en aquaculture aquaculture پژوهشي Orginal research papers <span lang="TR" style="font-size:11.0pt"><span calibri="" style="font-family:"><span style="color:black">The study investigates the antimicrobial effect of two herbal essential oils, the leaves of&nbsp;<em><span calibri="" style="font-family:">Origanum vulgare</span></em>&nbsp;and the bark of&nbsp;<em><span calibri="" style="font-family:">Cinnamomum verum</span></em>, separately and in combination against&nbsp;<em><span calibri="" style="font-family:">S. iniae</span></em>. The essential oils were extracted and their chemical composition was analyzed using gas chromatography-mass spectrometry. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using the broth microdilution method. The fractional inhibitory concentration (FIC) method was employed to analyze the combined effect of the essential oils. The MIC of&nbsp;<em><span calibri="" style="font-family:">O. vulgare</span></em>&nbsp;and&nbsp;<em><span calibri="" style="font-family:">C. verum</span></em>&nbsp;against&nbsp;<em><span calibri="" style="font-family:">S. iniae&nbsp;</span></em>were 0.976 mg/L and 100 mg/L, respectively, and the MBC value of&nbsp;oregano and&nbsp;cinnamon essential oils against&nbsp;<em><span calibri="" style="font-family:">S. iniae</span></em>&nbsp;were 125 mg/L and 6400 mg/L, respectively. The FIC index was 0.625, which shows that the combined performance of the two essential oils is additive. The major compounds in&nbsp;<em><span calibri="" style="font-family:">Origanum</span></em>&nbsp;and&nbsp;<em><span calibri="" style="font-family:">Cinnamomum</span></em>&nbsp;essential oils were phenol-2-methyl-5 (1-methylethyl) (48.61%), thymol (16.76%) and cinnamaldehyde (64.5%), respectively. Overall, the results demonstrate that both<i> </i>essential oils exhibit antibacterial effects against <i>S. iniae</i>. In conclusion, the combined essential oils of&nbsp;<em><span calibri="" style="font-family:">O. vulgare</span></em>&nbsp;and&nbsp;<em><span calibri="" style="font-family:">C. verum</span></em> may hold promise as a novel antibacterial agent for the potential treatment and control of streptococcosis caused by <i>S. iniae.</i></span></span></span> Streptococcosis, Fractional inhibitory concentration,Cinnamon, Oregano, Antimicrobial 409 426 http://jifro.ir/browse.php?a_code=A-10-4877-1&slc_lang=en&sid=1 S.M. Aghilian mobinaghilian65@gmail.com 100319475328460051348 100319475328460051348 Yes Department of Veterinary Medicine, Shabestar Branch, Islamic Azad University, Shabestar 5381637181, Iran S.S. Ghaemmaghami dr.ghaemmaghami@gmail.com 100319475328460051349 100319475328460051349 No Department of Veterinary Medicine, Shabestar Branch, Islamic Azad University, Shabestar 5381637181, Iran S. Hosseinzadeh s_hosseinzadeh2000@yahoo.com 100319475328460051350 100319475328460051350 No Department of Veterinary Medicine, Shabestar Branch, Islamic Azad University, Shabestar 5381637181, Iran A. Azarimatin azarimatinarmin@yahoo.com 100319475328460051351 100319475328460051351 No Department of Veterinary Medicine, Shabestar Branch, Islamic Azad University, Shabestar 5381637181, Iran