Competitive advantage of oral streptococci for colonization of the middle ear mucosa

The intermittent aeration of the middle ear seeds its mucosa with saliva aerosols and selects for a distinct community of commensals adapted to the otic microenvironment. We gained insights into the selective forces that enrich for specific groups of oral migrants in the middle ear mucosa by investigating the phylogeny and physiology of 19 strains enriched (Streptococcus) or transiently present (Staphylococcus, Neisseria and actinobacterial Micrococcus and Corynebacterium) in otic secretions. Phylogenetic analyses of full length 16S rRNA sequences resolved close relationships between the streptococcal strains and oral commensals as well as between the transient migrants and known nasal and oral species. Physiological functions that facilitate mucosal colonization (swarming motility, surfactant production) and nutrition (mucin and protein degradation) were widespread in all the otic cultivars, as was the ability of most of the isolates to grow both aerobically and anaerobically. However, streptococci stood out for their enhanced biofilm-forming abilities under oxic and anoxic conditions and for their efficient fermentation of mucosal substrates into lactate, a key metabolic intermediate in the otic trophic webs. Additionally, the otic streptococci inhibited the growth of common otopathogens, an antagonistic interaction that could exclude competitors and protect the middle ear mucosa from infections by transient pathobionts. These adaptive traits allow streptococcal migrants to colonize the otic mucosa and grow microcolonies with syntrophic anaerobic partners, establishing trophic webs with other commensals similar to those formed by the oral ancestors in buccal biofilms. ImportanceThe identification of a diverse microbiome in otic secretions from healthy young adults challenged the entrenched dogma of middle ear sterility and underscored previously unknown roles for oral commensals in the seeding of otic biofilms. By comparing the physiology of novel lineages of streptococci and transient (peri)oral species isolated from otic secretions, we identified adaptive behaviors that allow specific oral streptococcal species to successfully colonize the mucosa of the middle ear. We also describe antagonistic properties of the otic streptococci that help them outcompete transient nasal and oral migrants, including known otopathogens. This knowledge is important to predictively understand the functionality of the otic communities, their interactions with the host mucosa and the outcome of infections.