Nasal irrigation increases upper-airway microbial diversity: a prospective pilot study

Nasal irrigation is widely recommended for chronic rhinosinusitis and allergic rhinitis, but its effects on the upper-airway microbiome remain unclear.

To determine whether 30 days of buffered isotonic saline nasal irrigation alters bacterial diversity in the nasal cavity, nasopharynx, and oral cavity.

Ten volunteers performed twice-daily irrigation for 30 days. Swabs from the nasal cavity, nasopharynx, and posterior tongue were collected at baseline and day 30. Bacterial communities were analysed by 16S rRNA gene sequencing. The primary endpoint was change in nasal Shannon diversity; other microbiome, endoscopic, and correlation analyses were exploratory.

Nasal Shannon diversity increased significantly (median [IQR], 1.02 [0.80-1.48] vs 1.50 [1.37-1.66]; p = 0.040), whereas nasopharyngeal and oral diversity did not. Micrococcaceae, Lactobacillus, and Corynebacterium showed directional increases but did not survive FDR correction. Nasal mucosal erythema decreased (p = 0.039). No participant developed acute sinusitis or severe complications; transient mild discomfort occurred in three.

Thirty-day saline nasal irrigation was associated with increased nasal bacterial diversity and reduced mucosal erythema. These single-arm pilot findings are exploratory and warrant validation in larger controlled studies.