Alternoic acids A and B: antimicrobial esters of dienoic fatty acid from the marine sediment-derived fungus Alternaria rosae DY-79

Objectives
Bacterial resistance to current antibiotics is mounting at an alarming rate, making many conventional antibiotics ineffective and posing a serious worldwide health threat; thus, there is a crucial need for exploration of novel antimicrobial leads from unusual sources, particularly the marine environment, which produces unique chemical compounds with promising therapeutic potential. This study aims to purify antimicrobial compounds from the ethyl extract of the fungus Alternaria rosae DY-79, characterize their chemical structures, and evaluate their antimicrobial effects against pathogenic microbes.

Methods
The antimicrobial fraction of the fungal culture extract was partitioned, and purified, yielding two unreported compounds, alternoic acids A (1) and B (2). Their structural determination was conducted using NMR and (+)-HRESIMS spectral analyses. Antimicrobial activities were measured against Escherichia coli, Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Candida albicans.

Results
Alternoic acids A (1) and B (2) were identified as (4E,6E)-6-(methoxycarbonyl)-2,4-dimethylocta-4,6-dienoic acid and (3E,6E)-6-(methoxycarbonyl)-2,4-dimethylocta-3,6-dienoic acid, respectively. Compound 1 showed no MRSA activity, with inhibition zones of 14.2–17.2 mm against S. aureus, E. coli, and C. albicans (MICs: 21.3, 21.3, and 10.7 µg/mL, respectively). Compound 2 exhibited anti-MRSA activity (17.2 mm; MIC = 8.3 µg/mL), weaker zones against bacteria (12–12.2 mm; MICs = 21.3 µg/mL), and antifungal effects (17.3 mm; MIC = 10.7 µg/mL). Compared to imipenem (MRSA MIC = 0.75 µg/mL; S. aureus zone = 29.3 mm), compound 2 was less potent bacterially but showed antifungal activity comparable to clotrimazole (17.8 mm; MIC = 0.17 µg/mL). Significant differences (P < 0.001) were observed versus imipenem.

Conclusion
Compound 2 demonstrates notable anti-MRSA activity (weaker than imipenem), while both compounds show antifungal effects against C. albicans with inhibition zones like clotrimazole but weaker MICs. Alternoic acids A and B may serve as lead scaffolds for antimicrobial optimization, particularly for antifungal applications, warranting further mechanistic studies.