Effect of antibiotic prescribing on antibiotic resistance in individual children in primary care: prospective cohort study.

Objective: To assess the effect of community prescribing of an antibiotic for acute respiratory infection on the prevalence of antibiotic resistant bacteria in an individual child. Study design: Observational cohort study with follow-up at two and 12 weeks. Setting: General practices in Oxfordshire. Participants: 119 children with acute respiratory tract infection, of whom 71 received a lactam antibiotic. Main outcome measures: Antibiotic resistance was assessed by the geometric mean minimum inhibitory concentration (MIC) for ampicillin and presence of the ICEHin1056 resistance element in up to four isolates of Haemophilus species recovered from throat swabs at recruitment, two weeks, and 12 weeks. Results: Prescribing amoxicillin to a child in general practice more than triples the mean minimum inhibitory concentration for ampicillin (9.2 µg/ml v 2.7 µg/ml, P=0.005) and doubles the risk of isolation of Haemophilus isolates possessing homologues of ICEHin1056 (67% v 36%; relative risk 1.9, 95% confidence interval 1.2 to 2.9) two weeks later. Although this increase is transient (by 12 weeks ampicillin resistance had fallen close to baseline), it is in the context of recovery of the element from 35% of children with Haemophilus isolates at recruitment and from 83% (76% to 89%) at some point in the study. Conclusion: The short term effect of amoxicillin prescribed in primary care is transitory in the individual child but sufficient to sustain a high level of antibiotic resistance in the population.