Abstract:To assess whether long -term, low-dose Erythromycin treatment can change the bacteria resistance and the composition of respiratory microbiota in people with stable non-cystic fibrosis bronchiectasis. Methods A 6 -month, double -blind and placebo -controlled trial was conducted in adult patients with non-cystic fibrosis bronchiectasis and at least twice infective exacerbations in the preceding year. The 176 patients were randomly assigned into two groups, 86 in the placebo group and 90 in the Erythromycin group. Erythromycin ethylsuccinate 250 mg was given twice daily. The times of acute exacerbation, the changes of bacteria resistance and respiratory microbiota composition were compared between the Erythromycin group and the placebo group. Results Compared with placebo, Erythromycin reduced the rate of pulmonary exacerbations over the 6 months of study in the patients with -dominated infection, but not in those with P .Aeruginosa -nondominated infection. In the treatment group, the rate of Gram-negative bacteria resistance in respiratory tract had no obvious change, but the rate of Grampositive bacteria resistance significantly increased, especially to Macrolides. Sputum bacteria load in thetreatment group reduced significantly compared to the placebo group. The change in microbiota composition between baseline and month 6 was significantly greater in the Erythromycin group than in the placebo group. In the patients with baseline airway infection dominated by P.aeruginosa , Erythromycin did not significantly change microbiota composition. In those with infection dominated by organisms other than P.aeruginosa , Erythromycin caused a significant change in microbiota composition, representing a reduced relative abundance of Haemophilus influenzae and increased relative abundance of P.aeruginosa Conclusions Long-term oral low-dose Erythromycin increases the rate of resistance of Gram-positive bacteria in the lower respiratory tract of the treatment group, especially to Macrolides. Long-term Erythromycin treatment changes the composition of respiratory microbiota in patients with bronchiectasis. In the patients without P.aeruginosa airway infection, Erythromycin could not significantly reduce exacerbations but promote displacement of H. influenza by more Macrolide-resistant pathogens including P.aeruginosa and These findings argue for a cautious approach to chronic Macrolide use in patients without P.aeruginosa airway infection.