BIOPROSPECTING AND MODIFICATION OF MRSA-INHIBITING METABOLITES FROM RARE BOTANICAL SPECIES

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) poses a chronic public threat because it is resistant to conventional medications. In this study, we utilized 90 of the most important plant tissues from a collection of 300 rare plant species from the botanical garden of the University of Turku. The aim was to first investigate how the metabolite composition of the species is linked to the inhibition of MRSA bacterial growth. In the next step the metabolite extracts were treated in three separate ways that were thought to either remove or modify the MRSA-active compounds from the extracts. The changes in metabolite profiles were linked to the changes in MRSA inhibitions. The methodology involved the collection of two replicate samples per species, enzymatic oxidation of one replicate and freeze-drying all the tissue. The finely ground and weighed tissue was extracted with acetone/water (80/20, v/v) and the aqueous phase freeze-dried. Extracts were prepared in water, filtered and analysed using group-specific MS/MS methods with the UHPLC-DAD-MS/MS system, and for their oxidative activity and protein affinity. Of the treatments originally planned, enzymatic oxidation and nylon-membrane removal of the protein-active compounds were completed; the alkaline (pH 10) oxidation and its two combination treatments were not carried out, for reasons of time and technical constraints. The completed treatments were the ones tested against MRSA bacterial growth. Across the three completed treatments, the results followed a clear and interpretable trend. For MRSA, the mean inhibition of the 90 control extracts fell after enzymatic oxidation and fell further after nylon filtration, and the number of strong inhibitors dropped from 28 in the control to 16 after oxidation and 10 after filtration. Because nylon filtration removed the most activity, the large protein-binding tannins appear to carry much of the inhibition for most species, while a smaller group of extracts kept their activity through both treatments, marking those as the compounds least dependent on removable or oxidisable polyphenols. One species stood out: the Anacardiaceae member Pleiogynium solandri held strong inhibition of roughly 82 to 84% across control, enzymatic oxidation and nylon filtration alike, which points to a stable, treatment-resistant metabolite and makes it the clearest candidate for isolation and structural identification. It remains to be confirmed whether the metabolites removed by the treatments are the same ones responsible for the activity of the control extracts. The approach developed in this study could significantly help the future bioprospecting of MRSA-active metabolites, since multiple treatment types are likely to reveal the truly active metabolites better than studies using one type of sample only.