• Respiratory tracts includes both upper respiratory tract (URT) and lower respiratory tract (LRT). In normal healthy individual LRT is sterile. Microflora are only present in URT
• Although many microorganisms enter URT through air during breathing, most of them are removed by mucus lining and nasal hair.
• Microorganisms are trapped in mucus lining of respiratory tract and downward flow of mucus to nasopharynx into mouth and it is swallowed or spitted out. Mucus also contains lysozyme which is antimicrobial agent.
• Very few microorganism that can attach on the surface of mucus can form resident normal flora of URT.

1. Nose:

•  Staphylococcus aureus
• S. epidermidis
• Streptococci
• diphtheroids
• Haemophilus spp

2. Nasopharynx:

• Non-pathogenic strain of Streptococcus pneumoniae
• alfa haemolytic Streptococci
• Staphylococcus spp
• Neisseria species
• Corynebacterium species
• Haemophilus spp
• Micrococcus spp

3. Nasal sinus:

• Nasal sinus is sterile and the nasal secretion contains lysozyme that kills viruses and gram positive bacteria. However, some transient flora are present
• Staphylococcus spp
• Corynebacterium spp

4. Lower respiratory tract:

• sterile

Normal flora of Respiratory tract

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