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Abstract:It has been reported that methicillin-resistant strains of Staphylococcus aureus (MRSA) were slightly larger in coccal size than that of methicillin-susceptibe S. aureus (MSSA). However, the data in these reports were not exact because the observations by using microscope or transmission electron microscope on ultrathin sections were inadequate for measuring the coccal size.The coccal size was examined by using a scanning electron microscope. In addition, the width of cell walls was examined by using a transmission electron microscope on the ultrathin sections.Both mutant MSSA strains lost methicillin-resistance from MRSA strain and MRSA strains selected as heterogeneous strains from MSSA were used for measuring the morphological size compared with clinical strains of MRSA and MSSA.There were no significant differences in the coccal size and the width of the cell walls of MRSA, MSSA and mutant MRSA strains.However, the width of the cell walls of the MRSA strain selected from MSSA was about 85% larger than that of the parent MRSA strain. The effect might be influenced by the used antibiotics. These results revealed that the differences between MRSA and MSSA strains were not significantly