Bistability In The Protozoan Parasite Giardia lamblia

Giardia lamblia is an important protozoan parasite, the transmission of which from one host to the other, and survival outside the host are dependent on differentiation of vegetative trophozoites into infectious cysts (encystment). Like many protozoa, Giardia trophozoites differentiate into cysts in response to nutrient deprivation or other unfavorable environmental changes. Giardia, a lipid auxotroph, encysts when bile sequesters lipids that they require (Jarroll et al., 1981, Luján et al., 1996b). During encystment, Giardia trophozoites elicit a unique pattern of gene expression resulting in the synthesis and transport of cyst wall components ultimately resulting in the formation of a protective cyst wall.;Microorganisms that deal with constantly changing environmental conditions such as those experienced by G. lamblia in the host's intestinal tract have evolved additional strategies to increase their chances of survival. Here, evidence is presented that Giardia populations, in addition to forming cysts, employ bistability as a means of survival. Trophozoite populations respond heterogeneously to both encystment and vegetative growth; induction of encystment results in a bistable outcome where a proportion of the population encyst while a significant proportion remains undifferentiated thus producing a phenotypically heterogeneous population, and in the absence of exogenous bile in vegetative cultures a small, but significant, proportion of the population form cysts. The phenotypic heterogeneity observed prepares G. lamblia populations for survival in both favorable and unfavorable environments.;Even though heterogeneity ensures that the different members of Giardia's populations contribute to the survival of the population as a whole, there was no evidence that the sub-populations produced during encystment promote the heterogeneity observed; encysting and non-encysting trophozoites (within encysting cultures) neither exclusively promoted nor inhibited encystment in freshly induced trophozoites. In both cases, high bile conditions were required to produce statistically significant encystment levels. The evidence presented however shows that in addition to high bile conditions, the regulatory pathways for encystment can be spontaneously activated to achieve the heterogeneity observed in the vegetative cultures. There is, therefore, an element of stochasticity involved. The data also suggest that extracellular signaling molecules are neither produced nor used during the induction of encystment, which explains the lack of influence on encystment that occurred when cyst/encysting trophozoites were co-cultured with fresh trophozoites.;Signaling pathways induced during encystment also exhibit bistable properties; trophozoites induced to form cysts become committed after a short period of time and the production of encystment specific proteins is stably maintained when encystment is interrupted. After 3-6 h in inducing conditions, encysting trophozoites continue to encyst regardless of whether the inducing stimulus remains. Transfer of encysting cells into non-inducing conditions does not result in the interruption of encystment specific protein production, which suggests the involvement of regulatory pathways with the ability to "remember" a transient signal long after its removal. These observations suggest that the regulatory pathways induced during encystment exhibit hysteresis. This property enables encysting trophozoites to complete the encystment process should the triggering unfavorable condition(s) become sustained or prolonged. Bistability of the regulatory pathways induced during encystment also ensure that in situations where the presence of the inducing signal occurs only shortly, the induction process can be easily aborted to conserve energy and promote growth. Bistability appears to play an important role in Giardia's success as a parasite.