The interplay between antimicrobial activity and reactivity of isothiocyanates

This study aimed to determine antimicrobial activity (minimum inhibitory concentration, MIC; minimum bactericidal/fungicidal concentration, MBC/MFC) of novel ITCs against food spoilage and pathogenic Gram– bacteria, Gram+ bacteria, and fungi. The activity of the long-chain (C9) 9-(methylthio)nonyl ITC (9-MTITC), 9-(methylsulfinyl)nonyl ITC (9-MSITC), and 9-(methylsulfonyl)nonyl ITC (9-MSoITC) was determined for the first time. Due to the electrophilicity of ITCs, the activity of ITCs was evaluated in nucleophile-rich and nucleophile-poor growth media. ITCs reacted via conjugation with components in a nucleophile-rich growth medium at a rate of 39–141 μmol L−1 h−1, depending on their side chain configuration and temperature. The reaction rates were lowered by a factor of 2–21 when using nucleophile-poor growth media. Consequently, the activity of ITCs was generally improved, with MSITC and MSoITC being the most positively affected (activity increased by a factor of > 4). 9-MSITC and 9-MSoITC had good activity (MIC ≤ 25 μg/mL) against Gram+ bacteria and fungi. The short-chained (C3) analogues had good activity against Gram+ bacteria and Gram– bacteria. The highest bactericidal/fungicidal activity was obtained for 9-MSITC and 9-MSoITC (MBC/MFC 17.5–25 μg/mL). Overall, MSITC and MSoITC might be potential new natural food preservatives, but their reactivity with food matrix components should be considered