Cryptosporidium parvum DNA replication in cell-free culture.

The lack of robust methods for culturing Cryptosporidium parasites remains a major challenge and is hampering efforts to screen for anti-cryptosporidial drugs. In existing culture methods, monolayers of mammalian epithelial cells are inoculated with oocysts. The system supports an initial phase of asexual proliferation of the parasite. For reasons that are not clear, development rapidly declines within 2-3 days. The unexpected report of Cryptosporidium parvum culture in the absence of host cells, and the failure of others to reproduce the method, prompted us to apply quantitative PCR to measure changes in C. parvum DNA levels in cell-free cultures, and parasite-specific antibodies to identify different life cycle stages. Based on this approach, which has not been applied previously to analyze C. parvum growth in cell-free culture, we found that the concentration of C. parvum DNA increased by about 5-fold over 5 days of culture. Immuno-labeling of cultured organisms revealed morphologically distinct stages, only some of which reacted with Cryptosporidium-specific monoclonal antibodies. These observations are indicative of a modest proliferation of C. parvum in cell-free culture.