Comparing microbiotas in the upper aerodigestive and lower respiratory tracts of lambs

Background: Recently, the importance of the lung microbiota during health and disease has been examined in humans and in small animal models. Whilst sheep have been proposed as an appropriate large animal model for studying the pathophysiology of a number of important human respiratory diseases, it is clearly important to continually define the limits of agreement between these systems as new concepts emerge. In humans, it has recently been established that the lung microbiota is seeded by microbes from the oral cavity. We sought to determine whether the same was true in sheep. Results: We took lung fluid and upper aerodigestive tract (oropharyngeal) swab samples from 40 lambs (7 weeks old). DNA extraction was performed, and the V2-V3 region of the 16S rRNA gene was amplified by PCR then sequenced via Illumina Miseq. Oropharyngeal swabs were either dominated by bacteria commonly associated with the rumen or by bacteria commonly associated with the upper aerodigestive tract. Lung microbiota samples did not resemble either the upper aerodigestive tract samples or reagent-only controls. Some rumen-associated bacteria were found in lung fluids, indicating that inhalation of ruminal bacteria does occur. We also identified several bacteria which were significantly more abundant in lung fluids than in the upper aerodigestive tract swabs, the most predominant of which was classified as Staphylococcus equorum. Conclusions: In contrast to humans, we found that the lung microbiota of lambs is dissimilar to that of the upper aerodigestive tract, and we suggest that this may be related to physiological and anatomical differences between sheep and humans. Understanding the comparative physiology and anatomy underlying differences in lung microbiota between species will provide a foundation upon which to interpret changes associated with disease and/or environment.