An effective culture system for Ehrlichia (Cowdria) ruminantium comb. nov. was first established in 1985 and many stocks were subsequently isolated and propagated in vitro. A notable exception, however, was the Kumm isolate that resisted all attempts at in vitro culture until the successful experiment described here. In one experiment white blood cells were harvested from heparinized blood derived from a sheep infected with the Kumm isolate. The cells were added to DH 82 cells and incubated at 37degreesC. The high metabolic activity of the DH 82 cells necessitated that cell growth be retarded by the addition of cycloheximide. Colonies were first detected 19 days after culture initiation and, once the cultures were established, they could be passaged every 3 days. Bovine and sheep endothelial cells were readily infected with culture supernatant obtained from the infected DH 82 cells. In a further experiment, another sheep was infected, using a higher dose of the same batch of Kumm stabilate, and we attempted to infect several different cell lines: these were DH 82 cells, bovine aorta (BA 886) cells, sheep brain endothelial (SBE 189) cells and sheep fibroblastoid cells (E₂). Ten days after culture initiation, only the E₂ cells had become positive for E. ruminantium. Culture supernatant from the first cultured isolate (Kumm-1) was less virulent for mice than that of the second cultured isolate (Kumm-2) which killed all mice. Upon molecular characterization with E. ruminantium 16S probes, we found that Kumm-1 hybridized with a Senegal 16S genotype probe, whereas Kumm-2 hybridized only with an Omatjenne 16S genotype probe. The original stabilate used to infect the sheep hybridized with both probes. These results clearly indicate that two different stocks had been isolated in culture.
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