Isolation and semi-synthesis of enterocin analogues as novel antibiotics

The natural polyketide enterocin was first isolated in 1976 from two soil Streptomyces and was reinvestigated in this study for its potential use as an antibiotic. Previous reports show enterocin to have broad-spectrum bacteriostatic antibiotic activity. We show, however, that the related pyrone-containing molecules known as wailupemycins are stronger candidates for antibiotics than enterocin. For instance, in this study, four wailupemycins (wailupemycin F, wailupemycin G, p-hydroxywailupemycin F and p-hydroxywailupemycin G) were isolated from the enterocin-producing Streptomyces sp. MST-MA9095 on an optimised culture media. Except for wailupemycin G, these wailupemycins were ten-fold stronger antibiotics than enterocin with bactericidal activity against Micrococcus luteus (MIC 4-8 ug/mL) and Bacillus subtilis (16-32 ug/mL). Investigations into the mode of action of enterocin suggest that it plays a role as a gene regulatory molecule and that its antibiotic activity stems more from the presence of an a-pyrone moiety. Inhibition of secondary metabolism by enterocin (25 ug) has been demonstrated on several known producers including the producer of staurosporine, Streptomyces sp. MST-RA9773.
Enterocin was also shown to possess several instabilities not previously reported on. Most concerning was the intramolecular decomposition of enterocin to an isomer bereft of antibacterial activity. Additionally, enterocin was shown to be unstable under neutral and alkaline conditions, as well as in a range of solvents at room temperature. To circumvent its stability, 2-methoxyaminoenterocin was derived with a bactericidal rather than a bacteriostatic mode of action, albeit with inferior antibacterial activity (MIC 125 ug/mL). Semi-synthesis of enterocin at its 5-OH site also yielded four new esters all with reduced activity in comparison to enterocin. The structures of all semi-synthetic and isolated analogues of enterocin obtained in this study were confirmed with 2D NMR spectroscopic data and mass spectrometric data.