Antimicrobial and Antivirulence Potential of Fungal Bioactive Metabolites against Gram-Positive Bacteria
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The growing menace of antibiotic resistance demands immediate discovery of new antimicrobial agents. Fungi are a rich source of bioactive secondary metabolites. This study aimed to investigate the antimicrobial and antibiofilm potential of secondary metabolites from airborne Cladosporium species against Gram-positive bacteria. Fungal isolates were collected from atmospheric samples in Cairo, Egypt, using the settle plate technique. From 886 samples, 212 Cladosporium isolates were purified and identified through morphological and molecular (ITS sequencing) characterization, revealing ten species, with C. herbarum as the most prevalent. Antimicrobial activity was assessed via agar well diffusion and broth microdilution assays. Ethyl acetate extracts, particularly from C. cladosporioides-407, exhibited significant activity against pathogens, including Staphylococcus aureus and Bacillus cereus, with minimum inhibitory concentrations (MICs) as low as 64 μg/mL. The C. cladosporioides-407 extract demonstrated potent, dose-dependent inhibition of S. aureus biofilm formation by up to 60% in a crystal violet assay. Time-kill kinetics confirmed its bactericidal activity at 4x MIC, achieving a ≥3-log reduction in CFU/mL within 24 hours. A marked synergy (FICi=0.5) with amoxicillin was revealed by checkerboard assay. Bioassay-guided fractionation and HPLC-DAD analysis of active fractions identified known antimicrobial compounds, including cladosporin, coumarin, and questinol. Transmission electron microscopy of treated bacterial cells revealed severe ultrastructural damage, suggesting a mechanism of action involving cell wall and membrane disruption. These comprehensive findings underscore airborne Cladosporium species, particularly C. cladosporioides, as a highly promising source of synergistic antimicrobial and antibiofilm metabolites for combating resistant Gram-positive pathogens. This underscores their potential for development into novel therapeutic adjuvants to rejuvenate the efficacy of conventional antibiotics.
