Fungal blood: Mycology and natural products chemistry of endophytes from medicinal herbs

Recently, we initiated studies on the endophytic fungi of medicinal herbs, i.e. fungi that live asymptomatically within the tissues, to probe how the chemistry of such fungi may influence the chemistry (and perhaps, biological activity) of the medicinal herb. A prime example emerged with an endophytic Penicillium sp. (G85), which was isolated from surface sterilized stems of milk thistle [Silybum marianum (L.) Gaertn. (Asteraceae)] into axenic culture. Pure cultures of G85 produced a red exudate (also known as a 'guttation') on potato dextrose agar amended with antibiotics. Chemical analysis of the guttate, as well as the organic extract of a solid phase culture of this fungus, using a combination of UPLC-HRMS and NMR techniques, revealed a series of structurally related anthraquinone derivatives. Four known compounds, emodin, emodic acid, ω-hydroxyemodin, and isorhodoptilometrin, along with four new derivatives were isolated. The unsaturated nature of these compounds contributed the red color of the guttate. The pure compounds also exhibited quorum quenching activity against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA).