Background & Objectives: Brucellosis is an infectious disease caused by intracellular pathogens of the genus Brucella that have their natural reservoir in domestic and wild animals. Many studies show that herbal medicines have been used safely and successfully to treat bacterial diseases without significant side effects and drug resistance problems.

  Methods: In this study aquatic, alcoholic and acetonic extracts of Eucalyptus globulus leaves were prepared, then MIC and MBC of extracts for B. melitensis 16M and B. abortus S99 were determined by broth macrodilution and agar well diffusion methods. In animal model study, 5 × 10⁵ CFU/mL of Brucellae was injected intraperitoneally (i.p) to female BALB/c mice. After 24 hours, 0.5mL (equivalent MBC) of each Eucalyptus globulus extracts was injected (i.p) After 7 days, in spleen the colonies of brucellae were counted on Muller-Hinton agar as standard protocol. 

  Results: The MIC and MBC of Eucalyptus globulus for B. melitensis M16 and B. abortus S99 were 1:80 (10.81 mg/mL) and 1:40 (21.62 mg/mL) for aquatic extract, 1:1280 (0.64 mg/mL) and 1:640 (1.29 mg/mL) for acetonic extract, and 1:2560 (0.31 mg/mL) and 1:1280 (0.63 mg/mL), for ethanolic extract respectively. In culture of spleen supernatant (in vivo), after 48 hours, the average grown B. melitensis 16M colonies for aquatic, ethanolic and acetonic extracts were 5×10³ CFU/ml, 2×10² CFU/ml and 6×10² CFU/ml, respectively in comparison with control group (4×10¹⁰ CFU/ml). These results for B.abortus S99 were 3×10³ CFU/ml, 1×10² CFU/ml and 3×10² CFU/ml, respectively in comparison with control group (9×10⁹ CFU/ml) The results showed that bacterial load was significantly decreased in all experimental groups (p<0.01). 

  Conclusion : The results of in vitro and in vivo indicate that ethanolic and acetonic extracts of Eucalyptus globulus have more effective antimicrobial activity on B.melitensis M16 and B.abortus S99 than aquatic extract. It seems that the extracts Eucalyptus globulus can be used in treatment of human and animal brucellosis.