Effects of subminimal inhibitory concentrations of Erythromycin, Clindamycin, and Pristinamycin on the penicillinase production of Staphlyococcus aureus.

The influence of subminimal inhibitory concentrations of erythromycin, clindamycin, and pristinamycin on the penicillinase production of Staphylococcus aureus was tested in 12 strains. Of the 36 experiments performed, 16 (44%) showed a lack of influence, 10 (28%) displayed an increase, and 10 revealed a decrease in penicillinase activity. The maximal effect produced was generally induced by concentrations ranging from 1/4 to 1/32 the minimal inhibitory concentration, irrespective of the susceptibility of the strain to the drug. In spite of the fact that the drugs are closely related, they sometimes produced opposite effects on the same strain.