In-vitro activity of 29 antimicrobial agents against penicillin-resistant and -intermediate isolates of Streptococcus pneumoniae

Antibiotic resistance among isolates of Streptococcus pneumoniae is increasing worldwide. Optimal therapy, though unknown, should be guided by in-vitro susceptibility testing. Currently, vancomycin is the only approved antibiotic that is universally active against multiresistant S. pneumoniae. In-vitro activities were determined for 29 antimicrobial agents against 22 penicillin-intermediate S. pneumoniae (PISP) and 16 penicillin-resistant S. pneumoniae (PRSP) isolates. MICs were determined in cation-adjusted Mueller-Hinton broth with 3% lysed horse blood in microtitre trays. Antimicrobial classes tested included cephalosporins, penicillin, aminopenicillins, macrolides, quinolones, carbapenems and other antimicrobial agents. Among the classes of antimicrobial agents tested, wide differences in susceptibility were demonstrated for both PISP and PRSP. Of the cephalosporins, ceftriaxone and cefotaxime demonstrated the best in-vitro activity for both PISP and PRSP. Of the quinolones, clinafloxacin and trovafloxacin showed the greatest in-vitro activity. Rifampicin and teicoplanin demonstrated excellent in-vitro activity. Promising in-vitro results of newer agents, such as quinupristin/dalfopristin, ramoplanin, teicoplanin and linezolid may justify further evaluation of these agents in clinical trials.