Mohammad Kordkatouli - Department Biology, Gorgan Branch, Islamic Azad University, Gorgan , Iran
Leila Fozouni - Department Microbiology, Gorgan Branch, Islamic Azad University, Gorgan , Iran

BACKGROUND AND OBJECTIVESAcinetobacter baumannii is one of the most common and important causes of hospital- acquired infections. Due to its intrinsic resistance to antibiotics, A. baumannii can survive in the hospital environment for a long time and target hospitalized patients. Therefore, treatment and prevention of patients infected with these bacteria require identification of new antibacterial agents with no or fewer side effects and toxicity. The aim of this study was to investigate and compare the antibacterial effect of aqueous and ethanolic extracts of Silybum marianum on extensively drug-resistant A. baumanni clinical isolates.MATERIALS AND METHODSAntimicrobial susceptibility of the isolates was determined using the Kirby-Bauer method according to the Clinical and Laboratory Standards Institute document M۱۰۰ (۲۰۲۰). Antimicrobial activity of the ethanolic and aqueous extracts of S. marianum against extensively drug-resistant A. baumannii isolates was determined by agar well diffusion method. In addition, broth microdilution susceptibility testing was carried out to determine minimal inhibitory concentrations (MICs) of the extracts. Finally, active compounds with antimicrobial activity were identified by gas chromatography–mass spectrometry.RESULTS AND DISCUSSIONFrequency of the extensively drug-resistant, A. baumannii isolates was ۳۶.۳%. The MIC۹۰ of the aqueous extract of S. marianum was ۴۰۹۶ μg/mL, which was two times less than that of the ethanolic extract (۸۱۹۲ μg/mL). Similarly, the MIC۵۰ of the aqueous extract of S. marianum was significantly smaller than that of the ethanolic extract (P<۰.۰۵). According to the results, silybin and silychristin were the most abundant (۴۲.۰۴%) bioactive compounds in the aqueous extract of S. marianum.CONCLUSIONGiven the excellent antibacterial effects of the aqueous extract of S. marianum, it is recommended to investigate the potential application of these extracts and their bioactive constituents for production of pharmaceutical compounds and disinfectants.

Acinetobacter baumannii, drug resistance, Silybum marianum, Nosocomial infection