What is the difference between a bacterium and a rickettsia?

Question

The other day in a microbiology seminar, someone brought up how rickettsiae are technically bacteria but behave very differently. That got me curious, so I dug into it and found that while rickettsiae fall under the bacterial domain, they stand out because of their strict intracellular lifestyle, reliance on vectors like ticks, and reduced metabolism. This post breaks it all down clearly.

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Anttilampinen 2025-06-13T12:18:42+00:00 1 Answer 5 views
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  1. fleming
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    2025-07-13T14:49:51+00:00
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    Short version

    Rickettsiae are bacteria, but they are obligate intracellular parasites with several unusual traits that set them apart from free‑living bacteria.

    What makes rickettsiae different

    • Lifestyle: They must live and multiply inside eukaryotic cells, usually the endothelial lining of blood vessels. Ordinary bacteria can grow on agar plates.
    • Transmission: Spread mainly by arthropod vectors such as ticks, fleas or lice.
    • Metabolism: Small genomes (~1.1 Mb) missing many genes for ATP and amino acid synthesis; they import these from the host cell.
    • Cell envelope: Gram‑negative type but with a very thin peptidoglycan layer, so they stain poorly.
    • Culture: Need cell culture, embryonated eggs or animal models; they wont form colonies on routine media.
    • Antibiotic choice: Susceptible to tetracyclines and chloramphenicol; beta lactams work poorly because of the intracellular niche and slow growth.

    Similarities to other bacteria

    They still have bacterial ribosomes, DNA, reproduce by binary fission and possess a cell wall containing peptidoglycan.

    Bottom line

    A rickettsia is a highly specialised bacterium adapted to life inside host cells and to transmission by vectors. That explains why its diagnosis, culture and treatment differ from the garden variety bacterial infections.

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