Defence phytochemicals are produced de novo in the plant upon stress and have been acknowledged for their antimicrobial activity. Prenylated (iso)flavonoids are the main class of defence phytochemicals in the Fabaceae (e.g. soybean). By simultaneous malting and fungal elicitation of soybeans the amounts of prenylated (iso)flavonoids and consequently their antimicrobial activity can be enhanced. Broad diversity in the (iso)flavonoid skeletons, the number of prenyl groups, and the prenyl configurations can be found in nature. Evidence shows that these variations play a key role in their antimicrobial properties. Diprenylated (iso)flavonoids, bearing two prenyl chains, were among the most active antibacterials against Gram positive bacteria, e.g. L. monocytogenes, with minimum inhibitory concentrations (MIC) of &lt; 10 μg/mL. Furthermore, monoprenylated isoflavonoids showed the best anti-yeast activity, with a MIC of ≤ 15 μg/mL against Z. parabailii. Regarding the mode of action, it was shown that prenylated (iso)flavonoids can rapidly disrupt the integrity of the cytoplasmic membrane. Isothiocyanates (ITCs) are another class of defence phytochemicals released upon degradation of glucosinolates, as a defence response in the Brassicaceae family. Isothiocyanates bear an -N=C=S group, which makes them highly reactive towards nucleophiles. The antimicrobial properties of ITCs, mainly of allyl ITC (AITC), has been reported before, nonetheless, the antimicrobial properties and spectrum of activity of different ITC subclasses, bearing different side chains, have not been investigated before. Isothiocyanates (ITCs) with short chained methylsulfinyl (MS) or methylsulfonyl (MSo) groups showed good antimicrobial activity (MIC 25 μg/mL) against Gram negative bacteria (E. coli). ITCs with a long chained MS/MSo groups showed good antimicrobial activity (MIC ≤ 25 μg/mL) against Gram positive bacteria (B. cereus, L. monocytogenes) and fungi (S. cerevisiae and A. niger spores). The quantitative structure-activity relationships of these natural antimicrobials will be discussed and lead compounds for the development of novel antimicrobial strategies will be highlighted.

Antimicrobial activity of defence phytochemicals: Prenylated (iso)flavonoids and isothiocyanates

ACS Spring 2022

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