Rickettsias differ from chlamydias in that rickettsias:

• A. possess peptidoglycan in their cell walls
• B. can reproduce in arthropod vectors
• C. are obligate intracellular parasites of only humans
• D. do not require living cells for replication

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the nature of both Rickettsias and Chlamydias as obligate intracellular bacteria, meaning they require a host cell to reproduce.
2. Step 2: Review the structural differences, noting that Rickettsias possess peptidoglycan in their cell walls, whereas Chlamydias have a reduced or absent peptidoglycan layer.
3. Step 3: Examine the ecological and biological differences, focusing on the fact that Rickettsias can reproduce within arthropod vectors (such as ticks or lice), which serve as both hosts and transmission agents.
4. Step 4: Contrast this with Chlamydias, which typically do not involve arthropod vectors in their life cycle and primarily infect vertebrate hosts.
5. Step 5: Conclude that the key distinguishing feature in this context is the ability of Rickettsias to reproduce in arthropod vectors, setting them apart from Chlamydias.