Changing epidemiology of antimicrobial-resistant Streptococcus pneumoniae

Background. The impact of pediatric 7-valent pneumococcal conjugate vaccination (PCV-7) on the population of Streptococcus pneumoniae in the United States was examined by determining the serotypes, antimicrobial re-sistance profiles, and genetic relatedness of isolates from patients with invasive and noninvasive infections during the 2004–2005 respiratory illness season. Methods. Susceptibility testing, serotyping, and pulsed-field gel electrophoresis analysis were performed on 1647 S. pneumoniae isolates obtained from 41 US medical centers in 2004–2005 as part of a longitudinal anti-microbial resistance surveillance program. The results were compared with surveillance data from earlier periods. Results. From the 1999–2000 to the 2004–2005 respiratory illness season, the prevalence of isolates with intermediate penicillin resistance (minimum inhibitory concentration, 0.1–1 mg/mL) increased from 12.7 % to 17.9%, prevalence of penicillin-resistant isolates (minimum inhibitory concentration, 2 mg/mL) decreased from 21.5 % to 14.6%, and prevalence of isolates resistant to erythromycin increased from 25.7 % to 29.1 % among S. pneumoniae isolates. The prevalence of multidrug resistance among isolates did not change (22.4 % in 1999–2000 and 20.0 % in 2004–2005). Sixty different serotypes were recognized among the isolates from 2004–2005; predom-inant serotypes were 19A (14.5%), 3 (11.2%), 6A (7.1%), 19F (7%), and 11A (6%). Serotypes that were included in PCV-7 accounted for 16.3 % of isolates; 28.4 % of strains isolated had PCV-7–related serotypes, and the remainin