Antimicrobial and Antibiofilm Activities of the Fungal Metabolites Isolated from the Marine Endophytes Epicoccum nigrum M13 and Alternaria alternata 13A

Abstract

Epicotripeptin (1), a new cyclic tripeptide along with four known cyclic dipeptides (2-5) and one acetamide derivative (6) were isolated from seagrass-associated endophytic fungus Epicoccum nigrum M13 recovered from the Red Sea. Additionally, two new compounds, cyclodidepsipeptide phragamide A (7) and trioxobutanamide derivative phragamide B (8), together with eight known compounds (9-16), were isolated from plant-derived endophyte Alternaria alternata 13A collected from a saline lake of Wadi El Natrun depression in the Sahara Desert. The structures of the isolated compounds were determined based on the 1D and 2D NMR spectroscopic data, HRESIMS data, and a comparison with the reported literature. The absolute configurations of 1 and 7 were established by advanced Marfey's and Mosher's ester analyses. The antimicrobial screening indicated that seven of the tested compounds exhibited considerable (MIC range of 2.5-5 µg/mL) to moderate (10-20 µg/mL) antibacterial effect against the tested Gram-positive strains and moderate to weak (10-30 µg/mL) antibacterial effect against Gram-negative strains. Most of the compounds exhibited weak or no activity against the tested Gram-negative strains. On the other hand, four of the tested compounds showed considerable antibiofilm effects against biofilm forming Gram-positive and Gram-negative strains.

Keywords: Alternaria; Epicoccum; antibiofilm; antimicrobial; epicotripeptin; phragamide.

Figure 1

Structures of the fungal metabolites isolated from the E. nigrum M13 (1–6) and A. alternata 13A (7–16).

Figure 2

(A) Key COSY and HMBC correlations of compounds 1, 7, and 8; (B) molecular mechanics simulations of compound 7.

Figure 3

Biofilm inhibition effect of isolated pure compounds from E. nigrum M13 and A. alternata 13A against S. aureus, B. subtilis, E. coli, and P. aeruginosa. The biofilm was quantified using a microtiter plate and crystal violet assay. The bars on the graph represent the mean ± SD as a percentage of biofilm inhibition. Inactive compounds were not reported.