Chlamydial and rickettsial diseases pose a hazard to man and to domesticated and wild animals. The
virulence mechanisms which aid the establishment of these obligate intracellular parasites in the eukaryotic host
are still not within our grasp. Recent knowledge of the biochemical stratagem, the metabolic capabilities and the
genetic diversity of these microbes illustrate fundamental differences in ecology and evolutionary divergence.
The preferred site of intracellular residence determines the strategy for uptake, for nutrient assimilation and also
for evasion of the host's immunological defences. The Chlamydia, Rickettsia, and Coxiella are the most extensively
studied of the genera. Whereas the Ehrlichia and Cowdria are poorly understood, they are also the most
intriguing of the Rickettsiae. A number of antigenically and genetically distinct species are identified for the
genera Chlamydia, Rickettsia, and Ehrlichia, whereas the Coxiella and Cowdria may not represent such a wide
diversity. Recent information on the genetic heterogeneity of the chromosomal and plasmid DNAs of the strains
of Coxiella suggest the diversity is greater than was originally envisioned. New information regarding the
antigenic structure of Cowdria and their cellular tropisms suggests that they are closely related to the Ehrlichia.
In this review we compare the metabolic capabilities and the genetic diversity of these different intracellular
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