Chapter 16 : Amoeba and Slime Mold: Hosts of Virulence Evolution

Abstract:

This chapter summarizes the available information on amoeba-fungus interactions and speculates on the origins of fungal virulence for mammalian hosts. Other organisms such as slime molds have ameboid cells that behave like amoebae in movement and feeding. Amoebae and slime molds are grouped under the kingdom Protozoa. Various studies have reported specific types of interactions and responses when amoebae and fungi are brought into contact in the laboratory. Amoeba phagocytosis can be induced by arachidonic acid and prostaglandins, a phenomenon that may be highly relevant to fungus-amoeba interactions since many species of fungi produce arachidonic acid derivatives and prostaglandins. Although each of these observations was made under artificial laboratory conditions, they suggest certain specificity to amoeba-fungus interactions that buttresses the argument that these two types of organisms frequently interact in the environment. In the late 1990s, the author revisited the question of amoeba-fungus relationships and their potential impact in fungal virulence by analyzing the interaction of Acanthamoeba castellanii and Cryptococcus neoformans. Encapsulated C. neoformans cells were able to grow in the presence of A. castellanii, whereas nonencapsulated cells were killed. However, the observations with amoebae, slime molds, and nematodes suggest that traits associated with virulence for mammals also play a role in promoting the survival of C. neoformans following interactions with environmental predators. Fungi that are under selection by amoeba predation, that grow at mammalian temperatures, that tolerate slightly alkaline pH, and that have suitable adhesions may be able to establish themselves in a mammalian host.