Anti-Mycoplasma Activity of Bacilotetrins C–E, Cyclic Lipodepsipeptides from the Marine-Derived Bacillus subtilis and Structure Revision of Bacilotetrins A and B

Mycoplasma hyorhinis most commonly causes polyserositis and arthritis in swine and is a common contaminant during the cell culture in the laboratory. In our continuing research for diverse bioactive compounds from Bacillus subtilis 109GGC020, we discovered uncommon cyclic lipotetrapeptides showing inhibitory activities against M. hyorhinis with similar structures to previously reported bacilotetrins A and B. Bacilotetrins C–E (1–3), new cyclic lipodepsipeptides, were isolated from the EtOAc extract obtained from the fermentation of marine-derived Bacillus subtilis isolated from a marine sponge sample collected from the Gageo reef, Republic of Korea. The structures of 1–3, consisting of three leucine residues, one glutamic acid, and a β-hydroxy fatty acid, were elucidated by detailed analysis of 1D, 2D NMR, and HR-ESIMS data. The absolute configurations of the amino acids and β-hydroxy fatty acid were established by advanced Marfey’s method and Mosher’s method, respectively. The localization of L- and D-amino acids within the compounds was determined by retention time comparison of each purchased dipeptide standard to the partial hydrolysate products using LC-MS. Compounds 1–3 exhibited anti-mycoplasma activity, with an MIC value of 31 μg/mL, twofold stronger than that of the positive control, BioMycoX(®). Detailed analysis and comparison of the spectroscopic data between bacilotetrins A (4) and B (5) and 1–3 led us to revise the structures of 4 and 5.