P X AIRWAY MICROBIOME X The larynx, trachea, bronchi, and alveoli are traditionally considered normally sterile, particularly the distal airways and alveoli.

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Normal Flora of the Human Airway

Normal flora refers to microorganisms that normally inhabit the respiratory tract without causing disease.

1. Upper respiratory tract

The nose, nasopharynx, and oropharynx contain numerous microorganisms.

Common organisms include:

• Staphylococcus epidermidis

• Staphylococcus aureus — may be carried in the anterior nares

• Streptococcus viridans group

• Streptococcus pneumoniae — especially in the nasopharynx

• Neisseria species (non-pathogenic species)

• Corynebacterium species (diphtheroids)

• Haemophilus species — non-typeable strains may colonize the nasopharynx

• Moraxella catarrhalis

• Anaerobic bacteria, particularly in the oropharynx

• Various oral streptococci

2. Lower respiratory tract

The larynx, trachea, bronchi, and alveoli are traditionally considered normally sterile, particularly the distal airways and alveoli.

Small numbers of microorganisms may be detected in healthy lower airways with modern molecular techniques, but these are largely derived from the upper airway and do not represent the heavy colonization seen in the mouth and pharynx.

Key point for exams

Upper airway → colonized with normal flora.
Lower respiratory tract → traditionally considered sterile.

The normal flora of the upper airway helps prevent colonization by pathogens through competition for nutrients and attachment sites and production of antimicrobial substances.