The 100 Faces of Cryptosporidium parvum

Cryptosporidium parvum is a protozoan enteric parasite of humans and livestock. C. parvum infection mainly affects the ileum, where it has the potential to cause severe enteric disease. Drugs for the treatment of cryptosporidiosis are still not available and the biology and life cycle of C. parvum remain incompletely understood. The present study gives new insight into the parasite’s morphology, life cycle and host cell relationship. This study utilised light microscopy, scanning electron microscopy, transmission electron microscopy and labeling of C. parvum surface receptors to examine infected cell cultures, cell-free cultures and oocyst stocks of C. parvum. Hence, this study compared different culture and different microscopic examination methods to determine the most suitable way to examine C. parvum’s morphology. Cell-free culture did not provide additional information to this study. However, it served as a valuable comparison for life cycle stages detected in the supernatant above cells which are expected to occur in the intestinal lumen of infected hosts. Scanning electron microscopy was the most suitable tool for obtaining information on the parasite’s morphology, whereas transmission electron microscopy enabled a view into the interior of stages. Employing light microscopy in this study was essential to progressively monitor live samples and visualise stages in the supernatant above cells, which were not attached to host tissue. In the course of this study a protocol was developed, which enabled the visualisation of Cryptosporidium receptors on the surface of parasites and/or host cell material via immunogold labeling with scanning electron microscopy. For the first time, the entire range of C. parvum’s life cycle stages has been morphologically characterised (including their interactions with host cells) and presented in one study. A better understanding of the parasite’s biology, proliferation in host tissue and interactions with host cells will aid the drug development process. Recent electron micrographs acquired in the course of this study revealed new life cycle stages, provided new information about the parasite’s morphology and its relationship with host cells. New insight into the host cell invasion process of C. parvum sporozoites as well as merozoites I and II was obtained. Features of gliding motility of the invasive stages were visualised and explained. Phenomena including binary fission - commonly employed by bacteria for the production of two identical daughter stages from one parent stage - and syzygy - the pairing of gamonts to exchange genetic material, described in gregarines - , was observed and described. Extracellular gamonts and gamont-like stages were also characterised; developing from intracellular trophozoites to finally break host cell contact and take their place in the life cycle of C. parvum, travelling free in the intestinal lumen. The morphology of the two different types of oocysts has been described and findings on receptor expression in their outer membranes are presented. Furthermore, C. parvum receptors were identified in the apical membranes surrounding parasite stages. C. parvum surface receptors were also found on host cell microvilli in proximity to invading and/or gliding parasites. Additionally, the present study observed the effect that a C. parvum infection exerts on host tissue with respect to necrosis and apoptosis. This study also poses new ideas regarding the parasite’s host-dependent feeding behaviour.