Population studies of the tick-borne rickettsias, using RFLP variation, suggested that phylogenetic relationships between strains could be determined. This line of investigation was extended by the development of 16S rRNA based strain comparisons, supplemented later by 23S rRNA and 17kD surface antigen gene phylogenetic comparisons of Rickettsia, and 16S rRNA studies of members of the related intracellular group, Ehrlichia.

These studies laid the framework for a re-examination of the classification system for the genus Rickettsia, and for related members of the Rickettsiaceae. They have included an analysis of the ancient origin of the rickettsias, and the varying rates of evolution of the 16S and 23S rRNA genes and the 17kD antigen gene. They also include a consideration of the degree of gene sequence divergence which may be necessary for classification of a strain as a new species.

The methodological developments related to the study of Rickettsia, accomplished in my laboratory, also resulted in the initiation of a series of highly productive collaborations involving myself with Dr. Yasuko Rikihisa of the Department of Veterinary Pathobiology at Ohio State, and involving a visiting scientist, Mr. Wen Bohai, Associate Professor, Department of Microbiology, The Third Military Medical College, Qongqing, Peoples Republic of China. Mr. Wen originally came to my laboratory to obtain training in the use of molecular methods to study intracellular bacteria. Mr. Wen, Dr. Rikihisa and myself then initiated and have pursued a very aggressive set of analyses to understand the genetic relationships among a series of microorganisms thought to be related to the rickettsias being studied in my laboratory. We have been able to show that most of these newly described bacteria can be classified in or near the genus Ehrlichia, a group of special interest to Dr. Rikihisa, and have further shown that they are the "sister" group to Rickettsia. Additional parts of our studies have proven that a few other unclassified forms are only distantly related to our focal groups.