In vitro activity of clarithromycin against penicillin-susceptible and penicillin-resistant strains of Streptococcus pneumoniae in a pharmacodynamic simulation model

Clarithromycin has shown enhanced activity against Streptococcus pneumoniae , but increased resistance to macrolides has been observed in recent years. Our aim was to investigate its activity against strains of S. pneumoniae w ith variable susceptibility to this antibiotic and to penicillin, We determ ined killing curves using the Centriprep-10 pharmacodynamic simulation mode l, which permits using varying antibiotic concentrations to mimic a pharmac okinetic human profile in serum (corresponding to an oral dose of 500 mg), Four strains of S, pneumoniae were tested. In susceptible strains, clarithr omycin showed bactericidal activity (reductions of up to 2.97 log(10) cfu/m l of the initial inoculum), In resistant strains, clarithromycin showed a b acteriostatic effect (<1 log(10) cfu/ml reduction). Penicillin-susceptible strains showed higher reductions than penicillin-resistant strains. This ef fect is important owing to the high minimum inhibitory concentration (MIC) of one of the resistant strains (32 <mu>g/ml) More studies are needed to ex plain this bacteriostatic activity.