AZITHROMYCIN AND CLARITHROMYCIN INHIBITION OF 50S RIBOSOMAL-SUBUNIT FORMATION IN STAPHYLOCOCCUS-AUREUS CELLS

The ID50 values for azithromycin and clarithromycin inhibition of tran slation and of 50S assembly in Staphylococcus aureus cells have been m easured. For clarithromycin, 50% inhibition of growth occurred at 0.07 5 mu g/ml, and the effects on translation and 50S formation were equiv alent at 0.15 mu g/ml. The inhibition of these processes by azithromyc in was less effective, with an ID50 of 2.5 mu g/ml for growth and 5 mu g/ml for inhibition of translation and 50S formation, The additive ef fects of each of these drugs on translation and 50S formation account quantitatively for their observed influence on cellular growth rates, In macrolide-treated cells, there was also a direct relationship betwe en the loss of ribosomal RNA from the 50S subunit and its accumulation as oligoribonucleotides. These results are compared with the previous ly described effects of erythromycin on these same processes.