Effects of macrolides on ultrastructure of Staphylococcus aureus during postantibiotic phase.

The postantibiotic effects (PAEs) of macrolide antibiotics, such as midecamycin acetate (Miocamycin, MOM), erythromycin (EM), josamycin (JM) and clarithromycin (CAM), on Staphylococcus aureus and the ultrastructure of the pathogen during the postantibiotic phase were investigated. After exposure to 2 x MIC for 2 h, MOM showed the longest PAE of 3.9 h, while EM, JM and CAM showed PAE durations 1.2, 2.5 and 1.9 h, respectively. On examining the serum levels of these agents in man, the longest PAE of 2.4 h was induced by exposure to MOM. JM and CAM induced PAEs for durations of 1.4 and 1.3 h, but EM hardly induced the PAE. The ultrastructure was examined by transmission electron microscopy, and thick cell walls with an undulating outer layer and a multiple thick cross-section were observed for 4 h after exposure to 2 x MIC of MOM for 2 h. After exposure to 2 x MIC of EM, JM and CAM for 2 h, ultrastructural changes were observed for 1, 2 and 2 h, respectively. The size of these cells was about 1.5 to 2 times larger than the normal cells. Ultrastructural changes in S. aureus were observed during the PAE phase of each macrolide.