en Bacterial Disease Bacterial Disease پژوهشي Original research papers <p>&nbsp; In the present study, the effect of the high and low virulence <i>S. iniae</i> strains on oxidative activity and cytotoxic effects of trout spleen macrophages were studied. Spleen macrophages were collected from trout. Luminol was used as chemiluminigenic probe. Phorbolmyristate acetate (PMA) also was used as positive control activating agent of respiratory burst and significantly stimulated production of reactive oxygen species (ROS) by normal macrophages. Sensitivity of <i>S. Iniae</i> strains to H2O2 and cytotoxicity of bacterial strains for trout macrophages was also determined using the antibacterial effect of H₂O₂ and the release of lactate dehydrogenase from infected macrophages, respectively. The low virulence strain was killed approximately 6 times more by H₂O₂ compared to the high virulence strain. Results indicate that, high virulence in <i>S. iniae</i> appears to be correlated with higher macrophage cytotoxicity and resistance to ROS.</p> rainbow trout, Streptococcus iniae, reactive oxygen species, chemiluminescence, macrophage. 53 61 http://ijaah.ir/browse.php?a_code=A-10-35-17&slc_lang=en&sid=1 A. Nematollahi 1003194753284600947 1003194753284600947 Yes Division of Aquatic Animal Health and Diseases, Department of Food Hygiene and Quality Control, Faculty of Veterinary Medicine, Shahrekord University, Shahrekord, Iran A. Mohebbi 1003194753284600948 1003194753284600948 No Department of of Clinical Sciences, Faculty of Veterinary Medicine, Shahrekord University, Shahrekord, Iran A. Mokhtari 1003194753284600949 1003194753284600949 No Departments of Aquatic Animal Health and Diseases, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran S. K. Baruah 1003194753284600950 1003194753284600950 No Laboratory of Aquaculture and Artemia Reference Centre, Faculty of Bioscience Engineering, Ghent University, Belgium