Antimicrobial, antioxidant, and antimutagenic activities of selected marine natural products and tobacco cembranoids

Multidrug resistance (MDR) in microorganisms is a cause of major concern for clinicians and pharmaceutical industries. Continuous development of new antimicrobial drugs with multiple targets and potentials is expected to efficiently combat MDR in these microorganisms. In a continued exploration of new antimicrobial drug leads, 11 marine natural products, semisynthetic, or related synthetic analogs (1-11) and two tobacco cembranoids (12 and 13) were screened for their antimicrobial, antioxidant, and antimutagenic activities. Eight compounds showed varying levels of both antibacterial and antifungal activities. Compounds such as 17-O-methyllatrunculin-A, verongiaquinol, (1S,2E,4R,6R,7E,11E)-2,7,11-cembratriene-4,6-diol), and manzamine-A showed a broad spectrum of activity, inhibiting six of seven tested bacteria with zone of inhibition diameter from 9 to 30 mm. Four of these active compounds also showed antifungal activity. The findings of the in vitro time-kill assay of the most active compound, verongiaquinol, against Staphylococcus aureus indicated its subinhibitory effect at the level lower than the minimal inhibitory concentration (MIC) values (i.e., 2 and 4 μg/mL). At the MIC (8 μg/mL), bacterial cells were completely killed within 18 hours of incubation. DPPH free radical scavenging activity was demonstrated by five compounds in the range of 89.65-36.19% decolorization. Further, four compounds evaluated for their antimutagenic activity against the directly acting mutagens, methyl methanesulfonate and sodium azide, in Salmonella typhimurium strains, interestingly, showed no sign of mutagenicity. Verongiaquinol and manzamine A, in fact, reduced the mutagenicity by 50-75% at a dose of 5 μg/plate in different test strains. Our study seems to provide some novel antimicrobial leads with strong antioxidant potential and the associated ability of antimutagenicity.