Phyllosticta capitalensis: a novel source of Guignardone A, a high-affinity ligand for penicillin-binding proteins

Endophytic fungi from Pleurolobus gangeticus were isolated, clone purified, identified and subjected to liquid state fermentation. Ethyl acetate extracts of the fermentation product was prepared for antibacterial testing against gram-negative and gram-positive bacteria. Out of the four endophytes isolated, Phyllosticta capitalensis (DLa) showed strong antibacterial activity in the disc diffusion assay with maximum zone of inhibition (27.3 ± 0.6 mm). DLa also had a very low value for Minimum Inhibitory Concentration (10 µg/mL) against all the tested bacteria, indicating its strong broad-spectrum activity. The phytochemical characterisation of DLa showed a meroterpenoid named Guignardone A and three Cyclic dipeptides. In silico studies indicated strong binding affinity between Guignardone A and various penicillin-binding proteins (PBP1, PBP2, PBP2a, PBP3, PBP4 and PBP5) of the above tested bacteria. Further studies are ongoing for optimising the fermentation conditions of P. capitalensis for the enhanced production of Guignardone A.