Isolation and synthesis of a new cyclic tetrapeptide from marine-associated Bacillus sp. and its bacterial biofilm formation inhibitory activity.

A new cyclic tetrapeptide named Cereusitin B (1), along with a number of known cyclic dipeptides was isolated from the fermentation broth of a marine-associated Bacillus sp. I-2. The structure of Cereusitin B, cyclo-(L-phenylalanyl-trans-4-hydroxy-L-prolyl-L-leucyl-L-alanine), was determined through extensive analysis using high-resolution electrospray ionization mass spectrometry and NMR spectral data (1D, 2D NMR), and its identity was further confirmed by Marfey' method and chemical synthesis. The antimicrobial activity tests showed that compound 2 was moderately active against Candida albicans with a MIC value of 125 µg mL, while the other compounds exhibited no antimicrobial activity at the experimental concentrations. In addition, compound 1 showed the ability to inhibit the formation of S. aureus biofilm.