Étude des champignons endophytes d’une algue marine Ulva sp : isolement, identification et potentiel antimicrobien

Abstract

The growing resistance of bacteria to antibiotics is currently driving scientific research toward the exploration of new natural sources of antimicrobial molecules. Endophytic fungi associated with marine algae represent, in this regard, a still underexploited resource with significant biotechnological potential. This work aims to isolate, identify and evaluate the antimicrobial potential of endophytic fungi associated with the green marine alga Ulva sp. collected from three beaches in the wilaya of Mostaganem (Sidi Ladjel, Diancy and Stidia). Endophytic fungi were isolated on PDA medium, purified, and then identified based on macroscopic and microscopic criteria. Their antimicrobial activity was first screened through an antagonism test on Mueller-Hinton medium and the agar plug diffusion technique, against six pathogenic bacterial strains and one yeast. The most active isolates were subsequently subjected to solid- and liquid-state fermentation, followed by extraction of secondary metabolites with ethyl acetate. The antimicrobial activity of the obtained extracts was evaluated by the disc diffusion method, complemented by the determination of minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) using the broth microdilution method. Nine fungal isolates were obtained, belonging mainly to the genera Aspergillus, Alternaria and Penicillium. The antagonism test revealed no inhibition zone, whereas the agar plug diffusion technique allowed the selection of five active isolates, including B02.H and B1.R, exhibiting activity directed mainly against Gram-positive bacteria (Staphylococcus aureus and Bacillus cereus), with inhibition zones reaching up to 20 mm during preliminary screening. After fermentation and extraction, the disc diffusion tests confirmed this selectivity, with the solid extract of isolate B02.H showing an inhibition zone of 31.33 mm against B. cereus, exceeding that obtained with the antibiotic used as a positive control. These results confirm that endophytic fungi associated with Ulva sp. constitute a promising source of bioactive metabolites with antimicrobial properties, particularly against Gram-positive bacteria.