Background: The Bronchiectasis Severity Index (BSI) and FACED score assess the prognosis and severity of bronchiectasis (BE). Yet, its relationship with the lung microbiome is unknown. The aim of this study was to explore the relationship between lung microbiota and severity in BE patients colonized by Pseudomonas aeruginosa.

Methods: In this cross-sectional observational study, sputum was collected, and severity (BSI, FACED) was recorded. Alpha and beta diversities and linear discriminant analysis effect size (LEfSe) were estimated by 16S rRNA to determine differences among the severity groups.

Results: 27 patients were included (Table 1). No significant differences were found between alpha diversity and BE severity, but LEfSe showed differential abundance of Actinomyces hyovaginalis in severe patients grouped by the FACED score (Fig. 1).

Conclusions: Severity indexes do not reflect the loss of respiratory biodiversity. This is the first-ever description of A. hyovaginalis in human airways; in pigs, it has been associated with disseminated lung lesions (Aalbaek B et al. 2003). Thus, it could be a marker of severity and lung injury in patients with BE.