan Rezadoost Department of Phytochemistry, Medicinal Plants and Drug Research Institute, Shahid Beheshti University, Tehran, Iran. Mohammad Mehdi Feizabadi Department of Microbiology, School of Medicine, Tehran University of Medical Sciences, Poorsina street, Tehran, Iran. 2016 02 02 2016 02 03 Background and Objectives: Due to the importance of finding of new antibacterial agents, the antibacterial properties of Prosopis cineraria aerial parts were investigated using a bioassay guided fractionation scheme. Materials and Methods: The organic extract was prepared via maceration in methanol, followed by the fractionation using n-hexane and ethyl acetate. The MICs of fractions were determined against some human pathogenic bacteria using broth micro-dilution assay. The primary characterization and identification of bioactive substance(s) was based on a bio-autograph- ical method using HPTLC and flash chromatography in parallel with agar overlay assays. Finally the exact mass of effective compound(s) was determined by LC-MS. Results: The best antibacterial activities were related to the ethyl acetate fraction. The effective antibacterial compound of the plant were 2 substances with molecular weight of 348 and 184 Dalton that inhibited the growth of assessed Gram positive bacteria with MIC values lower than 125 to 62.5 µg/ml synergistically. Conclusion:  Further analysis using nuclear magnetic resonance could reveal the exact structure of these two antibacterial substances. These 2 effective antibacterial compounds could be applied as lead compound for synthesis of new antibacterial agents. https://ijm.tums.ac.ir/index.php/ijm/article/view/901 https://ijm.tums.ac.ir/index.php/ijm/article/download/901/554