Which of the following microorganisms commonly colonize portions of the human respiratory tract?

• A. Escherichia coli
• B. Streptococcus pneumoniae
• C. Lactobacillus acidophilus
• D. Clostridium tetani

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the question is asking which microorganisms commonly colonize the human respiratory tract. This means identifying normal flora or common pathogens found in that area.
2. Step 2: Recall that Escherichia coli is primarily found in the gastrointestinal tract, not the respiratory tract, so it is unlikely to be a common colonizer there.
3. Step 3: Recognize that Streptococcus pneumoniae is a well-known bacterium that commonly colonizes the upper respiratory tract and can sometimes cause respiratory infections.
4. Step 4: Note that Lactobacillus acidophilus is typically found in the gastrointestinal and vaginal tracts, not the respiratory tract.
5. Step 5: Understand that Clostridium tetani is an environmental bacterium associated with soil and wounds, not a normal colonizer of the respiratory tract.