Three New Diketopiperazines from the Previously Uncultivable Marine Bacterium Gallaecimonas mangrovi HK-28 Cultivated by iChip.

The in situ application of iChip cultivation in mangrove sediment from Hainan province, China, led to the isolation of a novel bacterial species Gallaecimonas mangrovi HK-28. The extract of G. mangrovi HK-28 exhibited antibiotic activity against the aquatic pathogen Vibrio harveyi, and its chemical constituents were further investigated by bioactivity-guided isolation. Three new diketopiperazines, gallaecimonamides A-C, were accordingly isolated from the AcOEt extract of the fermentation broth of G. mangrovi HK-28. The planar structures of gallaecimonamides A-C were determined using HR-ESI-MS together with 1D- and 2D-NMR. The absolute configurations of gallaecimonamides A-C were assigned by optical rotation, NOESY experiment and TDDFT ECD calculations. The in vitro antibacterial and antimalarial activities of gallaecimonamides A-C were assessed. Gallaecimonamide A was found to display antibacterial activity against V. harveyi with a MIC value of 50 μm. However, gallaecimonamides B and C showed no antibacterial activity against V. harveyi (MIC >300 μm). In addition, all the isolates did not exhibit any inhibitory activities against V. parahaemolyticus (MIC>300 μm) and Plasmodium falciparum W2 (EC50 >100 μg/mL).