Reference Work Entry

The Prokaryotes

pp 457-466

Introduction to the Rickettsiales and Other Intracellular Prokaryotes

  • David N. Fredricks


In the past, any small bacterium that required an intracellular environment for growth could be classified as belonging to the order Rickettsiales (Moulder, 1974). It is now clear that bacteria from diverse phylogenetic groups have adopted the intracellular niche, an example of convergent evolution. Most eukaryotic cells have defenses that prevent prokaryotes from invading and establishing a parasitic existence. Once these defenses are breached, however, an intracellular existence has many advantages for the suitably adapted bacterium. Within a host cell, bacteria are free from competition with most other microbes and benefit from host defenses directed against external threats. Intracellular bacteria may have access to host substrates and enzymes for metabolism. Host behaviors, such as blood feeding in insects or locomotion in vertebrate hosts, may aid the spread of intracellular bacteria. Therefore, it should not be surprising that bacteria from different phyl ...

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