Four new antibacterial xanthones from the marine-derived actinomycetes Streptomyces caelestis.

Four new polycyclic antibiotics, citreamicin theta A (1), citreamicin theta B (2), citreaglycon A (3), and dehydrocitreaglycon A (4), were isolated from marine-derived Streptomyces caelestis. The structures of these compounds were elucidated by 1D and 2D NMR spectra. All four compounds displayed antibacterial activity against Staphylococcus haemolyticus, Staphylococcus aureus, and Bacillus subtillis. Citreamicin theta A (1), citreamicin theta B (2) and citreaglycon A (3) also exhibited low MIC values of 0.25, 0.25, and 8.0 mu g/mL, respectively, against methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300.