We conducted a multicenter prospective study to document changes in nasopharyngeal carriage of Streptococcus pneumoniae, Haemophilus influenzae, and Branhamella catarrhalis during antibiotic therapy. A cohort of 629 children with respiratory tract infections underwent nasopharyngeal sampling before and after antibiotic treatment. Susceptibility testing, serotyping, arbitrarily primed polymerase chain reaction, and pulsed-field gel electrophoresis were used to compare pretreatment and

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... nt strains of S. pneumoniae. A significant decrease in carriage of all 3 species (especially S. pneumoniae and B. catarrhalis) was recorded. The increase in the proportion of penicillin-resistant pneumococci (PRP; 66% vs. 44%) was due to the decreased carriage of penicillin-susceptible pneumococci (71 of 629 vs. 176 of 629). The risk of PRP carriage in a given child did not increase. None of the children was found to harbor genetically related strains with increased minimum inhibitory concentrations. Given the multiple resistance of PRP, b-lactam antibiotic therapy also increased the incidence of macrolide-resistant strains, whereas macrolides selected both macrolide-and penicillin-resistant strains. During the past 10 years, there has been an alarming worldwide increase in antibiotic resistance among bacterial pathogens of the respiratory tract, particularly Streptococcus pneumoniae [1] . This is generally attributed to extensive use of antibiotics and the selection pressure they exert on bacterial strains of nasopharyngeal flora [2, 3] . This ecosystem is the reservoir for bacterial pathogens (Haemophilus influenzae, Branhamella catarrhalis, S. pneumoniae, etc.) involved in respiratory tract infections (RTIs) [4, 5] . Before 3 years of age, children are more likely than adults to harbor these 3 species in their nasopharynx and to receive antibiotic therapy [4, 6, 7] . Carriage of the same S. pneumoniae serotype generally lasts several weeks or months, and it is therefore not surprising that the highest rates of resistance are observed in young children [7] [8] [9] . The ecological impact of antibiotic therapy, especially in terms of increasing