Antimicrobial activity among multidrug-resistant Streptococcus pneumoniae isolated in the United States, 2001-2005

Infections caused by multidrug-resistant (MDR) Streptococcus pneumoniae remain a major concern when selecting an appropriate antimicrobial agent. In this analysis, 27 781 isolates of S pneumoniae collected from 2001 to 2005 in the United States were tested for MDR phenotypes. About 25% of all isolates were MDR, defined as resistant to 2 or more of the following agents: cefuroxime, a macrolide, penicillin, tetracycline (if available), and trimethoprim-sulfamethoxazole (TMP-SMX). There was a slight decreasing trend over time in multidrug resistance prevalence with erythromycin. Among MDR strains, the most common coresistance pair was erythromycin and TMP-SMX (74% of isolates, irrespective of resistance to other agents), although penicillin-erythromycin and penicillin-TMP-SMX coresistance patterns were also found in more than 56% of MDR strains. Resistance to 4 antimicrobial agents tested was observed in 33% of all antimicrobial-resistant isolates. Levofloxacin, which was used as a representative of the fluoroquinolone class, was active against at least 98% of all MDR isolates, and the minimum inhibitory concentration (90%) (MIC(90)) for this population was 1 microg/mL (identical to the total S pneumoniae, population). Multidrug-resistant isolates from 2003 to 2005 were found to be equally susceptible (98%) to other respiratory fluoroquinolones (gatifloxacin and moxifloxacin; data not shown), although only 88% of MDR isolates (from 2001-2005) were susceptible to ciprofloxacin. Careful monitoring of multidrug resistance patterns will help guide appropriate therapeutic selection and may provide early detection of changes in resistance to more potent agents.