[Annual changes in susceptibility of clinical isolates to midecamycin acetate]

To investigate annual changes in the susceptibility of clinical isolates to midecamycin acetate (MDM-AC), minimum inhibitory concentrations (MICs) of MDM-AC were determined for clinical isolates obtained from outpatients since 1985. MDM-AC-resistant strains of Staphylococcus spp. and Streptococcus spp. have shown similar degrees of resistance to midecamycin and josamycin. Regarding this as macrolide resistance, proportions of macrolide-resistant strains tended to increase for Staphylococcus aureus and Streptococcus pneumoniae but to decrease for Streptococcus pyogenes. 1. For S. aureus, 8% of the strains isolated in 1985, 20% in 1987 and 20% in 1989 were macrolide-resistant. Of these macrolide-resistant strains, 70% or more in 1987 and 80% or more in 1989 were methicillin-resistant S. aureus (MRSA). 2. For S. pneumoniae, 8% of the strains isolated in 1985, 12% in 1987 and 12% in 1989 were macrolide-resistant, indicating a tendency for resistant strains to increase annually. 3. For S. pyogenes, 8% of the strains isolated in 1985, 4% in 1987 and 0% in 1989 were macrolide-resistant, showing a decreasing tendency. 4. MDM-AC is still thought to be a clinically useful antibacterial agent because it still shows antibacterial activity against 80% or more of Gram-positive cocci clinically isolated in recent years and a low degree of induction of macrolide resistance in Staphylococcus spp.