3, 2, 1, go! Cryptosporidium counts down to sex

Abstract

Cryptosporidium is a leading cause of death from childhood diarrhea, but its biology is poorly understood. A recent study in PLOS Biology reveals hitherto unknown aspects of the parasite's life cycle that may lead to improvements in ex vivo culture.

Fig 1. Cryptosporidium infection initiates by ingestion of environmentally robust oocysts, which contain 4 haploid sporozoites.

Sporozoites excyst in the intestine and infect enterocytes in the intestinal epithelium. Here, parasites establish and are enclosed within a parasitophorous vacuole membrane and are separated from remainder of the host cell by an electron dense boundary. Division of this form (hitherto known as a type I meront) generates 8 merozoites (perhaps 6 in some other Cryptosporidium species), which egress and reinfect adjacent enterocytes where they each create 8 more merozoites. At the third such cycle, parasites differentiate into sexually committed merozoites that reinfect a new enterocyte to form a male or female gamont. A second proliferative cycle, involving a type II meront that produces 4 merozoites, was previously believed to be required for the generation of sexually committed parasites, but English and colleagues [4] find that no such form exists in their system in C. parvum. Male microgamonts undergo repeated mitotic divisions to each produce 16 microgametes, and subsequent fertilization between a microgamete and a female macrogamete results in a diploid zygote that undergoes meiosis to create 4 new haploid sporozoites. These sporozoites are encapsulated in an oocyst that can either be shed into the environment or can excyst to autoinfect the same host. Both thick- and thin-walled oocysts have been described microscopically, and some research suggests that only thick-walled oocysts are environmentally persistent.