Characterization of the mechanism of the Staphylococcus aureus cell envelope by bacitracin and bacitracin-metal ions.

Bacitracin is a metal-dependent dodecapeptide antipeptide produced by Bacillus species. Microcalorimetry was used to study the antimicrobial activity of bacitracin and bacitracin-metal ion complexation inhibited on Staphylococcus aureus at 37 degrees C. The affinity of metal ions binding to bacitracin was investigated by isothermal titration calorimetry and was as follows: Cu(II) >or= Ni(II) > Co(II) > Zn(II) >or= Mn(II). The metal ion binding affinity is not relative to the antimicrobial activity of bacitracin-metal complexation. Atomic force microscopic images revealed that the surface of S. aureus treated by bacitracin-Zn(II) was rather rough compared to that treated by bacitracin only. The central cell surface displayed small depressed grooves around the septal annulus at the onset of division. Bacitracin mainly inhibited the splitting system within the thick cross walls as seen by transmission electron microscopy (TEM). The inhibition mechanism of bacitracin may be relative to the assistance of Zn(II) coordination with the cell surface as seen by TEM. We can put forward that the activity of bacitracin only inhibited growth and division initially from the synthesis of the cell wall, especially the cell wall of the septal annulus. The divalent metal ions function to increase the adsorption of bacitracin onto the cell surface.