**2.5 Pharyngeal microbiota**

In the nasal, oral and pharyngeal human cavities live hundreds of microbial species, including between 25 and 40 families archeas, bacteria, amoebae and fungi, as evidenced in a wide range of cultures. The number of newly discovered species has increased considerably due to the discovery of noncultivable species [88]. A data published in the Human Microbiome Project (HMP), 5 main bacterial phyla have been identified in the pharynx: Firmicutes, Proteobacteria, Bacteroidetes, Fusobacteria and Actinobacteria. Interestingly, the pharyngeal microbiome is distinguished from other parts of the body (intestines, skin and vagina) by having more Bacteroidetes. The proportion of bacterial phyla in the pharyngeal microbiome comprises 27% of Bacteroidetes and only 10% of Proteobacteria, compared to the salivary microbiome, whose proportion corresponds to 9% of Bacteroidetes and 51% of Proteobacteria. These two phyla are mentioned because they are the main pathogens in human infections, for example, periodontitis is caused by Bacteroidetes and the most common Gram-negative pathogens (*Acinetobacter*, *Moraxella*, *Pseudomonas*, *Haemophilus*, *Klebsiella*, and *Legionella* spp) [89]. However, the two genera that dominate the micro-ecosystem of the pharynx are *Streptococcus* and *Prevotella* [90].

## *2.5.1 The pharyngeal microecosystem*

The most common bacterial genera in the human pharynx are *Prevotella*, *Capnocytophaga*, *Campylobacter*, *Veillonella*, *Streptococcus*, *Neisseria*, *Haemophilus*, which represent 9.72% to 1.26% of the bacteria in the normal microbiome, according to HMP data. [89, 90]. As there are few studies of the pharyngeal microbiome, many interactions between the components of the microecosystem are not clear, however, the interactions of microorganisms with factors of the local environment are characteristic of the microbiome. From the above, it can be assumed that the pharyngeal microbiome may share common characteristics of other human microbiomes [89].
