Inhibition of Peptidoglycan Biosynthesis by Ramoplanin

Ramoplanin, a new lipoglycopeptide antibiotic, inhibits cell wall peptidoglycan biosynthesis in gram-positive bacteria. In both Staphylococcus aureus and Bacillus megaterium, UDP-N-acetylmuramyl-pentapeptides (UDP-MurNAc-pentapeptides) accumulated at concentrations of ramoplanin close to the MIC, indicating that inhibition of peptidoglycan biosynthesis occurred after formation of cytoplasmic precursors. Susceptible bacteria bound or accumulated approximately 5 x 10(4) molecules of ramoplanin per cell, only 1/100th of the amount of vancomycin which binds to groups within peptidoglycan conforming to the pattern L-alpha alpha (amino acid)-D-alpha alpha-D-alpha alpha, suggesting that ramoplanin has a different target site. This was confirmed by in vitro studies involving a wall-membrane particulate fraction from Gaffkya homari in which peptidoglycan synthesis from UDP-MurNAc-tetrapeptide was inhibited by ramoplanin but not by vancomycin. The incorporation of peptidoglycan precursors into nascent peptidoglycan of a toluenized cell preparation of B. megaterium was inhibited by ramoplanin, indicating that the antibiotic acts at a step before transpeptidation. In vitro studies of a wall-membrane particulate fraction of B. megaterium indicated that ramoplanin did not prevent the formation of lipid intermediate I (undecaprenyl-P-P-MurNAc-pentapeptide) but inhibited the next reaction in which N-acetylglucosamine is transferred to that lipid intermediate. The high concentrations required to inhibit in vitro peptidoglycan-synthesizing systems probably reflect the high concentrations of target sites present. High concentrations of ramoplanin also damage certain properties of the cell membrane, but low concentrations only affected wall synthesis in intact bacteria without perturbing membrane function. These studies indicate that the primary target of ramoplanin is peptidoglycan biosynthesis and that the probable reaction inhibited is the N-acetylglucosaminyltransferase-catalyzed conversion of lipid intermediate I to lipid intermediate II.