Antimicrobial susceptibility of selected Essential Oils and their compounds against Streptococcus suis

Background: Streptococcus suis is an emerging zoonotic pathogen causing different diseases, in both humans and pigs. Generally, the control of diseases caused by this pathogen is based on antimicrobial therapy, but the development of bacterial resistance has led to look for new options. In this sense, the Essential Oils (EOs) constitute an alternative to the use of conventional antimicrobials. The activity of oregano, cinnamon, common thyme and red thyme EOs and their main active compounds (carvacrol, cinnamaldehyde and thymol) against 56 S. suis isolates from pigs (n=50) and human (n=6) was determined by broth microdilution. MIC50-90, MBC50-90 and the bactericidal index (MBC/MIC) were calculated. Also, the time-kill curve of each product against the S. suis P1/7 European reference strain was determined. Results: No differences in the MIC or MBC values were observed between all the tested products, which suggest a homogenous behaviour of S. suis, independently of their origin, organ of isolation or resistance profile. All the products showed a concentration-dependent and time-dependent killing activity and achieved the virtual eradication of S. suis (reduction of ≥ 4 log10) at suprainhibitory (2x-4x MIC) concentrations within the first 5 minutes of exposure, except cinnamaldehyde, that showed only bacteriostatic effect (reduction of < 3 log10). Conclusions: While all the tested products demonstrated an antimicrobial activity, red thyme and cinnamon followed by thymol showed the best results. It suggests that these products would be recommended as antimicrobials in veterinary medicine for the control of this zoonotic pathogen, although more pharmacology, toxicology, tolerability and formulation studies are necessary.