Microbiota of the Respiratory Tract

The upper respiratory tract is constantly exposed to airborne microbes, many of which are trapped by mucus and removed by immune defenses.

The anterior nares are typically dominated by commensal Staphylococci and Corynebacterium, which can competitively inhibit potential pathogens.

Deeper regions of the nasal cavity and the oropharynx host a broader range of bacteria.

Some resident Streptococcus species produce antimicrobial peptides that inhibit the growth of pathogens.

Interestingly, non-typeable Haemophilus influenzae can opportunistically become pathogenic when host defenses are weakened or mucosal barriers are disrupted.

It typically adheres to mucus and to non-ciliated epithelial cells, forming aggregates that mature into biofilms.

These bacterial cells can then contribute to chronic inflammation or localized infection.

The lower respiratory tract harbors transient microbes that enter with inhaled air and from the oropharynx.

Commonly detected genera include Prevotella and Veillonella. Some of these species survive the reactive oxygen species generated by host metabolism using detoxifying enzymes to protect their cellular structures and DNA.