Exploration of bioactive metabolites from Aspergillus austroafricanus as potential antibacterial agents against Klebsiella pneumoniae

Abstract

Marine-derived fungi are increasingly recognized as promising sources of bioactive metabolites with antimicrobial potential. This study investigated the antibacterial activity, chemical composition, and molecular identity of metabolites produced by Aspergillus austroafricanus isolated from marine stone samples collected from Oron River, Nigeria. The fungal isolate was identified using morphological characteristics and confirmed by internal transcribed spacer (ITS) region sequencing. Crude metabolites were extracted using an ethyl acetate–methanol (1:1) solvent system and evaluated against Klebsiella pneumoniae (PQ821874) using agar well diffusion, broth dilution, and bactericidal assays. Molecular analysis showed a 100% sequence similarity with A. austroafricanus, confirming taxonomic identity. The crude extract demonstrated strong antibacterial activity with inhibition zones ranging from 26–28 mm, outperforming standard antibiotics, including ceftriaxone and amoxicillin–clavulanic acid, against the tested strain. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the crude extract were 12.5 µL/mL and 50 µL/mL, respectively, indicating potent bacteriostatic and bactericidal effects. The M-fraction and H-fractions showed moderate antibacterial activity relatively comparable to the crude extract, suggesting synergistic interactions among metabolites in the crude extract. Gas chromatography–mass spectrometry (GC–MS) analysis revealed a diverse array of volatile bioactive compounds, predominantly fatty acids, esters, terpenes, and steroids, with phytol identified as the major constituent (13.447%). These findings highlight potentials of marine-derived A. austroafricanus metabolites as a source of anti-drug-resistant K. pneumonia treatment.