Long-term, low-dose treatment with erythromycin has only a modest disruptive effect on the bacteria that are normally present in the upper airways and their resistance to antibiotics, a study suggests. Because of their ability to reduce the frequency of pulmonary exacerbations, there has been a significant increase in the use of a class of antibiotics known as macrolides, which includes erythromycin, in the treatment of chronic respiratory conditions, including bronchiectasis and severe asthma. Long-term use of antibiotics is usually associated with bacteria developing a resistance against the antibiotic in question, resulting in unwanted effects. Although macrolide's effect on pulmonary exacerbations is unrelated to its antimicrobial activity (and instead related to the immune response of patients, an immunomodulatory action), there are concerns that its long-term use might have effects on important bacteria that normally inhabit the respiratory tract, known as commensal bacteria, which play key roles in safeguarding the respiratory system from pathogenic microbes. To address these concerns, in this study, "Impact of Long-Term Erythromycin Therapy on the Oropharyngeal Microbiome and Resistance Gene Reservoir in Non-Cystic Fibrosis Bronchiectasis," published in the journal mSphere, researchers aimed to investigate the effects of long-term macrolide therapy on the composition and function of commensal microbes of the respiratory system. They analyzed data from 84 adults who had been diagnosed with non-cystic fibrosis bronchiectasis and were enrolled in the Bronchiectasis and Low-dose Erythromycin Study (ACTRN12608000460303). Patients were divided into two groups, receiving either erythromycin or a placebo twice daily for 48 weeks. Throat swabs were collected at the beginning of the study and after 48 weeks of treatment. Swabs were analyzed to determine the microbes present and their resistance against macrolides. Results showed that at the beginning of the study, the microbes identified in the swabs were of the types that would normally be expected. However, after the 48-week treatment, the erythromycin group of patients had significant differences from the placebo-treated group. The erythromycin-treated group had higher levels of the potential respiratory pathogen Haemophilus parainfluenzae, but lower levels of Streptococcus pseudopneumoniae and Actinomyces odontolyticus. The clinical significance of these changes, however, is not yet clear, the researchers noted. In terms of antibiotic resistance, there were no significant increases in the number of patients who had bacteria with erythromycin resistance, but patients who already had erythromycin-resistant microbes showed an increase in resistance, as measured by resistance gene copies, following the treatment period. Researchers concluded that their results indicate that "changes in oropharyngeal microbiota composition resulting from long-term erythromycin treatment are modest and are limited to a discrete group of taxa," namely members of the genus Actinomyces. Furthermore, the team believes that the antibiotic resistance results "highlight the potential for the oropharynx to act as a reservoir for antimicrobial resistance."