THE ROLE OF PH AND TEMPERATURE IN THE DEVELOPMENT OF LEISHMANIA PARASITES

Protozoans of the genus Leishmania are obligate intracellular parasite s that cycle between the midgut of sandflies and the phagolysosomes of mammalian macrophages and therefore are exposed to extreme environmen tal changes. Recent evidence obtained from in vitro experiments indica tes that such environmental changes trigger a developmental program in the parasites. Thus, following heat shock, promastigotes from certain Leishmania species differentiate to amastigotes. Promastigotes also r espond to acidification of their environment by changing the expressio n of a number of genes. However, the combination of both low pH and hi gh temperature induces the transformation of the promastigote to the a mastigote in all Leishmania species examined to date. This review disc usses the role of pH and heat shock in gene regulation and its contrib ution to the differentiation processes in Leishmania spp. Cycling betw een cold-blooded insect vectors and the warm-blooded mammalian host is not unique to Leishmania spp., but typical to most parasitic protozoa . It is therefore likely that the mechanism of stress-induced differen tiation is shared by other mammalian parasites.