Antimicrobial activity of Origanum vulgare and Cinnamomum verum essential oils separately and in combination against Streptococcus iniae

The study investigates the antimicrobial effect of two herbal essential oils, the leaves of Origanum vulgare and the bark of Cinnamomum verum, separately and in combination against S. iniae. 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 O. vulgare and C. verum against S. iniae were 0.976 mg/L and 100 mg/L, respectively, and the MBC value of oregano and cinnamon essential oils against S. iniae 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 Origanum and Cinnamomum 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 essential oils exhibit antibacterial effects against S. iniae. In conclusion, the combined essential oils of O. vulgare and C. verum may hold promise as a novel antibacterial agent for the potential treatment and control of streptococcosis caused by S. iniae.