Metabolism and genetics of chlamydias and rickettsias

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 defenses. 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 bacteria.