Inhibition of ESKAPE bacterial pathogens by endolichenic fungi Nemania bipapillata and Xylaria badia associated with the fruticose lichen Ramalina spp.

Abstract

Endolichenic fungi (ELF), filamentous fungi that live asymptomatically within the lichen thalli, hold promise for combating ESKAPE bacteria — Enterococcus faecalis Schleifer and Kilpper-Bälz 1984, Staphylococcus aureus Rosenbach 1884, Klebsiella pneumoniae Trevisan 1887, Acinetobacter baumannii Bouvet and Grimont 1986, Pseudomonas aeruginosa Migula, and Enterobacter aerogenes Hormaeche and Edwards 1960 (current name: Klebsiella aerogenes Tindall et al. 2017). Twelve ELF from Ramalina were cultured in rice medium and potato dextrose broth (PDB), extracted with ethyl acetate, and assessed for antibacterial activity using a disc diffusion assay against ESKAPE bacterial pathogens. Specifically, 40 μL of 10 mg/mL crude extracts were impregnated onto each disc and placed on 15 mL Mueller-Hinton agar plates previously swabbed with 1 mL of bacterial suspension equivalent to a 0.5 McFarland standard. Crude culture extracts of Xylaria badia Pat. grown in PDB showed promising inhibitory activities against E. faecalis and S. aureus, with zones of inhibition (ZOI) of 18.17 mm and 21.70 mm, respectively. In addition, crude culture extracts of Nemania bipapillata (Berk. & M.A.Curtis) Pouzar grown in rice medium showed weak inhibition against A. baumannii (10.23 mm ZOI). These findings support the potential of endolichenic fungi, particularly X. badia and N. bipapillata, as sources of secondary metabolites active against ESKAPE bacterial pathogens.