Antimicrobial activity of selected essential oils against Streptococcus suis isolated from pigs

Abstract The inhibitory potential by contact and vapor of basil, cinnamon, clove, peppermint, oregano, rosemary, common thyme, and red thyme essential oils ( EO s) against 20 strains of Streptococcus suis was determined by the disk diffusion test. The broth microdilution method was used to determine the minimal inhibitory and minimal bactericidal concentration ( MIC and MBC ) of the four selected oils. Furthermore, the bactericidal power (ratio MBC / MIC ) was calculated. The EO s with the major potential in the disk diffusion method were red thyme, common thyme, oregano, and cinnamon (∅ mean 16.5–34.2 mm), whereas cinnamon did not show vapor activity. In the microdilution test, all the EO s showed notable antimicrobial activity ( MIC 90 and MBC 90 312.5–625 μg·ml −1 ) and a strong bactericidal power (ratio = 1). This is the first study that selects essential oils against S . suis . New studies about the possible synergic effect of EO s with antibiotics and about toxicity and efficacy in in vivo conditions are recommended.