Prior use of antibiotics is associated with carriage of resistant bacteria. Colonization by Streptococcus pneumoniae, Haemophilus influenzae, nonpneumococcal alpha-hemolytic streptococci (NPAHS), and Staphylococcus aureus was evaluated in children receiving antibiotic therapy for acute otitis media and in untreated, healthy control subjects. Children were randomly assigned to receive either amoxicillin/clavulanate (90 mg/kg per day) or azithromycin. Swabs were obtained before initiating therapy and again 2 weeks and 2 months after initiating therapy. We also obtained swabs from control subjects at the time of enrollment and 2 weeks and 2 months after enrollment. The decrease in the rate of carriage of S. pneumoniae and H. influenzae at 2 weeks was significant only in the amoxicillin/clavulanate group (P<.001 and P=.005, respectively). The rate of nasopharyngeal colonization with NPAHS among treated patients increased from 23% to 39% at 2 months (P=.01). This increase was similar for both treatment groups. These results suggest that the competitive balance between organisms is altered by antibiotic therapy.

The effect of antibiotic therapy on nasopharyngeal colonization by Streptococcus pneumoniae and Haemophilus influenzae was evaluated in children diagnosed with acute otitis media. Children were randomly assigned to receive either amoxicillin/clavulanate or azithromycin therapy, and nasopharyngeal swabs were obtained for culture before and after starting therapy. Amoxicillin/clavulanate therapy eradicated or suppressed all strains of S. pneumoniae susceptible to penicillin, 75% of strains with intermediate resistance, and 40% of strains resistant to penicillin. Azithromycin therapy cleared two-thirds of azithromycin-susceptible strains of S. pneumoniae but none of azithromycin-nonsusceptible strains. Selection for antibiotic-resistant strains in individual children was not observed in children who received amoxicillin/clavulanate therapy but was observed in 2 children who received azithromycin therapy. Carriage of H. influenzae was also reduced by antimicrobial therapy but more so by amoxicillin/clavulanate. Antibiotic therapy does not directly increase the number of resistant strains in the population but, by eradicating susceptible strains, allows greater opportunity for carriage and spread of resistant strains.

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