DOI: https://doi.org/10.21203/rs.2.24429/v1
Background The entrenched dogma of a sterile middle ear in health is incongruent with its periodic exposure to exhaled air when the tympanic (Eustachian) tube opens while swallowing, yawning or inhaling deeply. Tubal patency is brief but frequent to provide adequate aeration, equalize pressure across the eardrum and drain mucus down the sides of the oropharynx. It also provides a mechanism for aerial dispersal of microorganisms from the aerodigestive tract.
Method We designed a pilot study and received institutional approval to collect otic secretions that naturally drain on the sides of the oropharynx, behind the palatopharyngeal arch. This protocol bypassed the need to surgically access the middle ear through or around the eardrum, allowed us to collect samples from individuals with no underlying otic conditions, and prevented sample cross-contamination. As controls, we also collected samples from the center of the oropharynx and buccal mucosae seeded by saliva, which may serve as sources of microbial dispersal into the middle ear.
Results We sequenced 16S rRNA-V4 amplicons from otic, oropharyngeal and buccal samples collected from a cohort of 19 healthy young adults. The survey identified in the otic samples a diverse bacterial community with many oropharyngeal and buccal keystone taxa and most of the functional traits of the neighboring oral microbiomes. Neutral community models predicted a large contribution of oral dispersal to the composition of the otic microbiome as well as several taxa responsive to positive selection. This was further supported by the enrichment in the otic communities of obligate anaerobes of Bacteroidetes and Fusobacteria over facultative anaerobic Proteobacteria and Firmicutes. Furthermore, the prevalence of the anaerobic members decreased with the more frequent otic aeration predicted from the equalization training of scuba divers recruited to the study.
Conclusions These results challenge the long held view of a sterile middle ear and suggest instead that frequent seeding with oral microbes supports the establishment of a rich and robust otic community dynamically adapted to the episodic ventilation of the tympanic space.
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