In vitro efficacies of various isothiocyanates from cruciferous vegetables as antimicrobial agents against foodborne pathogens and spoilage bacteria

The antimicrobial activities of ten isothiocyanates (ITCs), namely sulforaphane, iberin, allyl-, benzyl-, methyl-, phenyl-, phenylethyl-, propyl-, 3-methylthiophenyl-, and 3-methylthiopropyl-ITC, were tested against a range of pathogenic and food spoilage bacteria. The activities of six ITCs were investigated for the first time. The growth of the bacteria was monitored by turbidimetry in a culture broth with increasing concentrations of ITCs. An antimicrobial efficacy index (IAE) was calculated based on the observed growth delay, reduction in maximum growth rate and reduction in population size. ITCs inhibited pathogenic and food spoilage bacteria including species from Bacillus, Escherichia, Klebsiella, Listeria, Salmonella, Serratia and Staphylococcus. Gram-negative bacteria were overall more sensitive to ITCs than their Gram-positive counterparts: the three most sensitive species with five ITCs, and two out of three most sensitive species with the other ITCs. Yet, the species of a bacterium seemed to play a more important role, since considerable variations in sensitivity were observed between species even within the same Gram type. All ITCs investigated displayed antimicrobial activity, but with levels that depended on the target bacteria and the molecule considered. The highest IAE values were obtained with Benzyl-ITC, followed by phenylethyl-ITC, but this was not necessarily the case with all aromatic ITCs. An aliphatic compound, 3-methylthiopropyl-ITC, was much more active than phenyl-ITC and 3-methylthiophenyl-ITC. Within a structural group, the activity of ITCs varied dramatically, and structural features, like the presence of a sulfinyl group, the molecule size, and the length of the hydrocarbon chain, seem to be of importance.