Purification of Cowdria ruminantium Organisms for Use in Genome Analysis by Pulsed‐field Gel Electrophoresis a

ABSTRACT: Cowdria ruminantium is an obligate intracellular rickettsial pathogen which is responsible for a tick‐borne disease of domestic and wild ruminants called heartwater or cowdriosis. Although several genes have been cloned and partially sequenced, the genome size, gross structure, and organization of the C. ruminantium genome is unknown. Genome analysis of the organism has been hindered because it is difficult to obtain C. ruminantium DNA free from contaminating host cell DNA, and this probably accounts for the lack of genome size data for this organism. In this study we investigated several methods for purifying C. ruminantium from bovine cellular contaminants and organisms of a relatively high purity were obtained. These were used to prepare Cowdria DNA which was analyzed by pulsed‐field gel electrophoresis (PFGE) and which revealed a genome approximately 1900 kbp in length plus an additional extra‐chromosomal fragment migrating with an apparent size of 815 kbp. This is the first time that the genome size of C. ruminantium has been determined and the first demonstration of an extrachromosomal element.