Molecular genetics of populations of intracellular bacteria: the spotted fever group rickettsiae.

The population structure of tick-borne rickettsiae show the following characteristics: (1) the amount of genetic differentiation between strains within subpopulations is very small. (2) The evolution of the subpopulations does not fit into models based on either host or geographic similarities, suggesting the need for more information on the frequency of vertical versus horizontal transmission of strains between hosts. (3) The species are highly clonal, with little evidence of genetic exchange between populations. (4) The dominant class of genetic change is single-nucleotide point mutation. No evidence for major rearrangements was observed. (5) Differentiation between species of the spotted fever group is equivalent to that seen between local strains of E. coli.