Exploring the Antimicrobial Potential of Hallachrome, a Defensive Anthraquinone from the Marine Worm (Polychaeta)

Antimicrobial resistance is a critical global health issue, with rising resistance among bacteria and fungi. Marine organisms have emerged as promising, but underexplored, sources of new antimicrobial agents. Among them, marine polychaetes, such as , which possess chemical defenses, could attract significant research interest. This study explores the antimicrobial properties of hallachrome, a unique anthraquinone found in the purple mucus of , against Gram-negative bacteria ( ATCC 25922, ATCC 9027), Gram-positive bacteria ( ATCC 29212, ATCC 6538, ATCC 12228), and the most common human fungal pathogen ATCC 10231. Antibacterial susceptibility testing revealed that Gram-negative bacteria were not inhibited by hallachrome at concentrations ≤2 mM. However, Gram-positive bacteria showed significant growth inhibition at 0.12-0.25 mM, while was inhibited at 0.06 mM. Time-kill studies demonstrated dose-dependent growth inhibition of susceptible strains by hallachrome, which exerted its effect by altering the membrane permeability of , , and after 6 h and after 24 h. Additionally, hallachrome significantly reduced biofilm formation and mature biofilm in , , and . Additionally, it inhibited hyphal growth in . These findings highlight hallachrome's potential as a novel antimicrobial agent, deserving further exploration for clinical experimentation.