Which of the following is a small bacterium that lives in lice, fleas, ticks, and mites?

• A. Rickettsia
• B. Bacillus subtilis
• C. Staphylococcus aureus
• D. Escherichia coli

Answer: (A)

Verified step by step guidance

1. Understand the question is asking to identify a small bacterium that specifically lives in lice, fleas, ticks, and mites, which are arthropod vectors.
2. Recall that Rickettsia is a genus of small, obligate intracellular bacteria known to inhabit and be transmitted by arthropods such as lice, fleas, ticks, and mites.
3. Recognize that Bacillus subtilis, Staphylococcus aureus, and Escherichia coli are not typically associated with living inside these arthropods; they have different ecological niches and modes of transmission.
4. Focus on the characteristics of Rickettsia: it is small, Gram-negative, and often causes diseases transmitted by arthropods, which matches the description in the question.
5. Conclude that the bacterium fitting the description of living in lice, fleas, ticks, and mites is Rickettsia.