Which of the following microorganisms are considered normal flora of the human respiratory tract?

• A. Clostridium botulinum
• B. Escherichia coli
• C. Streptococcus pneumoniae
• D. Staphylococcus aureus

Answer: (C)

Verified step by step guidance

1. Understand the concept of 'normal flora' or 'normal microbiota,' which refers to the microorganisms that normally reside on or within various parts of the human body without causing disease under normal conditions.
2. Identify the typical locations of normal flora in the human body, focusing on the respiratory tract, which includes the nose, throat, and upper airways.
3. Review the characteristics and common habitats of each microorganism listed: Clostridium botulinum is primarily found in soil and is not part of normal human flora; Escherichia coli is mainly a gut bacterium; Staphylococcus aureus can be part of the normal flora of the skin and nasal passages; Streptococcus pneumoniae is commonly found in the upper respiratory tract as part of the normal flora.
4. Compare each microorganism to the known normal flora of the respiratory tract and determine which are typically present without causing disease.
5. Conclude that Streptococcus pneumoniae and Staphylococcus aureus are considered part of the normal flora of the human respiratory tract, while Clostridium botulinum and Escherichia coli are not.