A growth study of Coxiella burnetii Nine Mile Phase I and Phase II in fibroblasts

Coxiella burnetii, a slow-growing, gram-negative, obligate intracellular bacterium, is the causative agent of Q fever in humans. The avirulent Phase II C. burnetii Nine Mile strain can invade and establish persistent infections in a wide variety of laboratory cell lines, and is generally considered to be easier to grow in culture than the wild-type Phase I organism. Efforts to improve Phase I organism yield in the BHK-21 cell line demonstrated that high CO2 conditions and the use of Dulbecco's modified Eagle's medium (DMEM) with 4.5 g/l glucose supplementation resulted in higher organism yields. Phase II organisms grown in the same cell line and conditions showed lower growth rates. Analysis revealed that increased average numbers of C. burnetii Phase I organisms within fibroblasts was due to higher growth rates within the hosts rather than to increased uptake or to increased cell-to-cell spreading. Addition of the nucleoside cytidine to the growth medium stimulated growth of Phase II but not Phase I organisms.